Nonlinear solid-state devices for light emission in the far UVC spectrum.

Abstract

The ultraviolet (UV) light source includes a light emitting element that generates light at a first frequency, a nonlinear optical element that receives the light at the first frequency from the light emitting element and generates UVC or far UVC light at a second frequency from the light at the first frequency, and an output coupling element that selectively outcouples the UVC or far UVC light from the nonlinear optical element as output light. Related light sources and methods of manufacture are also described.

Description

[Technical field]

[0001] This application relates to UV light sources, and in particular to deep UVC light sources and related devices and methods.

[0002] [Priority claim] This application claims priority to U.S. Provisional Patent Application No. 63 / 311,660, filed in the U.S. Patent and Trademark Office on February 18, 2022, and U.S. Provisional Patent Application No. 63 / 359,251, filed on July 8, 2022, the disclosures of which are incorporated herein by reference in their entireties. [Background technology]

[0003] A small, efficient ultraviolet (UV) light source in the wavelength range of about 200 nanometers (nm) to about 400 nm may be desirable for many applications. For example, UV lasers can be used for lithography in semiconductor manufacturing. Another application is the detection and classification of materials and substances, such as in mass spectrometry, since short wavelength radiation is easily absorbed by most materials. Photons in the UV-C (or UVC) wavelength range (e.g., about 200 nm to about 280 nm) can be used to disinfect disease-causing pathogens in the air and on surfaces while maintaining conditions safe for human exposure. For example, far-UVC light (about 200 nm to about 240 nm) may not penetrate the dead cell layer or tear film on the skin surface of the human eye, but can be effective against bacteria and viruses. In particular, far-UVC light can efficiently cause permanent physical damage to DNA, preventing bacteria, viruses, and fungi from replicating. Therefore, far-UVC light, which is safe for humans, can effectively kill disease-causing pathogens with little or no risk to humans because these wavelengths can be largely absorbed by the stratum corneum (the top layer of dead skin cells in the epidermis). Summary of the Invention [Problem to be solved by the invention]

[0004] However, operating in the far UVC wavelength range can present challenges. For example, there are few available light sources that are configured to operate in the far UV. Some conventional UV light sources are implemented by gas-based lamps. An important class of such lamps is called "excimer" (excited dimer) lamps, which use a mixture of reactive gases (such as F2 or Cl2) and inert gases (such as Kr, Ar or Xe) as the active medium. When electrically excited, the gas mixture produces quasi-molecular excited state dimers or "excimers" that have an energy level configuration that allows the generation of specific ultraviolet laser wavelengths. For example, some KrCl lamps can be used to generate far UVC light for medical applications. However, the inefficiency, large size, and significant cost of such lasers may make them unsuitable for use in many applications.

[0005] Also, high power ultrafast laser systems designed for laboratory use can generate nonlinear harmonics (e.g., second harmonic generation, third harmonic generation, fourth harmonic generation, and fifth harmonic generation) or parametric sum frequency generation to produce light in the far UV. Such systems can similarly be large (e.g., tabletop or macroscopic optical bench size), costly, and inefficient (e.g., producing less than 1 watt optical in the far UV).

[0006] Free-electron-pumped field emission lamps with hexagonal boron nitride (h-BN) targets may rely on vacuum-sealed bulbs to allow the electron beam to operate, but the power efficiency and reliability of such lamps may be unproven.

[0007] Semiconductor-based (e.g., based on the GaN material system) LED light sources have also been used to obtain UV-C light, for example, using phosphor-based wavelength conversion. Such sources typically suffer from short operational lifetimes and poor performance at emission wavelengths shorter than about 265 nm. Also, regulatory restrictions remain stringent, as uncertainties regarding human safety remain. [Means for solving the problem]

[0008] According to some embodiments, an ultraviolet (UV) light source includes a light emitting element configured to generate light at a first frequency, a nonlinear optical element configured to receive the light at the first frequency from the light emitting element and generate far-UVC light at a second frequency from the light at the first frequency, and an output coupling element selectively outcouples the far-UVC light from the nonlinear optical element as output light.

[0009] In some embodiments, the output coupling element selectively outcouples the far-UVC light in at least one direction different from the propagation direction of the light at the first frequency to provide the output light.

[0010] In some embodiments, the output light is substantially free of light at the first frequency.

[0011] In some embodiments, the nonlinear optical element, the light emitting element, and / or the output coupling element comprise elements of the same material system. In some embodiments, the nonlinear optical element comprises Aluminum Nitride (AlN). In some embodiments, the light emitting element and / or the output coupling element comprise III-Nitride based materials.

[0012] In some embodiments, the nonlinear optical element is or includes an optical cavity that is at least partially resonant at the first frequency.

[0013] In some embodiments, the nonlinear optical element has a ring configuration that forms an optical cavity.

[0014] In some embodiments, the nonlinear optical element includes a plurality of nonlinear optical elements positioned to receive light at the first frequency from the light emitting element.

[0015] In some embodiments, the system further comprises an input coupling element configured to receive light at the first frequency from the light emitting element, the plurality of nonlinear optical elements being disposed along the input coupling element.

[0016] In some embodiments, each of the nonlinear optical elements comprises different dimensions and / or materials. The output coupling element includes a plurality of output coupling elements, each configured to selectively outcouple the far-UVC light from each of the nonlinear optical elements.

[0017] In some embodiments, the optical cavity contains a light emitting element and a nonlinear optical element therein.

[0018] In some embodiments, the optical cavity has a linear or closed curve shape.

[0019] In some embodiments, the output coupling element includes at least one of a facet having a refractive index configured to selectively outcouple the far UVC light in a first direction or a grating having a diffraction order configured to selectively outcouple the far UVC light in a second direction different from the first direction.

[0020] In some embodiments, the nonlinear optical element and the output coupling element are combined into the same output element configured to outcouple far-UVC light at multiple locations or continuously along its length.

[0021] In some embodiments, the UV light source is configured to provide output light that has substantially no phase matching between the light at the first frequency and the far-UVC light at the second frequency.

[0022] In some embodiments, at least one of the nonlinear optical element and the output coupling element is configured to provide phase matching between the far-UVC light at the second frequency and the light at the first frequency.

[0023] In some embodiments, the light emitting element is a laser including a lasing cavity. The laser generates light at a first frequency. In some embodiments, the laser includes a III-nitride based material.

[0024] In some embodiments, the light emitting element further comprises one or more optical resonators that reflect light at the first frequency and that are disposed at the first and second ends of the lasing cavity.

[0025] In some embodiments, the nonlinear optical element receives light at the first frequency from an intracavity portion between the first end and the second end of the lasing cavity.

[0026] In some embodiments, the nonlinear optical element includes a first nonlinear optical element and a second nonlinear optical element disposed at a first end and a second end, respectively, of the lasing cavity.

[0027] In some embodiments, a saturable absorber within the lasing cavity generates light at the first frequency as a plurality of optical pulses at a predetermined pulse repetition frequency and duty factor.

[0028] In some embodiments, the at least one tuning mechanism tunes one or more operating characteristics of the nonlinear element based on the light at the first frequency.

[0029] In some embodiments, the monitoring element measures a characteristic of the output light and generates a feedback signal to a controller that operates the light-emitting element and / or the adjustment mechanism.

[0030] In some embodiments, the substrate has a light emitting element, a nonlinear optical element and an output coupling element on a surface thereof, and two or more of the light emitting element, the nonlinear optical element, the output coupling element, or their connecting waveguides overlap in a direction perpendicular to the surface of the substrate.

[0031] In some embodiments, the output coupling element comprises a plurality of output coupling elements configured to outcouple the far UVC light in respective directions to provide a desired far-field pattern in the output light.

[0032] In some embodiments, one or more sensors detect real-time conditions in the operating environment of the UV light source and send detection signals indicative of the real-time conditions to a controller that controls operation of the light-emitting elements based on the detection signals.

[0033] In some embodiments, the second frequency comprises a sum or a harmonic of the first frequency.

[0034] In some embodiments, the first frequency corresponds to a first wavelength in a range of approximately 400 nanometers (nm) to 480 nm, and the second frequency corresponds to a second wavelength in a range of approximately 200 nm to 240 nm.

[0035] In some embodiments, the light emitting element and the nonlinear optical element include respective elements disposed on a non-native substrate.

[0036] In some embodiments, the light emitting element and the nonlinear optical element are integrated into a monolithic structure.

[0037] In some embodiments, the UV light source comprises an array including a plurality of light emitting elements and a nonlinear optical element.

[0038] According to some embodiments, the light source has a monolithic structure including a light emitting element that generates light at a first frequency, and a nonlinear optical element that receives the light at the first frequency from the light emitting element and generates light at a second frequency from the light at the first frequency.

[0039] In some embodiments, the monolithic structure further comprises an output coupling element for selectively outcoupling light at the second frequency from the nonlinear optical element as output light, the output coupling element selectively outcoupling light at the second frequency in at least one direction different from the propagation direction of the light at the first frequency to provide the output light.

[0040] In some embodiments, the monolithic nonlinear optical element comprises aluminum nitride (AlN). In some embodiments, the monolithic light emitting element and / or output coupling element comprise III-nitride based materials.

[0041] In some embodiments, the light emitting element is a laser including a lasing cavity, and the nonlinear optical element receives light at the first frequency from an intracavity portion between the first and second ends of the lasing cavity.

[0042] In some embodiments, the second frequency of light is UVC light, such as far UVC light, hi some embodiments, the first frequency of light is visible light.

[0043] According to some embodiments, an ultraviolet (UV) light source includes a light emitting element that generates light at a first frequency, and a nonlinear optical element that includes aluminum nitride (AlN) that receives the light at the first frequency from the light emitting element and generates UVC light at a second frequency from the light at the first frequency.

[0044] According to some embodiments, the output coupling element is configured to selectively outcouple the UVC light from the nonlinear optical element as output light, in some embodiments in at least one direction different from the propagation direction of the light at the first frequency. The light emitting element and / or the output coupling element may, in some embodiments, comprise III-nitride based materials in a monolithic structure.

[0045] According to some embodiments, an ultraviolet (UV) light source comprises a light emitting element that generates light at a first frequency, and an optical cavity including a nonlinear optical element that receives the light at the first frequency from the light emitting element and generates UVC light at a second frequency from the light at the first frequency, the optical cavity at least partially resonating at the first frequency.

[0046] In some embodiments, the optical cavity is at least partially resonant at the first frequency and the second frequency.

[0047] In some embodiments, the optical cavity comprises a plurality of optical cavities, each optical cavity including a respective nonlinear optical element and configured to receive light at the first frequency from the light emitting element, hi some embodiments, the optical cavity is ring shaped.

[0048] In some embodiments, each of the optical cavities comprises different dimensions and / or materials, and the output coupling element comprises a plurality of output coupling elements each configured to selectively outcouple UVC light from each of the optical cavities.

[0049] In some embodiments, the nonlinear optical element and the output coupling element are combined into an output element configured to outcouple UVC light at multiple locations or continuously along its length.

[0050] According to some embodiments, the light source comprises a light emitting element that generates light at a first frequency, and a nonlinear optical output coupling element that receives the light at the first frequency from the light emitting element, generates light at a second frequency from the light at the first frequency, and outcouples the light at the second frequency as output light at multiple locations or continuously along its length.

[0051] In some embodiments, the light source is configured to provide output light with substantially no phase matching between the light at the first frequency and the light at the second frequency.

[0052] In some embodiments, the nonlinear optical output coupling element selectively outcouples light at the second frequency in at least one direction different from the propagation direction of light at the first frequency to provide output light.

[0053] In some embodiments, the nonlinear optical output coupling element includes or is coupled to an optical cavity that is at least partially resonant at the first frequency.

[0054] In some embodiments, the nonlinear optical output coupling element includes a plurality of alternating nonlinear optical element sections and output coupling element sections along its length.

[0055] In some embodiments, the nonlinear optical output coupling element comprises a first material and a second material that alters optical propagation at one of a first wavelength corresponding to a first frequency and a second wavelength corresponding to a second frequency, and that does not substantially alter optical propagation at the other of the first wavelength and the second wavelength.

[0056] In some embodiments, the nonlinear optical output coupling element is a waveguide with nanopores or defects having respective dimensions configured to scatter light at the second frequency without substantially affecting the propagation of visible light at the first frequency.

[0057] In some embodiments, the first frequency corresponds to a first wavelength in a range of approximately 400 nanometers (nm) to 480 nm, and the second frequency corresponds to a second wavelength in a range of approximately 200 nm to 240 nm.

[0058] Other devices, apparatus, and / or methods according to some embodiments will become apparent to one of ordinary skill in the art upon review of the following figures and detailed description, and all such additional embodiments, in addition to any and all combinations of the above embodiments, are intended to be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. [Brief description of the drawings]

[0059] [Figure 1A] FIG. 1 is a schematic block diagram illustrating a UV light source according to some embodiments of the present disclosure. [Figure 1B] FIG. 2 is a schematic block diagram illustrating elements of a UV light source in greater detail according to some embodiments of the present disclosure. [Figure 1C] 1 is a graph showing the emission range of light output from a UV light source according to some embodiments of the present disclosure. [Figure 2A1] FIG. 2 is a schematic perspective view showing elements of a vertical linear arrangement of UV light sources according to some embodiments of the present disclosure. [Figure 2A2] FIG. 2 is a side view showing elements of a vertical linear arrangement of UV light sources according to some embodiments of the present disclosure. [Figure 2B1] FIG. 2 is a schematic perspective view showing elements of a horizontal linear arrangement of UV light sources according to some embodiments of the present disclosure. [Figure 2B2] FIG. 2 is a top view showing elements of a horizontal linear arrangement of UV light sources according to some embodiments of the present disclosure. [Figure 2C1] 1 is a schematic top view illustrating elements of a UV light source in a spiral arrangement, according to some embodiments of the present disclosure. [Figure 2C2] 1 is a schematic top view illustrating elements of a UV light source in a spiral arrangement, according to some embodiments of the present disclosure. [Figure 3A1] FIG. 2 is a schematic block diagram illustrating elements of a UV light source including optical cavity enhancement, according to some embodiments of the present disclosure. [Figure 3A2] FIG. 2 is a schematic block diagram illustrating elements of a UV light source including optical cavity enhancement, according to some embodiments of the present disclosure. [Figure 3B1] FIG. 1 is a schematic perspective view showing elements of a UV light source including optical cavity enhancements in a horizontal linear arrangement according to some embodiments of the present disclosure. [Figure 3B2] FIG. 2 is a top view illustrating elements of a UV light source including optical cavity enhancements in a horizontal linear arrangement according to some embodiments of the present disclosure. [Figure 4A]FIG. 2 is a schematic block diagram illustrating elements of a UV light source including a nonlinear optical element in a ring cavity configuration according to some embodiments of the present disclosure. [Figure 4B] FIG. 2 is a schematic top view illustrating elements of a UV light source including multiple nonlinear optical elements in a sequentially arranged ring cavity configuration according to some embodiments of the present disclosure. [Figure 4C1] 1 is a schematic top view illustrating elements of a UV light source including a nonlinear optical element coupled to an intracavity portion of a light-emitting element according to some embodiments of the present disclosure. [Figure 4C2] FIG. 2 is a schematic top view illustrating an array of UV light sources, each of which includes a nonlinear optical element connected to an intracavity portion of a light-emitting element, according to some embodiments of the present disclosure. [Figure 4C3] 1 is a graph illustrating Vernier frequency selection to determine the optical cavity size (including height, width, and circumference / length) of a ring-shaped nonlinear optical element according to some embodiments of the present disclosure. [Figure 5A] FIG. 2 is a schematic block diagram illustrating elements of a UV light source in which the light-emitting element and the nonlinear optical element are provided within the same optical cavity, according to some embodiments of the present disclosure. [Figure 5B1] FIG. 13 is a schematic perspective view showing elements of a UV light source within the same optical cavity with output coupling elements implemented as reflective facets configured for selective light extraction in a horizontal linear arrangement according to some embodiments of the present disclosure. [Figure 5B2] FIG. 13 is a top view showing elements of a UV light source within the same optical cavity with output coupling elements implemented as reflective facets configured for selective light extraction in a horizontal linear arrangement according to some embodiments of the present disclosure. [Figure 5C1] FIG. 13 is a schematic perspective view showing elements of a UV light source within the same optical cavity with an output coupling element implemented as an optical grating configured for selective light extraction in a horizontal linear arrangement according to some embodiments of the present disclosure. [Figure 5C2]FIG. 13 is a top view showing elements of a UV light source within the same optical cavity with an output coupling element implemented as an optical grating configured for selective light extraction in a horizontal linear arrangement according to some embodiments of the present disclosure. [Figure 6A] FIG. 2 is a schematic block diagram illustrating elements of a UV light source in which the light-emitting element and the nonlinear optical element are provided in a closed loop or racetrack configuration within the same optical cavity, according to some embodiments of the present disclosure. [Figure 6B1] FIG. 1 is a schematic perspective view showing elements of a UV light source within the same optical cavity in a closed loop or racetrack configuration, with an output coupling element implemented as an optical grating configured for selective light extraction, according to some embodiments of the present disclosure. [Figure 6B2] FIG. 13 is a top view showing elements of a UV light source within the same optical cavity in a closed loop or racetrack configuration with an output coupling element implemented as an optical grating configured for selective light extraction according to some embodiments of the present disclosure. [Figure 7A] 1A-1C show exemplary drive signals for providing pulsed light output from light-emitting elements in a UV light source, according to some embodiments of the present disclosure. [Figure 7B] 1A-1C show exemplary drive signals for providing pulsed light output from light-emitting elements in a UV light source, according to some embodiments of the present disclosure. [Figure 7C] 1A-1C show exemplary drive signals for providing pulsed light output from light-emitting elements in a UV light source, according to some embodiments of the present disclosure. [Figure 8] FIG. 1 is a schematic perspective view illustrating respective elements of a UV light source arranged in a hybrid configuration on a common non-native substrate, according to some embodiments of the present disclosure. [Figure 9] FIG. 1 is a schematic block diagram illustrating an array of UV light sources, each of which includes a light-emitting element and a nonlinear optical element, according to some embodiments of the present disclosure. [Figure 10A]FIG. 1 is a schematic perspective view illustrating elements of a UV light source including an optical cavity enhancement with an output coupling element configured to provide distributed emission and selective light extraction in a horizontal linear arrangement according to some embodiments of the present disclosure. [Figure 10B] FIG. 1 is a top view illustrating elements of a UV light source including an optical cavity enhancement with an output coupling element configured to provide distributed emission and selective light extraction in a horizontal linear arrangement according to some embodiments of the present disclosure. [Figure 11A] FIG. 1 is a schematic top view illustrating an exemplary combination of various elements of a UV light source, including a nonlinear optical element connected to an intracavity portion of a light-emitting element in combination with a ring resonator at each end of the light-emitting element and an output coupling element configured for selective light extraction, in accordance with some embodiments of the present disclosure. [Figure 11B] 11 is a graph illustrating the Vernier frequency effect according to some embodiments of the present disclosure, which provides for selection of a subset of possible longitudinal modes supported by a light emitting element. [Figure 12] FIG. 1 is a schematic top view illustrating an example combination of various elements of a UV light source including a nonlinear optical element connected to an extra-cavity portion of the light-emitting element in combination with a ring resonator at each end of the light-emitting element, an adjustment mechanism, an output coupling element configured for selective light extraction, and an output monitor, in accordance with some embodiments of the present disclosure. [Figure 13] FIG. 1 is a schematic top view illustrating an exemplary combination of various elements of a UV light source including a nonlinear optical element connected to an extra-cavity portion of a light-emitting element including a saturable absorber element in combination with an adjustment mechanism, an output coupling element configured for selective light extraction, and an output monitor, in accordance with some embodiments of the present disclosure. [Figure 14] FIG. 1 is a schematic top view illustrating an example combination of various elements of a UV light source including multiple different or non-identical nonlinear optical elements 120 in a ring cavity configuration with respective output coupling elements, according to some embodiments of the present disclosure. [Figure 15]1A-1C are schematic top views illustrating example combinations of various elements of a UV light source in a physically overlapping configuration according to some embodiments of the present disclosure. [Figure 16] 1A-1C are schematic top views illustrating the inclusion of an output coupling element configuration of a UV light source that provides a desired far-field emission pattern, according to some embodiments of the present disclosure. [Figure 17] FIG. 1 is a schematic block diagram illustrating components of a sensor feedback based “smart” lighting device including a germicidal UV (GUV) light source communicatively connected to a sensor configured to feed back information to a controller of the GUV light source, according to some embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] DETAILED DESCRIPTION OF THE DRAWINGS The present disclosure provides a solution for generating electromagnetic radiation in the far-UVC wavelength band (approximately 200 nm to 240 nm, e.g., approximately 207 nm to 222 nm), and controlling associated illumination patterns. This may be useful for numerous applications, including (but not limited to) germicidal applications for disinfecting airborne and surface disease-causing pathogens, and for detecting trace amounts of chemical or biological species in various field environments (air, water, etc.), while remaining safe for human exposure and complying with human safety regulations and requirements. In particular, embodiments of the present disclosure provide solid-state systems and methods for generating coherent or non-coherent, collimated or non-collimated, electromagnetic, non-ionizing radiation in the far-UVC wavelength band using photonic integrated circuits (PICs) based on nonlinear optical processes. As used herein, "far-UV" or "Far UV" wavelength band or range refers to wavelengths greater than or equal to about 200 nm and less than or equal to about 240 nm, e.g., between about 200 nm and 230 nm (such that the radiation is non-ionizing in air).

[0061] Embodiments of the present disclosure enable the generation of light in the far UVC band using compact light sources based on semiconductor industry materials and processes, which allows for a rapid volume scaling reduction in cost that may not otherwise be available. Embodiments of the present disclosure can provide a far UVC light source that includes a semiconductor light emitting element, such as a pump laser (e.g., a III-nitride based laser diode), that generates light at a first wavelength (e.g., in the visible spectrum, also referred to herein as visible light). The semiconductor light emitting element is coupled to a nonlinear optical element (e.g., a monolithically integrated waveguide having nonlinear optical properties) to generate light that is the sum of the frequencies of the visible light (also referred to herein as sum frequency generation (SFG), e.g., second harmonic generation (SHG) of the frequency doubled light). Sum frequency generation can include both frequency doubling (combination of photons of the same wavelength) and optical parametric conversion (i.e., from the combination or difference of two photons of unequal wavelengths). The nonlinear optical element may be referred to herein as an SHG element, or more generally as an SFG element. Sum frequency generation or frequency doubling converts a portion of the visible light emitted by the light emitting element into far UVC light. In particular, some embodiments of the present disclosure provide a monolithic solid-state far UV photonic integrated circuit (PIC) (e.g., based on the AlN / GaAlN material system) that may be scalable to high volume, low cost, high WPE, and small form factor without the need for optical filters that identify or transmit only light in the far UVC wavelength range.

[0062] As used herein, the term monolithic structure or monolithic integration may refer to any arrangement of active elements (e.g., light emitting elements) and / or passive elements (e.g., waveguides or other optical coupling elements) in a monolithic structure without air interfaces or free propagation of light between the elements, including structures formed by epitaxial growth, wafer bonding, and / or microtransfer printing, or other forms of mass transfer for solid-state integration. Thus, a monolithic integration may include elements of the same material system or multiple materials, and may be provided on a native (e.g., growth) substrate or on a non-native substrate (different from the native or source substrate on which the elements are grown or formed). In contrast, a hybrid structure or hybrid integration may refer to an arrangement of separate or individual elements (e.g., respective semiconductor chips) with air interfaces between the elements, and / or an assembly of such individual elements on a non-native substrate. "Coupled" elements may refer to physical and / or optical coupling.

[0063] US Patent No. 9,159,178 describes the use of a semiconductor diode laser as a pump frequency that is passed once through a nonlinear crystal (BBO) and critically phase-matched by angle tuning. The publication "Periodically-Poled AlN for frequency doubling" by Sitar et al. describes AlN as a nonlinear material, but attempts to achieve phase matching by periodically poling the AlN using a macroscopic pump laser that is externally coupled with large optical losses into an AlN ridge waveguide.

[0064] Second (or third, fourth, etc.) harmonic frequency generation using nonlinear optical materials according to some embodiments may require multiple components or characteristics for efficient conversion. For example, a nonlinear crystal that is non-centrosymmetric and highly polarizable may result in a non-zero element of its second-order nonlinearity tensor, the higher the coefficient, the higher the conversion rate. The nonlinear crystal should be optically transparent at the wavelength of the frequency-doubled light, otherwise the crystal will absorb the newly generated light. Also, a pump light source that is coherent (second harmonic generation is a coherent effect associated with a single wavelength, therefore the pump laser can have a narrow linewidth with a sufficiently long coherence length) and high power (the output power of second harmonic generation is proportional to the square of the pump power, therefore the higher the power of the pump laser, the higher the conversion efficiency) may be used. In some embodiments, a pulsed laser, which generally has a higher peak pulse power than a continuous wave (CW) laser, may be preferred. In some embodiments, phase matching methods can be used to match the phase velocity of the pump wavelength to the phase velocity of the second harmonic wavelength such that coherent addition of the electric fields from both waves is maintained throughout the propagation length of the nonlinear crystal.

[0065] According to some embodiments of the present disclosure, a UV light source includes a light emitting element (e.g., a III-nitride based laser diode such as a blue pump laser diode) that generates light (e.g., visible light) at a first (fundamental) frequency or wavelength; a nonlinear optical element (e.g., a nonlinear optical crystal such as an SHG element) that is optically transparent to wavelengths below a desired output wavelength (e.g., in the UVC wavelength range of about 200 nm to about 280 nm, or the far UVC wavelength range of about 200 nm to about 240 nm) and generates UVC or far UVC light at a second frequency or wavelength based on sum frequency generation of the light at the first frequency; an input coupling element (e.g., a continuous waveguide or optical fiber or photonic wire bond that couples the radiation of a pump laser to the nonlinear optical crystal, also referred to herein as an input waveguide) that couples light from the light emitting element to the nonlinear optical element; and an output coupling element that selectively outcouples the UVC or far UVC light from the nonlinear optical element. In some embodiments, one or more elements can provide phase matching between the UVC or far-UVC light of the second frequency or wavelength and the fundamental (pump) frequency or wavelength of visible light.

[0066] FIG 1A is a schematic block diagram illustrating a UV light source 100 according to some embodiments of the present disclosure. As shown in FIG 1A, the ultraviolet (UV) light source 100 includes light emitting elements 110, 110' that generate light 111 of a first frequency (e.g., visible light 111'), nonlinear optical elements 120, 120' that receive the light 111, 111' from the light emitting elements 110 and generate light 121 of a second frequency (e.g., far-UVC light 121') based on sum frequency generation of the light 111, 111' of the first frequency, and an output coupling element 130 that selectively outcouples the light 121, 121' from the nonlinear optical elements 120, 120' as output light 131, 131'. Although the present disclosure will be described primarily with respect to the generation of far-UVC light (including wavelengths within about 200 nm to about 240 nm), it will be understood that embodiments of the present disclosure are not limited to the generation of far-UVC light. For example, some embodiments may include a nonlinear optical element 120 that generates light over a broader wavelength range, such as UVC light (including wavelengths within about 200 nm to about 280 nm).

[0067] In some embodiments, the light emitting element 110 may be a blue pump laser 110' that produces (high power) coherent radiation at wavelengths between about 400 nm and about 460-480 nm with good wall plug efficiency (optical power output per unit power consumption). In some embodiments, the laser 110' may be a laser diode (e.g., an edge-emitting laser or a vertical cavity surface-emitting laser (VCSEL)). However, other lasers (e.g., frequency-doubled fiber lasers) may also be used. The light emitting element 110 may be formed from or otherwise include a III-nitride based material (such as gallium nitride (GaN)).

[0068] The nonlinear optical element 120 can be configured to generate far-UVC light 121' at a second frequency based on sum frequency generation of the light at the first frequency output from the light emitting element 110. The second frequency can be a harmonic (e.g., an integer multiple) of the first frequency. The nonlinear optical element 120 can be a nonlinear optical crystal that is optically transparent to wavelengths equal to or less than the far-UVC output wavelength of about 200 nm to 240 nm. Examples of such nonlinear optical crystals include, but are not limited to, BBO, aluminum nitride (AlN), lithium niobate (LiNbO3), and the like.

[0069] AlN has not generally (to the best of the inventors' knowledge) been used to provide nonlinear optical elements. Rather, those skilled in the art of nonlinear optics (as distinct from those skilled in the art of semiconductor processing) have typically relied on bulk crystals that use, for example, angle tuning of birefringent materials to achieve phase matching. Typical suppliers of such bulk nonlinear crystals are similarly unaware of and do not sell AlN nonlinear optics. Also, bulk crystal AlN may be less suitable for operation in the UV wavelength range due to the large amount of point defects therein that can absorb relatively short wavelength light.

[0070] Thin film AlN can be optically transparent for wavelengths of light as short as 200 nm, but such thin films of AlN are limited to waveguide use. Research using thin film AlN for nonlinear optical conversion has typically been limited to longer wavelengths of light, for example, due to some inherent challenges with achieving similar results at shorter wavelengths of light. For example, polycrystalline thin films produced by sputtering may contain grain boundaries that cause absorption and scattering at short wavelengths of light (similar to point defects in bulk AlN). Fabrication of PICs in AlN with the fidelity required for acceptable nonlinear conversion may similarly be difficult at shorter wavelengths of light. Such challenges may manifest as losses in the nonlinear optical elements and thus may be a barrier to realizing the desired performance at short wavelengths. In particular, sufficient phase matching may be difficult to achieve, depending on the fidelity of the fabrication.

[0071] Some embodiments of the present disclosure arise from the recognition that providing higher intensity pump laser light to a nonlinear optical element can overcome the aforementioned optical loss challenges. The embodiments of the present disclosure provide various configurations for obtaining higher optical intensities in the nonlinear optical element, including (but not limited to) monolithic integration, cavity enhancement, and intracavity tapping as described herein. The embodiments of the present disclosure also address challenges related to phase matching, which can be more difficult at shorter wavelengths.

[0072] AlN may be advantageous for forming the nonlinear optical element 120 in combination (e.g., by epitaxial growth) with a III-nitride based light emitting element 110 material such as GaN, due to its lattice match or similarity in material processing with GaN. More generally, the nonlinear optical element 120 and the light emitting element 110 may include common elements or materials that belong to the same material system (e.g., AlN may be used as the nonlinear optical element 120' because it belongs to the same AlGaInN material system from which the GaN light emitting element 110' is formed). Certain, but not limited to, embodiments are described herein with reference to the AlN based nonlinear optical element 120 (and in some embodiments with reference to a light source in which the light emitting element 110, the nonlinear optical element(s) 120, and the coupling elements 115, 130 are all nitride based materials).

[0073] The output coupling element 130 may refer to an optical element that provides the output light 131 (e.g., far-UVC light 121') for propagation through free space. In some embodiments, the output coupling element 130 may be configured to provide selective light extraction such that the output light 131 may primarily include far-UVC light 121' of a second wavelength or frequency (and in some cases may be substantially free of visible light 111' of the first wavelength or frequency) in one or more directions different from the propagation direction(s) of the light 111 of the first wavelength or frequency. The output coupling element 130 may be implemented as part of a waveguide (also referred to herein as an output waveguide) and / or an edge facet in some embodiments. In some embodiments, the output coupling element 130 may be a grating that generates different propagation directions of the output light 131 as a surface emission, such as a surface normal (or near surface normal) emission. In some embodiments, the output coupling element 130 may be integrated into the nonlinear optical element 120 (also referred to herein as the nonlinear optical output coupling element 120 / 130).

[0074] FIG. 1B is a schematic block diagram showing in more detail the elements of a UV light source according to some embodiments of the present disclosure. As shown in FIG. 1B, the UV light source has a light emitting element 110 (shown as a pump laser 110′ outputting visible light 111′ having a wavelength λ0 between 400 nm and 480 nm) and a nonlinear optical element 120 (shown as an SFG element outputting light 121′ having a wavelength λ2 between 200 nm and 240 nm, e.g., ½λ0). Some subcomponents of the laser 110′ include a gain medium and an optical structure forming an optical cavity 125. The gain medium is pumped (electrically, optically, and / or by other means) to achieve population inversion, and “pump light” 111 (used herein to refer to the output of the laser 110′ or other light emitting element 110) experiences cavity effects and propagates through the elements. In the example of FIG. 1B, a simple Fabry-Perot cavity is shown with an asymmetry in reflectivity such that emission is favored on one side. While this embodiment shows a single pass of pump light 111 through the SFG element, generating SFG light (λ2) along the way, other embodiments can use optical cavity 125 at either (λ0 or λ2 or both) to generate resonant enhancement of the SFG element. That is, it is understood that this example is merely illustrative and can be used with other semiconductor diode lasers, including (but not limited to) edge emitters with cleaved facet end mirrors, distributed feedback grating structures, or photonic crystal structures, to generate the optical cavity 125. The exemplary diagram can also represent a vertical cavity surface emitting laser (VCSEL) where the gain is a multiple quantum well (MQW) structure and the end mirrors are dielectric Bragg reflectors (DBRs). In some embodiments, optional active or passive elements such as optical amplifiers, mode converters, etc. can be provided at the output of the light emitting element 110 to improve performance.

[0075] The input coupling element 115 couples the visible light 111′ from the light emitting element 110 to the nonlinear optical element 120. The input coupling element 115 can be a continuous waveguide that connects the radiation from the pump laser to the nonlinear optical crystal. Some embodiments can include optical coupling by non-waveguide means, e.g., free space propagation and focusing with lenses, optical fibers, etc. That is, the input optical coupling element can be implemented by any optical element that relays the light output from the light emitting element 110 to the nonlinear optical element 120.

[0076] In some embodiments, the UV light source can be configured to provide phase matching between the second frequency ω2 or second wavelength λ2 (also referred to herein as the SHG or SFG wavelength or wavelength range) of the far UVC light 121' generated by the nonlinear optical element 120 and the first frequency ω1 or first wavelength λ0 (also referred to herein as the fundamental or pump wavelength or wavelength range) of the visible light 111' generated by the light emitting element 110. For example, phase matching can be provided by implementing the nonlinear optical element 120 as a waveguide (which may or may not have optical resonance) configured such that the velocities of the propagation modes supported at the fundamental wavelength and the SHG / SFG wavelength are identical and thus phase matched. However, in some embodiments, other means of phase matching may be used, such as (but not limited to) birefringence in a periodically poled crystal (i.e., quasi-phase matching, type 0) or a nonlinear crystal for type 1 or type 2 phase matching. In other embodiments, the UV light source may be substantially free of phase matching (i.e., may be configured to provide output light without a method of phase matching or with relaxed phase matching requirements to match the propagation of the visible light 111' at the first frequency with the far UVC light 121' at the second frequency). For example, some embodiments may provide distributed selective outcoupling of the SFG / SHG light 121' such that the light at the second frequency does not propagate long distances in the waveguide and therefore the device may achieve satisfactory performance instead of or in the absence of phase matching.

[0077] FIG. 1C is a graph showing the emission range of light output from a UV light source according to some embodiments of the present disclosure. As shown in FIG. 1C, the output light 131 includes far UVC light 121' having a wavelength in the range of about 200 nm to 240 nm, e.g., about 207 nm or 222 nm, and may or may not include visible light 111'. UV light sources according to embodiments of the present disclosure may lack requirements regarding the nature or quality of the emitted SFG (e.g., far UV) light. Any generation of photons at the required wavelength in sufficient quantity to meet the purpose of the intended application may be used in the embodiments described herein. In other words, some embodiments may operate without specific requirements on the beam quality of the SFG light, or may not require a beam at all. The emitted SFG light may be divergent, unpolarized, and / or scattered. Thus, the embodiments described herein span the generation of light of all levels of quality (e.g., coherent or non-coherent, collimated or non-collimated) as long as the wavelength falls within a well-defined band (e.g., deep UV) based on second (or higher) harmonic generation from a coherent pump laser.

[0078] Referring again to FIG. 1A, embodiments of the present disclosure can monolithically integrate active optical components (such as light emitting element 110) and passive optical components (such as nonlinear optical element 120, low loss optical waveguide, resonant cavity, optical coupler, etc.) on a single chip (e.g., a single PIC) configured to generate human-safe far-UVC light 121′ without free-space propagation of light between the elements or components. Monolithic integration can be advantageous because it reduces losses in coupling light from active element 110 (e.g., laser 110′) to passive element 120 (e.g., SHG / SFG element 120′). As used herein, a “die” or chip can refer to a small block or body of semiconductor material or other substrate on which elements are fabricated, for example, to provide monolithic integration of active and passive optical elements. In other embodiments, the active and passive optical elements can be respective elements disposed or assembled on a non-native substrate. Such hybrid integration may include embodiments in which the light emitting element 110, the nonlinear optical element 120, and / or other passive components are packaged separately or on separate dies, and optical coupling is implemented by fiber or free space propagation.

[0079] In some embodiments, the output coupler(s) can be configured such that a desired wavelength of light (e.g., 220 nm) is preferentially supported and outcoupled (rather than the fundamental frequency or wavelength of the laser 110', e.g., 440 nm), referred to herein as selective light extraction or selective outcoupling. However, it will be understood that selective outcoupling of the far-UVC light 121 does not necessarily eliminate undesired wavelengths of light (e.g., pump light 111) in the output light 131. For example, the optical power of the visible light 111' may be an order of magnitude stronger than the SFG / SHG light 121', so that selective output of twice the SHG / SFG light 121' may provide an output light 131' that includes less SHG / SFG light 121' than the visible light 111'. The selective outcoupling of the far-UVC light 121 as the output light 131 can be provided in one or more directions different from the propagation direction of the pump light 111, in some embodiments.

[0080] While certain embodiments herein are described with reference to a laser emitting light 111' at 440 nm and an output coupler outputting light 121' at 220 nm, it will be understood that such particular emission wavelengths are mentioned by way of example only, and that any of the embodiments described herein (and / or components thereof) may be configured to emit or operate using other emission wavelengths such that the overall light output includes light in the far UVC wavelength range. Similarly, although described below primarily with reference to an exemplary implementation of a nonlinear optical element 120 for second harmonic generation (referred to as an SHG element), it will be understood that in any of the embodiments described and illustrated herein, the SHG element may be replaced by any nonlinear optical element 120 configured to generate light by sum frequency generation (referred to as an SFG element).

[0081] 2A1 and 2A2 are schematic perspective and side views, respectively, illustrating elements of a UV light source 200a (e.g., a vertical external cavity surface emitting laser (VECSEL)) in a vertical linear arrangement according to some embodiments of the present disclosure. FIGs. 2B1 and 2B2 are schematic perspective and top views, respectively, illustrating elements of a UV light source 200b in a horizontal linear arrangement according to some embodiments of the present disclosure.

[0082] The linear UV light source 200a, 200b (both the vertical arrangement shown in FIGS. 2A1 and 2A2 and the horizontal arrangement shown in FIGS. 2B1 and 2B2) includes a light emitting element 110 and a nonlinear optical element 120 integrated into a monolithic structure 190. Specifically, the linear UV light source includes a light emitting element 110 (e.g., a laser diode 110′ including a gain material configured to generate a light output in the blue portion of the visible spectrum having a wavelength of about 440 nm), an input coupling element 115, an optional semiconductor optical amplifier (SOA) having a gain of about 440 nm, a monolithically integrated nonlinear optical element 120 (e.g., an AlN waveguide having nonlinear optical properties configured for second harmonic generation of frequency doubled light in the far UV portion of the visible spectrum around 220 nm), and two facets 129-1, 129-2. The first facet 129-1 has a high reflectivity for both about 440 nm light and about 220 nm light, and the second facet 129-2 has a higher reflectivity for 440 nm light and a lower reflectivity for 220 nm light to provide selective outcoupling of far UVC light 121'.

[0083] 2C1 and 2C2 are schematic top views showing elements of a UV light source in a spiral arrangement, according to some embodiments of the present disclosure. The spiral UV light sources 200c-1, 200c-2 can integrate the nonlinear optical element 120 (and in some embodiments, the output coupling element 130) in a spiral-shaped waveguide 220 that can be adjacent (in FIG. 2C1) or extend around (in FIG. 2C2) the light-emitting element 110 (e.g., a laser diode 110' including an optical cavity 1105 configured to generate visible light 111' having a wavelength of about 440 nm). The embodiments of FIG. 2C1 and 2C2 can allow for a substantial increase in the optical path length while maintaining a relatively small footprint. Although bending the waveguide can increase radiation losses, this can be advantageous in some embodiments, particularly for embodiments where the far UVC light 121' can be outcoupled continuously or quasi-continuously along the length of the output element (also referred to herein as distributed emission), which can avoid or reduce phase matching requirements.

[0084] 3A1 and 3A2 are schematic block diagrams illustrating elements of UV light sources 300a-1, 300a-2 including optical cavity enhancement, according to some embodiments of the present disclosure. In the example of FIG. 3A1 and FIG. 3A2, the output coupling element 130 is configured to receive the far-UVC light 121′ from the optical cavity 125 that is at least partially resonant at a first frequency. For example, the optical cavity 125 can be optically resonant at a first (fundamental) wavelength / frequency of the visible light 111′, a second (e.g., harmonic) wavelength / frequency of the far-UVC light 121′, or both. The optical cavity 125 may, in some embodiments, include or surround the nonlinear optical element 120 (e.g., separate from the lasing cavity 105 of the laser 110′). By providing a nonlinear crystal inside the optical cavity 125, the efficiency of the conversion may be increased by providing many passes of the pump light 111 through the nonlinear crystal. In this way, the optical cavity 125 acts to optically "lengthen" the nonlinear optical element 120 (in terms of distance of optical propagation) beyond the physical dimensions of the nonlinear optical element 120. Alternatively, the cavity enhancement can be viewed as recycling the pump light 111 over multiple passes. Furthermore, the optical cavity 125 can significantly increase the pump light field intensity according to the quality (Q) factor of the cavity 125. Since the efficiency of nonlinear wavelength conversion (e.g., SFG / SHG) can depend monotonically on the pump field intensity, the optical cavity 125 can be used to improve the SFG / SHG efficiency for each of the passes.

[0085] In the example of FIG. 3A1, the nonlinear optical element 120 (or the optical cavity 125 in which the nonlinear optical element 120 is provided) is shown as resonating at the pump wavelength and non-resonating (or having high loss, i.e., emission) at the SHG / SFG (e.g., deep UV) wavelengths, also referred to herein as a single resonant configuration. In a single resonant configuration, the pump wavelength resonates such that its intensity is "boosted" (i.e., increased) within the optical cavity 125, but the SHG / SFG wavelengths are not substantially increased because the optical cavity 125 is not resonant at the SHG / SFG wavelengths. Without wishing to be bound by theory, in such a single resonant configuration, the requirement for phase matching may be reduced or relaxed. Phase matching may be sufficient to increase the intensity of the SHG / SFG wavelengths over a single pass over the length of the nonlinear optical element 120.

[0086] In the example of FIG. 3A2, the nonlinear optical element 120 (or the optical cavity 125 in which the nonlinear optical element 120 is provided) is shown as resonating with both the pump wavelength and the SHG / SFG (e.g., deep UV) wavelength, also referred to herein as a double-resonant configuration. When both the pump wavelength and the SHG / SFG wavelength are resonant with the nonlinear optical element 120, both the pump intensity and the SHG / SFG intensity may be increased in the optical cavity 125, and the phase matching requirements may become more stringent. In fact, double-resonant operation may not be possible without achieving phase matching between the two wavelengths. During each pass, the generated SHG / SFG light 121′ may be out-coupled (partially or completely) from the optical cavity 125. The optical cavity 125 may be configured to couple a maximum amount of pump laser light 111 while preventing the pump light 111 from leaking out on each pass.

[0087] More generally, embodiments of the present disclosure include implementations that are single-resonant or double-resonant. Although figures may show gaps between elements (e.g., FIG. 1B, FIG. 3A1, FIG. 3A2), it should be understood that illustration of such gaps may be provided merely to distinguish functional components from one another and does not imply that light necessarily propagates through the components or free space between the elements. For example, as discussed above, embodiments having monolithic integration of components may not have free-space optical propagation between the components, while embodiments having hybrid integration may include some free-space optical propagation.

[0088] 3B1 and 3B2 are schematic perspective and top views, respectively, illustrating elements of a UV light source 300b including optical cavity enhancement according to some embodiments of the present disclosure, and show implementations of the elements shown in FIG. 3A1 on a single chip in a rectilinear horizontal geometry.

[0089] In Figures 3B1 and 3B2, the nonlinear optical element 120 is implemented as an AlN waveguide and is provided within an optical cavity 125, indicated by facet #1 129-1 and facet #2 129-2. This optical cavity 125 is configured to recycle the pump light 111 to enhance the nonlinear effects of SHG / SFG and to avoid feeding the pump light 111 back into the pump laser diode 110' too heavily. The output coupling element 130 is implemented as a grating on a portion (e.g., some or all) of the nonlinear optical element 120. The grating is a diffraction order grating configured to selectively outcouple the SHG / SFG (e.g., deep UV) light with an efficiency that can be optimized for overall performance. A high level of out-coupling of the far-UVC light 121' provides a single resonant cavity (resonant only at the pump wavelength), while a low level of out-coupling of the far-UVC light 121' provides a double-resonant cavity (i.e., resonant at both the pump wavelength and the SHG / SFG wavelengths). As mentioned above, a double-resonant design may have more stringent requirements for phase matching and component design.

[0090] The gratings shown in Figures 3B1 and 3B2 are one of many possible implementations of an output coupling element 130 configured to selectively outcouple the SHG / SFG light 121' from the nonlinear optical element 120. For example, the output coupling element 130 may be or include at least one of a facet having a refractive index configured to selectively outcouple the far UVC light 121' in a first direction corresponding to its propagation direction, or a grating having a diffraction order configured to selectively outcouple the far UVC light 121' in a second direction different from the first direction. The second direction can be orthogonal to the first propagation direction of the visible light 111' from the light emitting element 110. For example, the grating or facet can be configured to outcouple the far UVC light 121' in a direction perpendicular to the surface of the substrate 101 (native or non-native) having a UV light source thereon.

[0091] 4A is a schematic block diagram showing elements of a UV light source 400a including a nonlinear optical element 120 in a ring cavity configuration, according to some embodiments of the present disclosure. This is another design variation in which the nonlinear optical element 120 uses a ring-shaped optical cavity instead of a linear cavity, as shown in plan view, to exploit the resonance effect to enhance SHG / SFG.

[0092] As shown in FIG. 4A, the nonlinear optical element(s) 120 may be implemented in a ring configuration that defines the optical cavity 125, rather than being disposed within the optical cavity 125. The ring cavity may be circular, as shown for simplicity, or may be oval, elliptical, or other ring or closed shape. The nonlinear optical element 120 may be an AlN-based ring cavity in some embodiments. Pump light 111 at a first frequency ω1 is partially or fully coupled from an input coupling element 115 (shown as being in an input waveguide) into the optical cavity 125 defined by the nonlinear optical element 120. The SHG / SFG light 121′ is outcoupled (optionally) from the nonlinear optical element 120 to an output coupling element 130 (shown as including an output waveguide). The higher the outcoupling of the SHG / SFG light 121′ from the optical cavity 125, the lower the quality factor (Q) of the cavity, and therefore the closer the cavity is to a single resonance. More generally, the optical cavity 125 can be at least partially resonant at both the pump wavelength and the SHG / SFG wavelength.

[0093] In some embodiments, multiple nonlinear optical elements 120 (e.g., crystals or cavities) may be provided for each pump laser 110', each with a separate output coupling element 130. For example, since the input coupling element 115 may not transmit 100% of the pump light 111' to the first SHG element, multiple different SHG / SFG elements 120' or cavities may be arranged to receive light from a common input coupling element 115 and thus be pumped by a single pump laser 110'. That is, since the input coupling element 115 may be imperfect, any light that does not couple into the first SHG / SFG element 120' or cavity may be provided to the next or subsequent SHG / SFG element 120' or cavity. Thus, each subsequent SHG / SFG element 120' or cavity may receive the "leftover" light that was not coupled into the previous SHG / SFG element 120' or cavity. Further embodiments may use a photonic integrated circuit (PIC) to split the pump light 111 before distribution across the various SHG / SFG elements 120 ′ or optical cavity 125 .

[0094] 4B is a schematic top view illustrating elements of a UV light source 400b including multiple nonlinear optical elements 120 arranged sequentially in a ring cavity configuration, according to some embodiments of the present disclosure. As shown in FIG. 4B, the UV light source 400b includes an input coupling element 115 (shown as a waveguide) configured to receive visible light 111′ from the light emitting element 110, and multiple nonlinear optical elements 120 are arranged sequentially along the waveguide to receive the visible light 111′ from the light emitting element 110.

[0095] As mentioned above, the coupling from the waveguide to the nonlinear optical element 120 may be less than 100% efficient. In fact, to increase the Q of the ring cavity, the coupling ratio should not be too large. In other words, a larger coupling ratio may mean that a large portion of the pump light 111 may leak back into the waveguide from the ring every cycle around the ring cavity. It may therefore be advantageous to provide multiple nonlinear optical elements 120 along the length of the waveguide, each of which "tap" the pump light 111 that was not coupled to the previous nonlinear optical element 120 in the placement sequence. In particular, FIG. 4B shows a photonic integrated circuit (PIC) in which a light emitting element 110 (e.g., a blue (about 440 nm) single mode laser 110') is integrated into a linear (e.g., AlN) waveguide as an input coupling element 115. The input coupling element 115 is coupled to one or more nonlinear optical elements 120 (e.g., AlN ring resonators) each having a respective optical cavity 125 that generates SHG / SFG light 121' in the far-UVC (approximately 220 nm) wavelength range. The far-UVC light 121' from each nonlinear optical element 120 is then coupled into a respective linear (e.g., AlN) waveguide, each of which includes either a low reflectivity (for the far-UVC light 121') exit facet or a grating as the output coupling element 130.

[0096] Thus, FIG. 4B illustrates (i) splitting or splitting the pump light 111 into multiple SHG / SFG elements 120′, and (ii) configuring the design of each subsequent SHG / SFG element 120′ to improve or optimize overall device performance (as also shown in more detail in FIG. 14 ). Also, as shown in FIG. 4B , the nonlinear optical elements 120 do not necessarily have to be identical to each other. For example, some embodiments can provide different coupling ratios to the subsequent SHG / SFG ring cavities, as illustrated by the final SHG / SFG ring (far right in FIG. 4B ) having different geometries. More generally, each of the nonlinear optical elements 120 can have different dimensions and / or even different materials, and the output coupling element 130 can include multiple output coupling elements 130, each configured to selectively outcouple the far-UVC light 121′ from each of the nonlinear optical elements 120 as output light 131′.

[0097] 4C1 is a schematic top view illustrating an element of a UV light source 400c including a nonlinear optical element 120 coupled to an intracavity portion 105i of a light-emitting element 110 according to some embodiments of the present disclosure. FIG 4C2 is a schematic top view illustrating an array 499 of UV light sources 400c each including a nonlinear optical element 120 coupled to an intracavity portion 105i of a light-emitting element 110 according to some embodiments of the present disclosure.

[0098] As shown in Figures 4C1 and 4C2, the (ring) cavity resonant SHG / SFG element 120' is coupled to a pump laser 110' on the same chip or substrate 101 (e.g., native substrate) in Figure 4C1, and has multiple chips in an array 499 on a substrate 101 (e.g., native substrate or non-native substrate) in Figure 4C2. Each nonlinear optical element 120 is positioned and configured to receive visible light 111' from an intracavity portion 105i between a first end and a second end of the respective lasing cavity 105 (also referred to herein as an "intracavity tap" configuration), as opposed to the configuration shown in the previous embodiment where the nonlinear optical element(s) 120 are positioned to receive optical output from an end of the lasing cavity 105 (also referred to herein as an "extracavity tap" configuration). That is, the terms intracavity coupling or tapping and extracavity coupling or tapping may be used herein to distinguish the relative position of the nonlinear optical element 120 with respect to the light emitting element 110 for optical coupling to the nonlinear optical element 120.

[0099] As shown in FIG. 4C1, in an intracavity tap configuration, the light output from the laser 110′ or other light emitting element 110 may traverse only one interface to be input to the optical cavity 125 of the nonlinear optical element 120, and thus a relatively high intensity intracavity light 111′ may be incoupled into the nonlinear optical element 120. In an extracavity tap configuration, the light output from the laser 110′ or other light emitting element 110 must pass through the end of the lasing cavity 105 (or other optical interface of the light emitting element 110) and then traverse a waveguide or other input coupling element 115 to be input into the optical cavity 125 of the nonlinear optical element 120. Because at least two optical interfaces between elements may be present in an extracavity tap configuration (e.g., a waveguide having respective interfaces with the lasing cavity 105 and the nonlinear optical element 120), the light input into the nonlinear optical element 120 may be of lower intensity compared to an intracavity tap configuration. Also, in the extra-cavity tap configuration, the light tapped by the nonlinear optical element 120 propagates in a single direction (the direction of output from the laser 110') relative to the nonlinear optical element 120. However, in the intra-cavity tap configuration, the light propagates in two directions (between opposite ends of the lasing cavity 105), as indicated by the double-headed arrows.

[0100] FIG. 4C3 is a graph showing the Vernier frequency selection that determines the size (including height, width, and circumference / length) of the optical cavity 125 of the ring-shaped nonlinear optical element 120 according to some embodiments of the present disclosure. As shown in FIG. 4C3, the size of the optical cavity 125 can be adjusted to correspond to a free spectral range (FSR) that has a resonance that matches (only) a particular mode of the pump laser 110'. By increasing the FSR over the spectral width of the gain of the laser, it may be possible for the nonlinear optical element 120 to modify the operation of the pump laser 110', thereby forcing more (or even all) of its intracavity power into the frequency associated with the SHG / SFG cavity, thereby increasing the efficiency of the system. For example, the laser 110' can be modified to single mode operation by providing an SHG / SFG ring 120' at the edge of the lasing cavity 105 of an otherwise multimode laser 110'.

[0101] Although the intracavity tap configuration is described herein for generating far UVC light 121' where high SHG / SFG efficiency may be most important, it may be used for generating light at other wavelengths. Also, although the nonlinear optical element 120 is coupled inside the lasing cavity 105 in Figures 4C1 and 4C2, it should be understood that the SHG / SFG optical cavity 125 (ring) is a separate optical cavity 125 from the lasing cavity 105. In other words, the embodiment of Figures 4C1 and 4C2 shows two separate or separate cavities per UV light source, namely the lasing cavity 105 of the pump laser 110' and the optical cavity 125 of the nonlinear optical element 120.

[0102] 5A is a schematic block diagram illustrating elements of a UV light source 500a in which the light-emitting element 110 and the nonlinear optical element 120 are provided within the same or a shared optical cavity 125, according to some embodiments of the present disclosure. That is, the nonlinear optical element 120 may be integrated into the same single cavity as the gain material of the pump laser 110′, such that the cavity of the laser 110′ (i.e., the lasing cavity 105) or other light-emitting element 110 is shared with the nonlinear optical element 120 providing SHG / SFG. It will be appreciated that the shared optical cavity 125 configuration (also referred to herein as an intracavity SHG / SFG configuration) illustrated in FIG. 5A is different from an “intracavity tap” configuration in which the nonlinear optical element 120 is coupled inside the lasing cavity 105, but the optical cavity 125 of the nonlinear optical element 120 is different from the lasing cavity 105.

[0103] As shown in FIG. 5A, the shared optical cavity 125 may only resonate at the pump wavelength, i.e., in a single resonant configuration. Since the SHG / SFG wavelengths may not be able to pass through the gain material (the gain material is likely to be absorptive and not transparent at the SHG / SFG wavelengths), gain at the SHG / SFG wavelengths may not be necessary. That is, the shared optical cavity 125 does not need to be double-resonant at both the first (pump) frequency and the second (SHG / SFG) frequency, since the second harmonic frequency may be absorbed by the gain region and therefore may not increase in intensity due to optical resonance. Instead, the shared optical cavity 125 may provide the SHG / SFG light 121′ to the output coupling element 130, which is configured to selectively outcouple the SHG / SFG photons as output light 131′ in a manner similar to that shown and described in other embodiments herein.

[0104] Advantages of this approach include allowing the optical field at the fundamental (pump) wavelength to be much higher than the optical field output from the light emitting element 110. The shared optical cavity 125 may be designed or configured with as high a Q as possible (e.g., up to about 100% reflectivity of the output coupling element 130) at the pump wavelength to increase or maximize the intracavity field intensity. However, in some embodiments, the Q may be reduced (e.g., the output coupling element 130 may have less than 100% reflectivity at the pump wavelength) to outcouple some of the pump wavelength for other purposes.

[0105] 5B1 and 5B2 are schematic perspective and top views, respectively, illustrating elements of a UV light source 500b within the same or shared optical cavity 125 with output coupling elements 130 implemented as reflective facets configured for selective light extraction, according to some embodiments of the present disclosure. Specifically, FIGs. 5B1 and 5B2 show implementations of the elements shown in FIG. 5A on a single chip, in a rectilinear horizontal geometry.

[0106] As shown in Figures 5B1 and 5B2, the UV light source 500b includes, in a horizontal linear arrangement, a light emitting element 110 implemented as a laser diode 110' configured to generate light in the blue portion of the visible spectrum around 440 nm, a nonlinear optical element 120 (monolithically integrated with the light emitting element 110) implemented as an AlN waveguide 120' having nonlinear optical properties configured to generate frequency doubled light in the far UVC portion of the visible spectrum around 220 nm, a first facet having high reflectivity for both about 440 nm and about 220 nm light, and an output coupling element 130 implemented as a second facet having a higher reflectivity for 440 nm light and a lower reflectivity for 220 nm light (to provide selective outcoupling of the far UVC light 121'). The first and second facets define a shared optical cavity 125 that is resonant (i.e., singly resonant) for 440 nm light. An input coupling element 115 having low reflectivity for both about 440 nm and about 220 nm light is provided between the light emitting element 110 and the nonlinear optical element 120. Optionally, in some embodiments, a semiconductor optical amplifier (SOA) having gain at about 440 nm can amplify the output of the light emitting element 110 and provide the resulting light to the input of the nonlinear optical element 120.

[0107] 5C1 and 5C2 are schematic perspective and top view diagrams, respectively, illustrating elements of a UV light source 500c within the same or shared optical cavity 125 with an output coupling element 130 implemented as an optical grating configured for selective light extraction in a horizontal linear arrangement, according to some embodiments of the present disclosure. The UV light source 500c may be similar to the UV light source 500b of FIGS. 5B1 and 5B2, but includes additional elements that may be used to implement a shared or intracavity SHG / SFG configuration in a linear horizontal geometry on a single chip.

[0108] As shown in Figures 5C1 and 5C2, the UV light source 500c includes a light emitting element 110 (e.g., a laser diode 110' configured to generate light in the blue portion of the visible spectrum around about 440 nm), a nonlinear optical element 120 (e.g., an AlN waveguide 120' having nonlinear optical properties monolithically integrated with the light emitting element 110 and configured to generate frequency-doubled light in the far UVC portion of the visible spectrum around about 220 nm), and an (optional) semiconductor optical amplifier therebetween (having gain at about 440 nm). The first facet (having high reflectivity for both about 440 nm and 220 nm light) and the second facet (having high reflectivity for at least about 440 nm light, and in some embodiments, for both about 440 nm and 220 nm light) define a shared optical cavity 125 that is resonant (i.e., singly or doubly resonant) for at least 440 nm light. The output coupling element 130 is implemented as a second (or other order) grating configured to selectively outcouple 220 nm light from the optical cavity 125, while 440 nm light (i.e., the pump wavelength) is highly confined.

[0109] Further embodiments of the present disclosure may provide both the nonlinear optical element 120 and the light emitting element 110 (e.g., laser gain medium) within the same or shared optical cavity 125, which may have a ring or other closed curved shape (also referred to herein as a "racetrack" configuration, which includes non-rotationally symmetric closed loops of any shape), with light propagating in one or more directions (e.g., a single direction or opposing directions).

[0110] FIG. 6A is a schematic block diagram showing elements of a UV light source 600a in which the light-emitting element 110 and the nonlinear optical element 120 are disposed within the same or shared optical cavity 125 having a closed-loop or racetrack (e.g., rectangular with semicircles at each end) configuration, according to some embodiments of the present disclosure.

[0111] As shown in FIG. 6A, the optical cavity 125 has a closed curve shape that optically couples the laser gain medium therein to the SHG / SFG element 120′, for example, by a curved waveguide without reflective facets. The SHG / SFG wavelengths can be selectively outcoupled either as they are generated (e.g., along a portion of the length of the optical cavity 125, also referred to herein as distributed emission) or after they are generated. This can be accomplished, for example, by implementing the output coupling element 130 as a partially reflective mirror or facet, a distributed Bragg reflector (DBR), or a second-order diffraction grating to generate a surface emission (e.g., in a direction orthogonal to the light propagation direction(s) around the closed loop). Meanwhile, the fundamental wavelength can be completely confined within the optical cavity 125 with an increased or maximum possible quality factor (Q).

[0112] 6B1 and 6B2 are schematic perspective and top views, respectively, illustrating elements of a UV light source 600b in the same or shared optical cavity 125 having a closed loop or racetrack configuration with an output coupling element 130 implemented as an optical grating configured for selective light extraction, according to some embodiments of the present disclosure. As shown in FIG. 6B1 and 6B2, the UV light source 600b includes a light emitting element 110 (e.g., a GaN laser diode 110′ configured to generate light in the blue portion of the visible spectrum around about 440 nm) and a nonlinear optical element 120 (e.g., an AlN waveguide having nonlinear optical properties monolithically integrated with the light emitting element 110 and configured to generate frequency-doubled light in the far UVC portion of the visible spectrum around about 220 nm), with curved waveguides (e.g., AlN / GaN waveguides) optically coupling the respective ends of the light emitting element 110 and the nonlinear optical element 120, with no reflective facets between them. Avoiding or eliminating the use of reflective facets may allow for higher instantaneous pulse intensities by avoiding limitations associated with catastrophic mirror damage.

[0113] The output coupling element 130 is implemented as a second (or other order) grating configured to selectively outcouple 220 nm light from the optical cavity 125. For example, the output coupling element 130 can include an optical structure having a grating pitch configured based on the wavelength of the light to be outcoupled. The output coupling element 130 can be configured to direct the SHG / SFG wavelengths orthogonal to or otherwise out of a plane defined by the light propagation direction(s) around the closed loop forming the surface emitting device, while the pump wavelength continues to propagate within the closed loop defined by the optical cavity 125. In some embodiments, the output coupling element 130 can also be a nonlinear optical element 120 (e.g., an AlN element having an optical structure with a desired grating pitch) to selectively outcouple the far-UVC light 121′ as it is generated. In some embodiments, the optical cavity 125 can further include a section configured to form a saturable absorber 1305 configured to generate pulses of light at the pump wavelength.

[0114] In particular, although some embodiments are described by way of example with reference to continuous wave (CW) operation of the pump laser 110', some embodiments may operate the pump laser 110' in a pulsed mode, which may enable operation of the device at higher field strengths than CW. That is, the light emitting element 110 in any of the embodiments described herein may be a laser diode 110' configured to be driven continuously or in a pulsed manner.

[0115] 7A, 7B, and 7C show example drive signals configured to provide a pulsed light output from the light-emitting element 110 in the UV light source, according to some embodiments of the present disclosure. With reference to FIGS. 7A-7C, the SHG / SFG conversion may be higher when the (instantaneous) pump power is higher. Thus, by maximizing the peak-to-average ratio of the pump laser 110′, in other words, by operating the pump laser 110′ to provide a pulsed light output, the overall efficiency may be improved.

[0116] As shown in FIG. 7A, one way to obtain pulsed light output is by direct modulation, i.e., by providing a pulsed drive signal to the pump laser 110′, thereby activating and deactivating the laser diode 110′ to emit light with a higher intensity or pulsed output power than would be acceptable if operated continuously, for example, with a pulse width of about 1 ns to 5 ns, over a desired pulse repetition frequency (PRF) and duration or duty factor (DF). That is, some embodiments can achieve pulsed operation through the use of an electrical drive circuit that provides short, high current pulses to the diode laser 110′. In this way, the laser 110′ can be “quasi-CW” for short periods of high current application, but the low duty factor of the drive current can allow for higher transient operating power.

[0117] Another way to obtain pulsed optical output is to design or configure the laser diode 110' to generate a pulsed output even when driven continuously, as shown in FIG. 7B. One way to achieve this is by configuring the pump wavelength to be passively (or actively) mode-locked, for example, by controlling the PRF and DF of the laser diode 110' based on the length L of the optical cavity 125. FIG. 7B shows that a pulse "train" with a short pulse width on the order of about 1 ps (or less) can be achieved at a high pulse repetition frequency (e.g., c / L) to provide a higher intensity or output pulse power than can be achieved by CW. For example, some embodiments can provide a saturable absorber 1305 inside the lasing cavity 105 to achieve passive mode-locking. Some embodiments can achieve pulsed operation by active Q-switching.

[0118] As shown in FIG. 7C, the above or other pulsing strategies can be combined to achieve even higher peak pulse powers and therefore higher nonlinear conversion efficiencies. Specifically, FIG. 7C illustrates a drive signal based on a combination of the method shown in FIG. 7A (direct modulation) and the method shown in FIG. 7B (continuous modulation) to provide a pulsed light output by, for example, using an electrical drive circuit to activate and deactivate the laser diode 110' to emit light with a desired or predetermined PRF and DF, and in combination with a saturable absorber 1305 in the lasing cavity 105 to provide a pulse width as short as about 1 ps (or less). Also, although illustrated herein with reference to a specific lasing cavity, it will be understood that other types of lasers may be used in any of the embodiments described herein. For example, a grating may be provided at the end of the laser gain medium to form a distributed feedback laser (DFB) emitting in a single longitudinal mode. In some embodiments, the grating may be provided in a waveguide separate from the laser gain region to form a distributed Bragg reflector (DBR) laser emitting in a single longitudinal mode. More generally, embodiments of the present disclosure can employ multiple operating methods for light emitting devices (e.g., continuous wave and / or pulsed in various ways or combinations thereof) and laser configurations (e.g., DFB, DBR), some of which can achieve higher SHG / SFG efficiencies.

[0119] To monolithically integrate the light emitting element 110 with the nonlinear optical element 120, embodiments of the present disclosure may utilize various fabrication techniques to combine different materials. For example, some embodiments may utilize heterogeneous integration methods such as micro-transfer printing (MTP) to couple the laser 110' and the nonlinear crystal when both are microscopic in size (e.g., with dimensions of about 0.5 μm to about 1000 μm). Microfabrication techniques may enable direct end-to-end coupling of the two optical components without the use of extra optical elements.

[0120] In some embodiments, epitaxial regrowth can be used on top of an appropriately patterned existing waveguide as an alternative to microassembly to monolithically integrate the two sets of materials into a single waveguide. An example of such a concept is shown in Figures 2A1 and 2A2, where the gain material of the laser diode 110' (e.g., GaN or other III-nitride material) is formed, a section of the GaN is patterned and etched away, and an AlN layer or other nonlinear optical element 120 material is grown (e.g., by MOCVD or MBE) such that a high-quality optical interface is achieved and the waveguide material is changed without modifying the physical cross-sectional dimensions of the waveguide. In some embodiments, some or all of the elements of the UV light sources described herein (e.g., the light emitting element 110, the nonlinear optical element 120, the output coupling element 130, the optical cavity 125, and one or more waveguides between them) can be nitride-based materials.

[0121] Further embodiments may use MTP, pick-and-place, or other assembly techniques to place separate active and passive optical elements on a non-native substrate, also referred to herein as hybrid integration. For example, each light emitting element 110 of one material may be formed on a non-native substrate that is different from the source substrate of either the light emitting element 110 or the non-linear optical element 120, and optically coupled to the non-linear optical element 120 of a different material or material system.

[0122] 8 is a schematic perspective view illustrating each element of a UV light source 800 arranged in a structure or configuration on a common non-native substrate 801 according to some embodiments of the present disclosure. As shown in FIG. 8, the UV light source 800 may be implemented as a photonic integrated circuit (PIC) that includes each element described herein as separate components assembled on the common substrate 801. In particular, the light emitting element 110 (e.g., a laser diode 110′ including a gain material configured to generate a light output in the blue portion of the visible spectrum having a wavelength of about 440 nm), the non-linear optical element 120 (e.g., an AlN, BBO, or lithium niobate based waveguide having non-linear optical properties configured for second harmonic generation of frequency doubled light in the far UV portion of the visible spectrum around 220 nm), and the output coupling element 130 (e.g., a facet having a higher reflectivity for 440 nm light and a lower reflectivity for 220 nm light to provide selective out-coupling of the far UVC light 121′) may be fabricated as respective elements and disposed on the non-native substrate 801. In some embodiments, an optional semiconductor optical amplifier (SOA) with gain around 440 nm can amplify the output of the light emitting element 110 and provide the resulting light to the input of the nonlinear optical element 120 .

[0123] In the example of the "hybrid" integration of FIG. 8, the optical coupling between the respective elements can be implemented by optical fiber or free space propagation (with an air interface between them). However, the UV light source 800 can be implemented by any combination of the fabrication techniques described herein, including (but not limited to) epitaxial regrowth, wafer bonding, micro-transfer printing, pick-and-place, lithography, etc. Although described herein with reference to monolithic integration of certain embodiments, it will be understood that any of these embodiments can be assembled using the hybrid configurations described herein. That is, any of the embodiments described herein can be assembled as a monolithic structure (no air interface between components) or a hybrid structure (with an air interface between two or more components).

[0124] Similarly, it will be appreciated that embodiments of the present disclosure may include various types of optical cavities 125 and feedback structures that can provide a high quality factor Q for efficient operation. Examples of possible optical microcavities may include, but are not limited to, linear Fabry-Perot cavities with polished facet end mirrors, linear Fabry-Perot cavities with dispersive (dielectric) Bragg reflector end mirrors, linear optical Fabry-Perot cavities with distributed feedback gratings, linear optical cavities 125 with various photonic crystal designs, ring cavities fabricated by a waveguide that closes on itself, and ring cavities fabricated by circular or elliptical 2D or 3D disk structures.

[0125] Also, while the embodiments herein are described primarily with reference to optical second harmonic generation (SHG) from a pump laser 110' (e.g., from blue light at about 400 nm to about 480 nm) generating emission at about 200 nm to about 240 nm, it will be understood that embodiments of the present disclosure may also include implementations in which higher order harmonic generation (e.g., third harmonic generation, fourth harmonic generation, and / or fifth harmonic generation) is applied to the pump laser 110' or other light 111 (including light at wavelengths suitably higher than 400 nm to 480 nm) to generate output light 121 at 200 nm to 240 nm. That is, it will be understood that second harmonic generation or frequency doubled light as described herein may more generally be referred to as sum frequency (including harmonic doubling) light generation, using a nonlinear optical element 120 implementing an optical frequency doubler or other nonlinear frequency conversion device in any of the embodiments described herein.

[0126] In some applications, more power in the far UVC wavelength range may be desired than can be produced by a single UV light source, in which case multiple UV light sources (e.g., arranged in an array) may be provided on a common substrate 101 (native or non-native).

[0127] FIG. 9 is a schematic block diagram illustrating an array 900 of UV light sources 100, each including a light emitting element 110 and a nonlinear optical element 120, according to some embodiments of the present disclosure. As shown in FIG. 9, the array 900 includes multiple light emitting elements 110 and nonlinear optical elements 120 on a common substrate 901. For example, some embodiments can utilize heterogeneous integration methods, such as micro-transfer printing (MTP), to efficiently fabricate a large array of similar or identical UV light sources 100 on a non-native substrate 901 to increase the total optical power output from a single package. In other embodiments, rather than singulating and packaging each UV light source separately and then aggregating the UV light sources on a common (i.e., non-native) substrate at the system level, an array of identical (or non-identical) devices can be fabricated on the same chip (i.e., on the same native substrate) before packaging the chip. Although illustrated as an array 900 of UV light sources 100 similar to that shown in FIG. 1B, it will be understood that any of the UV light sources described herein can be implemented in an array form.

[0128] 10A and 10B are schematic perspective and top views, respectively, illustrating elements of a UV light source 1000 including an optical cavity enhancement with an output coupling element 130 configured to provide distributed emission and selective light extraction, according to some embodiments of the present disclosure. As used herein, distributed emission may refer to a configuration of a nonlinear optical element 120 and an output coupling element 130 in which light (e.g., far-UVC light 121′) is extracted continuously or semi-continuously when generated at multiple locations along the length of the components, rather than from one specific point or location. The distributed emission may or may not be collimated and may or may not be coherent.

[0129] As shown in Figures 10A and 10B, the UV light source 1000 includes a light emitting element 110 (e.g., a laser diode 110' configured to generate light in the blue portion of the visible spectrum around 440 nm) and a nonlinear optical element 120 (e.g., an AlN waveguide having nonlinear optical properties monolithically integrated with the light emitting element 110 and configured to generate frequency-doubled light in the far UVC portion of the visible spectrum around 220 nm). An optional SOA amplifies the output of the light emitting element 110 and provides it to the input of the nonlinear optical element 120. In the embodiment of Figures 10A and 10B, the nonlinear optical element 120 and the output coupling element 130 are integrated into an output element 120 / 130 configured to outcouple the far UVC light 121' at multiple locations or continuously along its length. 10A and 10B, the output element 120 / 130 includes a waveguide material having alternating sections or regions in which the SHG / SFG light 121' is alternately generated (by the SFG / SHG section) and outcoupled (from the output coupling section). More generally, the output element 120 / 130 includes a plurality of nonlinear optical element 120 and output coupling element 130 regions (e.g., in a periodic or other alternating arrangement) that are configured to provide continuous or semi-continuous extraction or distributed emission of the SHG / SFG light 121' along the length of the output element 120 / 130 (also referred to herein as the nonlinear optical output coupling element 120 / 130).

[0130] An output element 120 / 130 having multiple integrated nonlinear optical sections and output coupling sections may be advantageous in that typical requirements or constraints regarding phase matching between the SHG / SFG wavelength and the fundamental wavelength may be relaxed or may not be necessary. By semi-continuously extracting and recovering the SHG / SFG light 121' from the output element 120 / 130, it may be possible to relax or eliminate constraints related to phase matching between the SHG / SFG light 121' and the pump light 111 in the waveguide, since the SHG / SFG wavelength is not expected to co-propagate with the fundamental wavelength. That is, the SHG / SFG field intensity is not expected to build up in the waveguide, but rather the SHG light generated is outcoupled (at a certain rate) to the outside world as it is generated. Thus, the output light 131 may include primarily the SHG / SFG light 121' and may, in some instances, be substantially free of the pump light 111.

[0131] Thus, the UV light source 1000 is not phase matched (i.e., not configured to match a first phase of the visible light 111' with a second phase of the far-UVC light 121'). Although illustrated in the examples of Figures 10A and 10B with reference to a horizontal linear arrangement of the UV light source 1000, it will be understood that the output element 120 / 130 including integrated nonlinear optical and output coupling elements 130 (e.g., arranged in alternating fashion along its length) may be used in one or more other embodiments described herein, such as embodiments including a closed curve / racetrack optical cavity 125 configuration, a helical shaped waveguide, or other output coupling element 130 configuration.

[0132] 10A and 10B show the output element 120 / 130 as including a second-order grating that selectively outcouples the SHG / SFG light 121' from the waveguide while confining the pump light 111 within the waveguide, embodiments of the present disclosure can include other types of output coupling elements 130 having wavelength-dependent optical properties configured for distributed emission or (semi-)continuous extraction of the SHG / SFG light 121'. Some additional output element 120 / 130 configurations for selective extraction of the SHG / SFG light 121' include, but are not limited to, dielectric material interfaces or stacks configured for wavelength-dependent transmission, curves or tapers in the waveguide geometry to obtain wavelength-dependent effects.

[0133] For example, in some embodiments, the output element 120 / 130 can be configured to provide periodic poling of the SHG / SFG material (e.g., AlN) to achieve “quasi” phase matching. In particular, the output element 120 / 130 can include alternating regions of AlN, each having a different height (relative to the substrate 101) and surface roughness. To address poor performance as a waveguide at λ0, a capping layer index-matched at λ0 can be provided on top of the alternating AlN regions. That is, the output element 120 / 130 can include multiple periodically poled nonlinear optical sections of a first material and an index-matched capping layer of a second material different from the material of the nonlinear optical sections. Phase matching at the SHG / SFG frequency (e.g., λ0 / 2) may not be required (so that the need for phase matching may be mitigated or eliminated) since the SHG / SFG light 121′ can be scattered from the output element 120 / 130 using this configuration.

[0134] More generally, the output element 120 / 130 configured to provide a distributed emission as described herein may include any optical structure (or combination thereof) configured to confine light at the fundamental wavelength (λ0) output from the light emitting element 110 and emit or outcouple light at the SFG / SHG wavelength (e.g., λ0 / 2). As one example, in some embodiments, the output element 120 / 130 may be a waveguide that includes a nanopore or defect with dimensions (or band gap) configured to affect only the SFG / SHG wavelength (e.g., λ0 / 2) and not substantially affect the fundamental wavelength. That is, the output coupling element 130 may be implemented as a waveguide that includes a nanopore or defect with dimensions configured to be index mismatched at the second (SHG / SFG) frequency of light but not substantially affect the propagation of light at the first (fundamental) frequency.

[0135] For example, in some embodiments, the output element 120 / 130 may be a waveguide that includes or incorporates two (or more) different materials having respective optical indices that are matched at the fundamental wavelength λ0 but mismatched at the SHG / SFG wavelength (e.g., λ0 / 2) and roughened (e.g., at the interface between the materials) such that light at the fundamental wavelength is confined therein (also referred to herein as a confined mode) while light at the SHG / SFG wavelengths is scattered away from the output element 120 / 130.

[0136] Conversely, in some embodiments, the output element 120 / 130 may be a waveguide that includes or incorporates two (or more) different materials, where the two materials are index matched at the SHG / SFG wavelength (e.g., λ0 / 2) but mismatched at the fundamental wavelength λ0, such that the first material provides a confined mode for the fundamental wavelength λ0, while the SHG / SFG wavelengths can occupy a mode in the second (optically "thicker") material. Roughening or other scattering structures may be provided on top of the second material (e.g., opposite the interface with the first material) such that the SHG / SFG wavelength (e.g., λ0 / 2) is preferentially scattered out of the waveguide, while the fundamental wavelength λ0 remains confined within the higher index material. That is, the output coupling element 130 may be implemented as a waveguide including a first material and a second material having relative dispersion curves configured such that their first and second optical indices of refraction are matched at one of the first (fundamental) and second (SHG / SFG) frequencies but mismatched at the other.

[0137] As yet another example, in some embodiments, rather than combining the nonlinear optical element 120 with the scattering or output coupling element 130 over the length of the output element 120 / 130, the output element 120 / 130 can be a waveguide that includes separate or separate sections of the nonlinear optical element 120 and the output coupling element 130. The SHG / SFG sections 120' can be relatively short (along the propagation direction of the fundamental wavelength light 111) such that phase matching may not or may not be required over the length of the optical element. A relatively long output coupling section 130 can be provided after each SHG / SFG section 120' (with respect to the propagation direction of the fundamental wavelength light 111) and can include different first and second materials configured to scatter or outcouple light at the SHG / SFG wavelength (e.g., λ0 / 2) from the waveguide while confining light at the fundamental wavelength (λ0). The sequence of alternating SHG / SFG sections 120' and output coupling sections 130 may be repeated (e.g., periodically) along the propagation direction of the fundamental wavelength light 111 to increase SHG / SFG. The materials of the SHG / SFG sections 120' and output coupling sections 130 may be selected such that the optical index at the fundamental wavelength (λ0) is matched and confined over all periods of the structure. That is, at the SHG / SFG wavelength (e.g., λ0 / 2), the output elements 120 / 130 may be alternated between the SHG / SFG sections 120' and the output coupling sections 130 to extract the SHG / SFG wavelength of light, while at the fundamental wavelength (λ0), the output elements 120 / 130 may be continuous and confine the fundamental wavelength light everywhere along the propagation direction.

[0138] Some photonic integrated circuits (PICs) described herein may be based on the GaInAlN material system and may be scalable to large volumes, leveraging the extensive growth and manufacturing infrastructure deployed for the manufacture of white LEDs. The PICs described herein may be configured to emit engineered monochromatic output at one or more selected wavelengths between 200 nm and 240 nm, which in some embodiments may eliminate the use or need for optical filters, providing significant cost savings. The light emitting element 110 may be a laser 110' emitting light in the wavelength range of 400 nm to 480 nm (blue / violet), and the nonlinear optical element 120 (which may be implemented as an engineered waveguide) may sum or double the frequency of the light input from the laser 110' based on SFG or SHG such that far-UVC light 121' is generated at the desired wavelength. The light is then outcoupled from the chip in an out-of-plane direction (e.g., substantially perpendicular to its surface), similar to the emission of a vertical cavity surface emitting laser (VCSEL). Furthermore, the power output can be increased beyond what a single device could achieve by simply designing a PIC with a monolithic array of devices on a single chip, similar to a VCSEL array.

[0139] Referring again to FIGS. 4C1-4C3, an exemplary PIC architecture according to some embodiments of the present disclosure includes a light emitting element 110 implemented as a linear single frequency pump laser diode 110′ (in this example, an AlGaN laser diode) coupled to a resonator nonlinear optical element 120 (which in the example of FIG. 4C1 is ring-shaped and formed from AlN), which is coupled to an output coupling element 130 implemented as a waveguide (in this example, AlN) that extracts far-UVC light 121′. The ring-shaped nonlinear optical element 120 may also be referred to herein as a ring resonator. However, it should be understood that the nonlinear optical element 120 need not be ring-shaped and other nonlinear optical element 120 designs may be used in the embodiments described herein.

[0140] As shown in FIG. 4C2, each emitter of the emitter array 499 includes a laser 110' (e.g., configured to emit light at 440 nm, which may be more generally referred to herein as input light) that significantly increases the internal light intensity and couples a portion of that light into an adjacent nonlinear optical element 120 (AlN ring resonator) that resonates in this mode. The coupling (indicated by the arrows in FIG. 4C1 showing the propagation direction of the laser light 111) may be in-plane (i.e., along the xy axis of the plan view or figure) or vertical (i.e., perpendicular to the page or off-page), and in some embodiments may even cross a gap (e.g., on the order of microns) between the laser 110' and the nonlinear optical element 120. The positive sign represents the electrical anode of the laser 110' and the negative sign represents the electrical cathode. Respective mirrors or other low-loss reflective elements may be provided at both ends of the laser 110'.

[0141] In a particular embodiment, the laser 110' may be implemented using an integrated waveguide or other integrated lasing cavity 105 (shown as bidirectional) in some embodiments. Pump (e.g., 440 nm) light coupled into an AlN nonlinear optical element 120' (ring resonator) generates far UVC (e.g., 220 nm) light due to the nonlinear response of AlN. This second harmonic generation (SHG) or sum frequency generation (SFG) process increases the far UVC (e.g., 220 nm) light more efficiently due to the high Q of the cavity at the frequency of the pump light. The far UVC (e.g., 220 nm) light is (selectively) outcoupled from the nonlinear optical element 120' (ring resonator) and in some embodiments is coupled to an adjacent output waveguide, also across a gap (e.g., on the order of microns) between them. The output waveguide may be formed, for example, from AlN, SiO2, or other materials, and may or may not be linear in some embodiments. For example, the ring resonator 120' may have separate nonlinear and (optional) output coupling sections in some embodiments. The far-UVC light (or, more generally, the SHG / SFG light 121') may be output as output light 131' propagating in a direction perpendicular to or out of the page by respective output coupling elements 130 at either end of the output waveguide to provide surface emission (similar to a VCSEL). The output waveguide may be narrower (e.g., less than 100 nm wide) than the lasing cavity 105 in some embodiments.

[0142] It should be noted that the 440 nm light can be coupled in two directions, from the laser 110' to the ring resonator (forward coupling) and backward (reverse coupling). Reverse coupling can provide the advantage that by coupling two optical cavities 125 (lasing cavity 105 and SHG / SFG cavity), it can be ensured that the single longitudinal lasing mode of the laser 110' is the correct mode required to pump the SHG process. This can ensure that only one supported frequency is matched to both resonators, as shown by the graph shown in FIG. 4C3. The top graph shows the resonance of the laser 110' (including 440 nm light), the middle graph shows the resonance of the ring resonator (including 440 nm light and 220 nm higher order light resulting from second harmonic generation or frequency doubling), and the bottom graph shows the synchronous resonance of the far UV output waveguide (including 220 nm light). The physical sizes of the devices shown herein are not to scale. The cross-sectional dimensions of each element can be configured to provide the desired wavelength, modal requirements, phase matching, and the like.

[0143] Further embodiments are described below with reference to various configurations of single emitters or UV light sources. As noted above, it will be understood that any of the configurations described herein may be implemented with an array including multiple emitters that may or may not be of the same or identical emitter configuration. That is, any of the UV light source configurations herein may be combined in any manner.

[0144] 11 is a schematic top view illustrating an example combination of various elements of a UV light source 1100 including a nonlinear optical element 120 coupled to an intracavity portion 105i of the light-emitting element 110 in combination with a ring resonator at each end 105a, 105b of the light-emitting element 110 and an output coupling element 130 configured for selective light extraction, according to some embodiments of the present disclosure. In the example of FIG. 11, the light-emitting element 110 is implemented as a laser diode 110′ having one or more optical resonators 1105 at opposing first and second ends of a lasing cavity 105 (shown as including an AlGaN waveguide).

[0145] As shown in Figure 11, the nonlinear optical element 120 is implemented as an SHG / SFG element 120' having a ring-shaped optical cavity 125 that is intracavity tapped into the lasing cavity 105 of a laser diode 110' as the pump laser 110'. The longitudinal modes of the pump laser 110' can be controlled by a dual ring optical resonator 1105 positioned at each end of the linear waveguide that provides the lasing cavity 105. The optical resonators 1105 (Si x N y The optical resonator 1105 (shown as a dual ring resonator) is configured to reflect light of the first (fundamental) frequency of the laser 110' and thus acts as a wavelength-selective "mirror" (with dimensions that determine a particular longitudinal mode frequency) for single-frequency laser feedback without the use of facets, coatings, gratings, and / or other reflective elements. The optical resonator 1105 or reflector is an element of the active pump laser 110' (e.g., configured for reflectivity for light between 400 nm and 480 nm) rather than the nonlinear optical element 120. That is, the laser diode 110' includes a dual ring element as a wavelength-selective mirror that forms the lasing cavity 105.

[0146] 11B, the Vernier frequency effect provides a selection of only a subset of the possible longitudinal modes supported by the laser 110', since the SHG / SFG optical cavity 125 is also coupled into the laser cavity. SHG / SFG (e.g., 220 nm) light can be coupled back from the nonlinear optical element 120 cavity into the lasing cavity 105 (i.e., coupling can be in the forward / output direction as well as in the reverse direction) and coupled into the optical resonator 1105 (Si x N y Note that the mirrors may not be 100% reflective, so that there may be losses at the ends of the lasing cavity 105 .

[0147] 11, the output coupling element 130 is implemented as a waveguide configured to provide a lower cutoff frequency such that light at the fundamental wavelength (e.g., 440 nm) is confined within the waveguide while light at the SHG / SFG wavelength (e.g., 220 nm) can escape. By selectively out-coupling light from the nonlinear optical element 120, the Q of the ring-shaped optical cavity 125 at the SHG / SFG wavelength can be kept low. In some embodiments, the output coupling element 130 can be configured to out-couple the SHG / SFG wavelength light semi-continuously, for example, if the efficiency of the output coupling element 130 is particularly high. In some embodiments, the output coupling element 130 can conformally extend along at least a portion of the nonlinear optical element 120 (e.g., wrap partially around the ring) such that the SHG / SFG wavelength light can be extracted semi-continuously, thereby relaxing (or eliminating) the phase matching requirement.

[0148] 12 is a schematic top view illustrating an example combination of various elements of a UV light source 1200 including a nonlinear optical element 120 coupled to an extra-cavity portion of a light emitting element 110 in combination with a ring resonator 1105, an adjustment mechanism 1225, an output coupling element 130 configured for selective light extraction, and an output monitor element 1245, according to some embodiments of the present disclosure. In FIG. 12, the nonlinear optical element 120 is implemented by multiple (e.g., first and second) SHG / SFG elements 120′ (e.g., two AlN ring resonators) positioned adjacent respective first and second ends of a lasing cavity 105 (shown as including an AlGaN waveguide).

[0149] The first SHG / SFG element 120' and the second SHG / SFG element 120' are positioned relative to the lasing cavity 105 in an external cavity tap configuration, such that light of a fundamental wavelength (e.g., 440 nm) is output from the laser 110' at respective first and second ends of the lasing cavity 105, propagating unidirectionally at each end. Because the laser 110' is configured to emit light at both ends, the first SHG / SFG element 120' and the second SHG / SFG element 120' can be identical (or similar). By providing a first SHG / SFG element 120' and a second SHG / SFG element 120' at each end 105a, 105b of the lasing cavity 105, the coupling of SHG / SFG light 121' into the lasing cavity 105 can be reduced, thereby allowing a larger (or maximum) percentage of light (e.g., closer to 100%) to be coupled into the SHG / SFG element 120' (compared to an intra-cavity tap configuration).

[0150] The nonlinear optical element 120 may be disposed adjacent both ends of the lasing cavity 105 such that the coupling between the laser 110′ and the nonlinear optical element 120 may be in-plane (i.e., along one or more directions parallel to the surface of the substrate 101) and / or may be at least partially stacked on the laser 110′ (e.g., at least partially overlapping the ends of the lasing cavity 105 in a vertical direction perpendicular to the surface of the substrate 101) such that the coupling between the laser 110′ and the nonlinear optical element 120 may be vertical (i.e., perpendicular to the surface of the substrate 101). In such a vertically overlapping arrangement, most or all of the fundamental wavelength light 111 that is not reflected by the dual ring optical resonator element is coupled into the nonlinear optical element 120 (e.g., an AlN ring resonator), which may improve overall device efficiency. Also, the SHG / SFG light 121′ may not be coupled back into the lasing cavity 105, which may also improve efficiency.

[0151] One or more mirror elements (Si x N y An optical resonator 1105 (shown as an optical resonator 1105) is provided at or near the end of the lasing cavity 105. The SHG / SFG light 121' may be coupled (in-plane or vertically) from the first SHG / SFG element 120' and the second SHG / SFG element 120' to respective output waveguides and output by respective output coupling elements 130 at respective ends of the output waveguides. The output coupling elements 130 may be facets, gratings, or other optical elements configured to outcouple the SFG / SHG light 121' in a direction substantially perpendicular to the surface of the substrate 101 (or otherwise out of the plane shown in the illustrated plan view) to provide surface emission.

[0152] FIG. 12 also illustrates at least one tuning mechanism 1225 configured to tune one or more operating characteristics of the nonlinear element based on the light output from the light emitting element 110. The tuning mechanism(s) 1225 can include thermal heaters, electro-optical (EO) devices, and / or other devices used to change the thermal, electrical, and / or optical properties of the nonlinear optical element 120 (e.g., an AlN ring resonator) to more closely match or correspond to the emission wavelength of the light output from the light emitting element 110, which may vary or drift depending on the operating environment (e.g., changes in temperature) and / or manufacturing tolerances. For example, the wavelength tuning mechanism 1225' may be provided by a thermal or electro-optical element configured to tune the resonance of the nonlinear optical element 120 to correspond to the output of the laser 110'. In some embodiments, the wavelength tuning mechanism 1225' can be a gold or other thermally conductive metal plate or element configured to change the resonance wavelength in response to heating. In another example, the wavelength tuning mechanism 1225' can include one or more thermoelectric cooling (TEC) elements configured to change the resonance wavelength in response to cooling.

[0153] Such a wavelength tuning mechanism 1225 may be implemented to allow for imperfections or variations in the output of the laser 110', such as wavelength drift with changes in operating temperature, and may be used in any of the embodiments described herein as well. Also, while illustrated by way of example as being used to tune the characteristics of the nonlinear optical element 120, the tuning mechanism 1225 may also be used to tune other elements of the UV light source (one or more (Si x N y) the wavelength-selective optical resonator 1105 and / or the light-emitting element 110 itself. For example, the wavelength-selective optical resonator 1105 may include a respective tuning mechanism 1225 in which the reflected laser emission 111 is tuned to match the characteristics of the nonlinear optical element 120 for second harmonic or sum frequency generation. The wavelength tuning (thermal or electro-optical) described herein may also be used to compensate for variations in the coupling gap (e.g., between the laser 110′ and the nonlinear optical element 120 and / or between the nonlinear optical element 120 and the output waveguide) and / or the refractive index of one or more optical elements. That is, the tuning mechanism 1225 may be used with multiple optical elements described herein.

[0154] 12 further illustrates a monitor element 1245 configured to measure a characteristic of the output light 131 (e.g., detect a power level) and generate a feedback signal to a controller configured to operate the light-emitting element 110 and / or the regulating mechanism 1225. For example, the monitor element 1245 may be implemented as an integrated photodiode configured to detect or monitor the field strength or power of the SHG / SFG light 121′ output to provide a feedback signal to other system components, such as laser drive and / or regulating elements. One or more monitor photodiodes 1245 may similarly be integrated at various locations within the UV light source 1200 to monitor the light output from the light-emitting element 110 and provide a feedback signal to one or more controllers. Such a monitor element 1245 may similarly be used in any of the embodiments described herein.

[0155] 13 is a schematic top view illustrating an exemplary combination of various elements of a UV light source 1300 including a nonlinear optical element 120 coupled to an extra-cavity portion of a light-emitting element 110 including a saturable absorber element 1305 in combination with an adjustment mechanism 1225, an output coupling element 130 configured for selective light extraction, and an output monitor element 1245, in accordance with some embodiments of the present disclosure. It will be understood that these and other illustrated implementations are examples, and that embodiments of the present disclosure may include various other combinations of the illustrated (and other) components.

[0156] The UV light source 1300 of FIG. 13 may be similar in some aspects to the UV light source shown in FIG. 12, and therefore the description of similar elements will not be repeated for brevity. However, in FIG. 13, the light emitting element 110 (e.g., AlGaN pump laser 110′) may include end mirror facets or other elements that define the lasing cavity 105 without using a double ring reflector. In addition, a saturable absorber element 1305 is integrated or otherwise provided within the lasing cavity 105. The saturable absorber 1305 is configured to induce the laser 110′ to operate in a mode-locked manner (passively or actively) or otherwise generate light at the fundamental wavelength as multiple optical pulses. In particular, the saturable absorber 1305 is configured to generate higher power pulses, which may increase the SHG / SFG conversion efficiency (e.g., by utilizing second-order nonlinearity in the SHG / SFG element 120′). The saturable absorber 1305 may be substantially transparent at higher lasing intensities, allowing the lasing cavity 105 to preferentially support modes in which the optical power is pulsed. It is understood that embodiments herein may include any type of saturable absorber material configured based on the desired output wavelength of the light emitting element 110 (e.g., wavelengths between 400 nm and 480 nm), or may include an artificial saturable absorber, such as a Kerr lens structure with an aperture.

[0157] As with the other embodiments, the SHG / SFG element 120' is implemented as a resonant cavity (shown in FIG. 13 as an AlN-based ring-shaped cavity) that is overcoupled (e.g., near 100% collection) to the output of the pump laser 110' at both ends of the lasing cavity 105 (i.e., in an extra-cavity tap configuration). Each wavelength tuning mechanism 1225' is used to tune the operating characteristics of the SHG / SFG element 120' (e.g., an AlN ring resonator) to match the emission wavelength of the laser 110'. Also, one or more output monitors 1245 (e.g., photodiodes) provide feedback signals (e.g., to a drive controller / electronics) to adjust the drive current and / or tuning elements based on the detected optical power from the light emitting element 110 (i.e., fundamental light) and / or the output coupling element 130 (i.e., SHG / SFG light 121'). More generally, elements, features, arrangements, and / or other features described herein with respect to one embodiment may be combined with elements, features, arrangements, and / or other features of other embodiments, unless stated otherwise.

[0158] Further embodiments of the present disclosure are described below with reference to one or more design features configured to improve the yield of an emitter that uses cavity-enhanced SHG to provide a far-UVC light output 131'. These embodiments are described herein by way of example with reference to a light emitting element 110 implemented as a laser diode 110' configured to emit light having a predetermined wavelength, for example, 440 nm light. The configuration of the laser 110' may be according to any of the embodiments described herein, including, but not limited to, a ridge or buried slab waveguide, mirror elements on its ends, a distributed feedback (DFB) configuration, and / or dual ring reflector elements at its respective ends 105a, 105b.

[0159] FIG. 14 is a schematic top view showing an example combination of various elements of a UV light source 1400 including multiple different or non-identical nonlinear optical elements 120 in a ring cavity configuration with respective output coupling elements 130, according to some embodiments of the present disclosure. As shown in FIG. 14, the emission from the laser diode 110' is directly coupled into a linear (in these examples, unidirectional) input waveguide as the input coupling element 115, and the fundamental wavelength light 111 propagates in a single (or a few) mode(s) through the input coupling element 115. The input coupling element 115 (at the laser output) can be a 440 nm input waveguide or a single mode fiber. The nonlinear optical element 120 is provided by a ring-shaped SHG / SFG element 120' (shown as multiple AlN ring resonators, but more generally multiple nonlinear optical elements 120) along the edge or side of the length of the input waveguide 115. The ring-shaped SHG / SFG element 120' can be fabricated from a variety of nonlinear optical materials (in these examples, AlN, but other embodiments may use different materials). The ring-shaped SHG / SFG element 120' can be configured to be double-resonant at both the fundamental frequency and higher order frequencies or wavelengths (e.g., 440 nm and 220 nm), and the ring design can be configured to provide high Q for a single mode at both the fundamental frequency ω1 or wavelength (e.g., 440.0 nm) and the higher order frequency ω2 or wavelength (e.g., 220.0 nm).

[0160] In certain embodiments, it may be important that the waveguide be configured to support modes that are exactly 2.0 times related at the wavelengths of the two modes supported to provide improved or best possible SHG conversion efficiency. The double resonant cavity of the ring-shaped SHG / SFG element 120' may be configured to have a gap between its edge and the edge of the adjacent input waveguide, and the size of the gap may be configured and carefully controlled to improve or maximize the coupling of 440 nm light from the input waveguide to the ring-shaped SHG element. Additionally, on the other side of the ring-shaped SHG element (bottom, as shown), one or more linear output waveguides are fabricated and configured to collect the outcoupled 220 nm light therefrom. In some embodiments, each output waveguide (in the illustrated embodiment, one for each ring-shaped SHG / SFG element 120') is configured such that only the desired wavelength of light (the SHG / SFG frequency or wavelength, e.g., 220 nm) is supported (and not the fundamental frequency or wavelength, e.g., 440 nm) because the critical frequency of the output waveguide is greater than the fundamental frequency.

[0161] Without being bound by theory, some advantages of providing multiple nonlinear optical elements 120 can be explained as follows: The design of the ring-shaped nonlinear optical element 120 may be such that a single or each cavity can provide very high SHG conversion efficiency, but the manufacturing tolerances of the radius, coupling gap(s) and loss of each ring-shaped nonlinear optical element 120 may be difficult to maintain for each emitter. Therefore, multiple nominally identical (or substantially similar) nonlinear optical elements 120 along the edge of the waveguide may be provided for redundancy, each of the nonlinear optical elements 120 may have slightly different dimensions (e.g. radius R, supported frequency ω, gap to the waveguide, etc.). Thus, even if only one of the multiple nonlinear optical elements 120 provides sufficient SHG efficiency, the entire device (laser + waveguide(s) + nonlinear optical element 120) can operate as intended.

[0162] That is, because manufacturing tolerances of structures or elements of the UV light source 1400 may present challenges, some embodiments may include multiple variations of the SHG / SFG elements 120' to address differences in manufacturing tolerances. Specifically, as the pump light 111 is outcoupled from the laser 110' and propagates through the input waveguide, the geometry (including dimensions and shapes) of one or more (or each) of the SHG / SFG elements 120' may vary by design or due to manufacturing variations. With different geometries, each SHG / SFG element 120' may have a different amount or level of coupling to the waveguide or may have a different resonant frequency, such that one or more of the plurality of SHG / SFG elements 120' may be particularly well matched (with respect to operating wavelength) to the pump laser 110', while one or more other elements of the plurality of SHG / SFG elements 120' may not be equally well matched to the pump laser 110'. Thus, only a subset of the multiple SHG / SFG elements 120' may contribute to a majority of the SHG / SFG light 121' generated by the UV light source 1400. Alternatively, as temperature or operating conditions change, one or more of the SHG / SFG elements 120' may move in and out of ideal alignment with the pump wavelength (i.e., due to variations in operating characteristics over time or duration of operation of the UV light source). Overall, the redundancy provided by multiple variations of the SHG / SFG elements 120' may contribute to the robustness of the overall performance of the UV light source 1400.

[0163] In other words, to address issues regarding manufacturing tolerances, the multiple nonlinear optical elements 120 may be intentionally fabricated (e.g., along the length of the input coupling element 115 or the lasing cavity 105) with one or more different sizes, shapes, or even materials such that at least one of the nonlinear optical elements 120 may have a desired dimension that provides high conversion efficiency for the fundamental wavelength of the light output from the light emitting element 110. The remaining (i.e., non-matching) nonlinear optical elements 120 may not be used if they are insufficiently coupled to the input waveguide or do not provide a high enough SHG / SFG efficiency, and the fundamental wavelength light 111 that is coupled to the non-matching ring-shaped nonlinear optical element 120 may be returned to the input waveguide or outcoupled. Alternatively, the non-matching nonlinear optical elements 120 may be intentionally removed, destroyed, or disabled by some additional process step after fabrication.

[0164] Because the overall area or footprint of the UV light source 1400 may be dictated by the length of the laser 110' and the width of each nonlinear optical element 120 (i.e., along a direction perpendicular to the length of the laser 110' or the waveguide), the inclusion of "extra" or unused nonlinear optical elements 120 of nominally identical or similar configuration may not require substantially more area on the chip. That is, the use of multiple nonlinear optical elements 120 may have relatively little cost, especially when the size of the SHG / SFG elements 120' is small compared to the dimensions of the pump laser 110' and / or the waveguide (e.g., a fraction of the length). However, the inclusion of additional nonlinear optical elements 120 may increase the likelihood of achieving desired device functionality and / or high conversion efficiency. In this manner, providing multiple ring-shaped or other nonlinear optical elements 120 with one or more different sizes, shapes, and / or materials along the length of the waveguide may result in higher device yields than may be conventionally possible, with little or no size penalty.

[0165] More generally, in the example of FIG. 14 , each of the nonlinear optical elements 120 may have different sizes, shapes, and / or materials, and the output coupling element 130 may include multiple output coupling elements 130, each configured to selectively outcouple far-UVC light 121′ as output light 131′ from each of the nonlinear optical elements 120. As mentioned above, variations of the embodiments described herein may be implemented with a similar layout or configuration, but include (a) different types of lasers 110′ used to pump the system or materials used for the lasers 110′, such as GaN, AlGaN, InGaN, etc.; (b) different materials used for the nonlinear optical elements 120, such as AlN, lithium niobate, BBO, and / or other nonlinear materials; (c) different shapes or designs of the waveguide itself (e.g., some embodiments may use ridge waveguides with flat edges, while other embodiments may use slabs of high index material with guiding provided by lateral variations in the cladding, etc.); (d) different substrates, such as AlN, GaN, Si, sapphire, or other materials; and (e) different fabrication methods for heterogeneously integrating the various components (e.g., monolithic fabrication using epi regrowth, wafer bonding, and / or microtransfer printing). Any or all of the various combinations of the embodiments encompassing the concepts described herein are included in the above-described embodiments using multiple nonlinear optical elements 120 along the same waveguide.

[0166] Further embodiments of the present disclosure may similarly provide multiple nonlinear optical elements 120 along one or both sides of the input waveguide (at the laser output), but may remove or omit the respective output waveguide (illustrated in FIG. 14 as providing SHG / SFG light 121′ from the nonlinear optical element 120 to the output coupling element 130). Rather, the SHG / SFG light 121′ generated by the nonlinear optical element 120 may escape by radiative bending loss from the ring-shaped nonlinear optical element 120. In general, the smaller the bending radius, the greater the radiative loss from the ring-shaped waveguide. Thus, the ring-shaped nonlinear optical element 120 may be configured to take into account the radiative bending loss when considering its radius. 14 (which maintains low radiation losses for outcoupling the SHG / SFG light 121′ through the bottom output waveguide as output light 131′), further embodiments can be configured to emit a portion or up to all of the SHG / SFG light 121′ as output light 131′ via the bending radius of each ring-shaped nonlinear optical element 120. Without being bound by theory, some advantages of eliminating each output waveguide and relying on radiation bending emission to collect the SHG / SFG light 121′ may include reduced fabrication complexity by eliminating at least one component of the overall device (e.g., the 220 nm output waveguide(s)) whose dimensions may otherwise require strict precision in terms of size and positioning relative to the nonlinear optical element(s) 120.

[0167] That is, while Figure 14 shows the use of multiple SHG cavities in an "external cavity tap" configuration, providing an output waveguide to outcouple the SHG light, other embodiments can vary by providing an "intra-cavity tap" of the pump laser 110' (instead of using a waveguide) and / or a radiated or scattered output of the SHG / SFG light 121'. Furthermore, these embodiments can use a nonlinear optical element 120 that is not a ring cavity, but instead is some other type of whispering gallery mode cavity, or some type of linear cavity.

[0168] The omission of output waveguide element(s) for collection of the SHG / SFG light 121′ outcoupled from the nonlinear optical element 120 indicates that the output light 131 may be emitted laterally (i.e., in-plane emission relative to the plan view above) rather than surface emission in a direction perpendicular to the plane in the plan view shown above (or alternatively with some out-of-plane components). Thus, in some embodiments, one or more angled reflectors may be integrated around the exterior (e.g., circumference) and / or interior of the ring. Such angled reflectors (not shown) may extend around and / or within the periphery or circumference of the nonlinear optical element 120 in the plan view. For example, in a cross-sectional view, the angled reflectors may be triangular reflectors on either side of the nonlinear optical element 120. It will be appreciated that some out-of-plane emission components may also be incorporated into the SHG / SFG light 121′ Poynting vector (propagation direction) depending on the shape of the sidewalls of the ring-shaped nonlinear optical element 120 (which may be trapezoidal in cross section). That is, the sidewalls of the nonlinear optical element 120 can have a significant amount of angle in cross section such that light can be emitted with some upward / out-of-plane component. In other words, the waveguide sidewall angle of the nonlinear optical element 120 can be controlled or otherwise configured in some embodiments to optimize the desired emission.

[0169] It will be appreciated that the concept of SHG / SFG emission via bending losses described in the above embodiments can be similarly implemented in any of the embodiments described herein. For example, any of the embodiments including the ring-shaped nonlinear optical element 120 described herein can be configured to allow light generated by the nonlinear optical element 120 to escape via radiative bending losses from the ring-shaped nonlinear optical element 120 (e.g., by omitting the output coupler(s) and using one or more angled reflectors for light extraction in the desired direction).

[0170] Further embodiments of the present disclosure may similarly provide multiple nonlinear optical elements 120, but in an intracavity configuration that receives input light directly from the lasing cavity 105 on one side thereof in combination with one or more output waveguides (e.g., one for each nonlinear optical element 120) along the other side. In particular, further embodiments may similarly include an SHG / SFG light 121′ output waveguide that outcouples SHG / SFG light 121′ (e.g., 220 nm) from each ring resonator, as shown in FIG. 14 , but instead of introducing the fundamental wavelength light 111 into the nonlinear optical element 120 by a unidirectional wave from the laser 110′, the nonlinear optical element 120 may be positioned along the waveguide that provides the lasing cavity 105 of the laser 110′. That is, although shown in FIG. 14 as including an input coupling element 115 implemented as an input waveguide, the input coupling element 115 may be omitted and light from the light emitting element 110 may be coupled directly into the nonlinear optical element 120 from within the cavity of the (active) laser 110′ itself, rather than from a separate passive input waveguide. Such an embodiment may be more complex in terms of the design of the laser 110′ itself (the presence of one or more rings represents a loss term in the energy balance of the laser 110′), but may be advantageous in that a higher intensity pump power (e.g., at 440 nm) may be provided. The remaining aspects of this embodiment may be the same or similar to the previous embodiments, including variations with respect to the materials, substrates, and / or fabrication methods described herein.

[0171] Further embodiments of the present disclosure may similarly provide multiple nonlinear optical elements 120 configured to receive input light on one side thereof in combination with one or more output waveguides (e.g., one for each nonlinear optical element 120) along the other side. However, the nonlinear optical elements 120 may be implemented in shapes other than rings (e.g., other rotationally symmetric shapes such as disk (e.g., microdisk) or sphere (e.g., microsphere) shapes). The modes supported by such structures may differ from the modes of a ring-shaped nonlinear optical element 120, but the overall concept and advantages of using multiple resonators remain the same as the previous embodiments.

[0172] Further embodiments of the present disclosure may similarly include multiple nonlinear optical elements 120 (e.g., ring resonators) configured to receive input light on one side thereof, in combination with one or more output waveguides (e.g., one for each nonlinear optical element 120) along the other side, using a nonlinear optical element 120 (e.g., ring resonator) directly coupled to the intracavity region 105i of the lasing cavity 105 of the laser diode 110'. However, the laser diode 110' (to which multiple ring-shaped nonlinear optical elements 120 are coupled) may be implemented as a ring laser rather than a linear ridge laser. Coupling to a ring laser may be advantageous in terms of the type of modes in which coupling can be realized. Coupling to a ring laser as the light-emitting element 110 may also be configured to couple only to modes propagating in a single direction (as opposed to multiple directions as can be obtained from a linear laser with standing waves).

[0173] Further embodiments of the present disclosure may include at least one nonlinear optical element 120 configured to receive input light on one side thereof in combination with at least one output waveguide (e.g., one for each nonlinear optical element 120) along its other side. However, optical coupling between elements may be achieved at least in part by lateral overlap of the nonlinear optical element(s) 120 with the components for input and / or output optical coupling such that at least two of the components (e.g., the light emitting element 110, the input coupling element 115, the nonlinear optical element 120, or the output coupling element 130) are not in the same plane, also referred to herein as multilayer integration.

[0174] 15 is a schematic top view illustrating an example combination of various elements of a UV light source 1500 in a physically overlapping configuration for multi-layer integration, according to some embodiments of the present disclosure. In FIG. 15, the coupling geometry differs from some embodiments described herein in that the respective portions of the SHG / SFG elements 120' physically overlap with the input and output waveguides, but are not located in the same or coplanar plane (i.e., the elements are offset in the vertical or Z direction (in-plane or out-of-plane) relative to each other).

[0175] Advantages provided by the multi-layer integration arrangement shown in FIG. 15 include that the critical dimension at which optical coupling occurs is in the vertical or Z direction (i.e., out-of-plane in the plan view shown), which may be easier to control by many manufacturing methods. It will be understood that the vertical overlap of two or more components as shown in the embodiment of FIG. 15 may include any and all methods of integrating or fabricating multiple components such that two or more do not share the same plane. Exemplary methods include wafer bonding, micro-transfer printing, micro-assembly, self-assembly, and / or polishing and epitaxial regrowth. Additional variations of vertically overlapping elements beyond those shown in FIG. 15 may also be used for coupling. Such coupling configurations may include, but are not limited to, vertical overlapping evanescent wave coupling, lateral evanescent coupling, the use of gratings to direct light into or out of the plane of the waveguide, butt (end-to-end) coupling, and / or the use of tapered element shapes (e.g., waveguides with non-uniform thickness) that enhance coupling. More generally, the UV light sources described herein may include two or more of a light emitting element 110, an input coupling element 115, a nonlinear optical element 120, and an output coupling element 130 overlapping in a direction perpendicular to a surface of a substrate 101 having the light emitting element 110, the nonlinear optical element 120, and the output coupling element 130 thereon.

[0176] Further embodiments of the present disclosure can extend the Vernier frequency selection strategy through the use of an intermediate cavity (e.g., a ring or other shaped cavity between the lasing cavity 105 and the optical cavity 125 of the nonlinear optical element 120) to include various coupling ring configurations that can select only a single mode over an even larger free spectral range (FSR). Doing so can help concentrate the energy from the pump laser into the single mode that is doubled, thus increasing efficiency.

[0177] Some embodiments can be configured to provide the ability to switch the coupling on and off by providing an electro-optic or thermo-optic material between the two rings (to electrically or thermally shift the refractive index). For example, an electro-optic or thermo-optic material may be provided between a ring laser and a ring-shaped nonlinear optical element.

[0178] Some embodiments may include a saturable absorber in the ring laser to induce a pulsed mode.

[0179] Some embodiments may include one or more secondary rings as "filters" to provide a wider free spectral range and to match particular ring modes to particular SHG / SFG ring modes. For example, at least one passive oscillator may be provided as a secondary ring that receives the light outcoupled from the ring laser and outcouples a subset of the light to a nonlinear optical element for second harmonic generation.

[0180] Some embodiments may include multiple nonlinear optical elements disposed partially or substantially around the periphery of a laser, for example, multiple ring-shaped nonlinear optical elements 120 disposed around a ring laser.

[0181] Some embodiments may include one or more secondary rings as filters within a larger nonlinear optical element, partially or substantially positioned around the periphery of a laser. For example, a large radius ring-shaped nonlinear optical element extends around a ring laser, which in some embodiments may be filtered by one or more ring filter oscillators to enable mode selection from an SHG ring with high mode density.

[0182] Some embodiments may include multiple ring lasers per nonlinear optical element. For example, multiple ring lasers may be arranged around the periphery (or circumference) of a ring-shaped nonlinear optical element. Due to coupling between the rings, the ring lasers may all be forced into the same phase or mode. In some embodiments, the ring lasers may be turned on or activated sequentially, allowing the first ring laser to set the phase of the remaining ring lasers. Some embodiments may include a wavelength tuning mechanism configured to independently provide local temperature or electric field tuning of each ring laser, which may provide another degree of freedom.

[0183] As mentioned above, some conventional designs of UV light sources can be disadvantaged by coupling losses between active and passive components and / or conversion efficiency, but embodiments of the present disclosure can provide higher nonlinear conversion efficiency by using an optical cavity 125 to increase the number of passes the pump laser 110' makes through the material (effectively recycling the unconverted pump light 111). These advantages have been demonstrated at other wavelengths. For example, optical microresonators fabricated from AlN have demonstrated SHG / SFG conversion efficiencies of over 17000% / W (up to 10% absolute conversion efficiency for 10 mW input) and third harmonic conversion efficiencies of 180% / W2, despite using a fundamental wavelength of 1540 nm [9-12]. These results demonstrate that AlN has a sufficiently high nonlinear response (4 pm / V vs. 7 pm / V) to provide very high conversion efficiencies, especially when cavity enhancement is also used to increase or maximize the intensity of the fundamental wavelength.

[0184] Further embodiments of the present disclosure may provide a way to control the far-field pattern of the output light 131 (e.g., far-UVC light 121') outcoupled from the nonlinear optical element 120 described herein. In some applications, control of the spatial distribution of emitted irradiance over an area of ​​illumination (or equivalently, the angular distribution of emitted intensity over a field) may be important to performance. In fact, visible lighting products support entire industries dedicated to shaping and shaping illumination patterns. For UV applications, there may be a similar need to control this "far-field pattern" to provide a germicidal effect to the far-UVC output light 131', which may depend, for example, on spreading the germicidal UV over the application area in an optimal manner.

[0185] Some embodiments of the present disclosure can include an output coupling element 130 implemented as a second-order diffraction grating configured to outcouple light from an in-plane PIC and project it over a range of angles surrounding the normal surface vector. The design or configuration of the diffraction grating can provide some ability to modify how wide an angle the light is spread over and the uniformity of the emission intensity within the range of emission angles.

[0186] In further embodiments, the photonic integrated circuit may include structures configured to split the light generated on-chip into multiple channels, with each channel having its own output coupling element 130 that may or may not include a second-order diffraction grating. Figure 16 is a schematic top view illustrating a UV light source output coupling element configuration 1600 configured to provide a desired far-field emission pattern, according to some embodiments of the present disclosure.

[0187] In FIG. 16, the output coupling element 130 is implemented as multiple output coupling elements 130-1, 130-2, 130-3, 130-4 configured to outcouple the far UVC light 121′ as output light 131 in respective directions (e.g., at various angles in or out of the plane of the page or substrate 101, as shown) to provide output light 131 having a desired far-field pattern. One or more waveguides 160 may be coupled between the nonlinear optical element 120 and each output coupling element 130, and the output light 131 may be beam split by each waveguide and output coupling element 130 to provide respective channels of light output. The multiple output coupling elements 130 may be configured to direct light upward (e.g., away from the surface or substrate 101), but in a direction that is not necessarily or perfectly perpendicular (e.g., not necessarily 90 degrees) to the surface or substrate 101. By centering or providing the emission pattern at the end of each channel to provide output light 131 (e.g., far-UVC light 121') in different directions (coherently or incoherently), the combined overall light pattern can be designed to suit the requirements of the desired application. The number of channels for which this configuration can be used can be as few as one or two, with the upper limit limited only by the practicalities of PIC design (e.g., several thousand or more).

[0188] Two specific subclasses of multiple output channel configurations described herein include arrays of UV light sources and coherent light combinations. In an array of UV light sources, the light output of any one UV light source may or may not be split to multiple output couplers. However, because the device is intended to be operated as part of a larger array of nominally identical devices, the output coupling element 130 of each individual UV light source in the array (or elements) may be individually modified so that the combined far-field pattern of the entire array meets desired specifications.

[0189] In coherent combination of light, each individual UV light source can have its respective light output split into at least two different channels. Because the output light 131 emitted from each of the two or more channels (e.g., far-UVC light 121') originates from the same coherent light source (e.g., visible light 111' output from laser 110'), the output light 131 from each channel can maintain a fixed phase relationship. Thus, the far-field emission pattern generated by each emission channel (one per UV light source) can be subject to coherent effects (similar to those used for light beam steering). In other words, some embodiments can utilize coherent combination of the output from each channel of multiple individual UV light sources to obtain a desired emission pattern. However, when multiple UV light sources are operated as a very large array, or as an array with separate or different output coupling element configurations, it may be unlikely that phase coherence can be maintained between the UV light sources, so that the field pattern generated by the collective light output across the array may include an incoherent combination of light fields.

[0190] The embodiments of the present disclosure may differ from some conventional designs in several ways. For example, some embodiments of the present disclosure integrate active and passive components on the same chip (e.g., using components of the same material system, such as nitride-based materials) so that optical losses between devices are reduced or minimized. In addition, some embodiments of the present disclosure may specifically target 440 nm to 220 nm conversion, focusing on conversion efficiency, as opposed to designs that may attempt to fabricate coherent polarized lasers (beams) with narrow linewidths, which may not be necessary for some applications. Also, in contrast to previous demonstrations of SHG generating 220 nm light, PICs according to some embodiments of the present disclosure leverage resonant cavity enhancement to increase or maximize the intensity of the fundamental wave, and therefore increase or maximize efficiency. The output light provided by the embodiments of the present disclosure may be collimated or uncollimated, may be coherent or incoherent, and may be emitted as a beam or as a distributed emission. Some embodiments may use one or more of the following technology elements in various combinations (but are not limited to): wavelength conversion using nonlinear optics (SHG / SFG), use of AlN-based nonlinear optical elements 120 for wavelength conversion, selective outcoupling of far UVC wavelengths, use of AlN-based waveguides or waveguides including PICs, use of optically resonant microcavities, monolithic integration of active and passive components, and optical output of the light emitting element 110 that does not include the fundamental wavelength.

[0191] The embodiments of the present disclosure described herein may thereby reduce the cost and increase the (power) efficiency of generating far-UVC light 121', which may be advantageous in providing a cost-competitive source of disinfecting light that may be widely deployed to combat airborne (and surface) pathogens. Furthermore, by providing UV emission in the far-UVC range (e.g., about 200 nm to about 240 nm), embodiments of the present disclosure may be used to actively eliminate pathogens from the air while people are present, in contrast to the traditional use of UV wavelengths for disinfection purposes in wavelength ranges that are harmful to humans (e.g., greater than about 240 nm to about 400 nm).

[0192] Further embodiments of the present disclosure provide devices configured to generate electromagnetic radiation in the far UVC spectrum to provide germicidal effects while also complying with human safety regulations and requirements. Germicidal light sources configured to operate in the far UVC wavelength range may be advantageous in that (i) rates of disinfection of pathogens may be higher, and (ii) from a human safety perspective, acceptable irradiation levels may be higher (and perhaps infinite or unlimited) as compared to the remaining wavelengths in the UV spectrum.

[0193] In light of safety regulations and / or concerns, it may be advantageous to operate a GUV light source only when necessary and / or at a power level, duty cycle, and / or spatial illumination pattern optimized to minimize the risk of airborne pathogen transmission. Achieving such operation may require sensing of operating conditions and / or other information in real time.

[0194] The embodiments of the present disclosure described herein can provide real-time actionable information to the GUV light source by integrating sensors into the GUV system operation, either physically or via a communication network. In particular, some embodiments of the present disclosure provide a sensor feedback based "smart" lighting device that includes a GUV light source communicatively connected to various types of sensors, the sensors configured to feed back information to a controller of the GUV light source to enable algorithmic decision making and optimized operation.

[0195] The integration of the sensor(s) and the GUV light source into a single device can be advantageous with respect to the capabilities and range of operation of the GUV illumination product, allowing the detected operating conditions to be provided to a controller in real time and allowing control of the operation of the GUV light source according to the detected operating conditions. GUV illumination and illumination can thereby be optimized, i.e., with respect to increasing or maximizing the effectiveness of the GUV with respect to its ability to disinfect, while reducing or minimizing any overall GUV light output with the aim of staying within safety limits, extending GUV life, and reducing or minimizing the impact of UV light on the surrounding environment.

[0196] In contrast, some conventional GUV systems may not be configured to detect or control operation based on existing operating conditions. Rather, such conventional GUV systems may be operated in a limited number of states, typically "on" or "off" illumination states. Furthermore, such conventional GUV systems may require manual intervention to change the operating conditions of the GUV light source. Some GUV systems are driven by autonomous robots used to disinfect surfaces within enclosed rooms. These autonomous robots may employ sensors in conjunction with the operation of the UV light, but the sensors are typically directed to controlling the operation of the autonomous robot, not the optimization of GUV illumination in a dynamic environment.

[0197] Also, while sensors may traditionally be used in conjunction with typical visible lighting, embodiments of the present disclosure are directed to operation in the UV spectrum, where (a) availability and cost of lighting is scarce and (b) concerns regarding human safety are particularly high. In GUV applications, the types of sensors used and the reasons for using them may differ from those in general (visible) lighting applications. For example, sensors that may guide the use of GUV lighting may include various forms of air quality sensors (aerosol detectors, pathogen detectors, etc.) to determine the degree of need or effectiveness of GUV lighting, including the relative intensity at which the lighting fixture should be operated. Instead of or in addition to the use of these sensors, 3D time-of-flight cameras or other positional sensors that can both detect movement and quantify occupancy levels within a given space may be used to moderate the amount of GUV lighting provided to stay within regulatory limits. In any of these cases, the distinction from the type and sophistication of any sensors incorporated into general lighting is significant.

[0198] Some elements of embodiments of the present disclosure may integrate a sensor suite into the GUV lighting devices described herein. Figure 17 is a schematic block diagram illustrating components of a sensor feedback based "smart" lighting device including a germicidal UV (GUV) light source communicatively connected to a sensor 1750 configured to feed back information to a controller 1701 of the GUV light source, according to some embodiments of the present disclosure.

[0199] In particular, FIG. 17 illustrates an illumination device 1700 including a GUV light source 100′ configured to generate and emit electromagnetic radiation in the germicidal region of the UV spectrum. In some embodiments, the GUV illumination device 1700 can be configured to provide emission in the far UVC spectrum from about 200 nm to about 240 nm (e.g., about 222 nm). The GUV illumination device 1700 can include a UV light source (such as the UV light source 100′), a controller 1701, and one or more sensors 1750 configured to detect real-time conditions within an operating environment of the UV light source 100′ and provide detection signals to the controller 1701 indicative of the real-time conditions. The controller 1701 is configured to control operation of the light-emitting elements 110 of the GUV light source 100′ based on the detection signals.

[0200] The GUV light source 100' may be implemented using a solid-state system that generates coherent or non-coherent electromagnetic non-ionizing radiation in the far-UVC wavelength band based on nonlinear optical processes and using photonic integrated circuits (PICs), as described above in "Nonlinear Solid State Devices For Optical Radiation In Far-UVC Spectrum" by Fisher et al., the disclosure of which is incorporated herein by reference. Alternatively, the GUV light source 100' may be implemented by any of the UV light sources (e.g., 100, 200, 300, etc.) or arrays (e.g., 499, 900) described herein. For example, the GUV light source 100' may include a light emitting element 110 implemented by a pump laser 110' (e.g., a III-nitride based laser diode such as a blue pump laser diode) or a light emitting diode (LED) configured to generate visible light 111', and a nonlinear optical element 120 (e.g., a nonlinear optical crystal) configured to receive the visible light 111' from the light emitting element 110 and generate a second frequency of far-UVC light 121' based on the first frequency of the visible light 111' (e.g., based on SHG or SFG). The nonlinear optical element 120 may be optically transparent to wavelengths equal to or less than the desired output wavelength (e.g., the far-UVC wavelength range). An input coupling element 115 (e.g., a continuous waveguide connecting the emission from the pump laser 110' or LED to the nonlinear optical crystal) may be configured to couple light from the pump laser 110' to the nonlinear optical element 120. In some embodiments, phase matching may be provided between the SHG / SFG light 121' and the fundamental (pump) wavelength light 111'. The output coupling element 130 is configured to selectively or in combination with the visible light 111' from the nonlinear optical element 120 (i.e., such light output is either only the far-UVC light 131', or only the far-UVC light 131' at a second frequency, or in combination with the visible light 111' at a first (fundamental) frequency) outcouple as output light 131'.However, it will be understood that embodiments of the present disclosure may be used for sensor feedback based control of other GUV light sources.

[0201] 17, a sensor suite including one or more sensors 1750 of various types is configured to detect real-time conditions in the operating environment of the GUV light source. Communication between the sensor suite and the GUV light source 100' may be provided by a controller 1701 and / or other communication connections. The communications / controller 1701 is configured to communicate information obtained by the sensors 1750 back to the GUV device 1700 to control the operation of the GUV light source 100' for light generation 131.

[0202] The sensors 1750 are thereby configured to provide information feedback that improves or optimizes the operation of the GUV illuminator 1700 for a desired application. The sensors 1750 can also be configured to detect and communicate information for purposes other than the operation of the GUV illuminator 1700. Examples of possible sensors 1750 include, but are not limited to, air quality sensors (such as humidity sensors, temperature sensors, VOC sensors, chemical sensors (CO2, CO, etc.), specific substance sensors, and aerosol sensors), biological sensors such as virus or pathogen detectors; radar sensors, e.g., to assess distance to objects; 2D camera sensors, e.g., to assess conditions within an area of ​​operation including personnel and occupancy; 3D camera or lidar systems, e.g., to measure distance to objects, occupancy, movement, etc.; illumination sensors, e.g., to assess the intensity of GUV illumination within a field of view over time; and / or passive infrared (IR) or other motion sensors.

[0203] The communication channel 1702 between the sensors 1750 and the controller 1701 may be bidirectional so that information from the GUV light source 100' can be shared with the sensor suite 1750 to obtain a more accurate measurement of the environment. That is, the controller 1701 may be configured to control the operation of the GUV light source 100' based on information or data output from the sensors 1750 and / or to control the operation of the sensors 1750 based on the operation and / or light output 131 of the GUV light source 100'.

[0204] Additionally, the components of the GUV illumination device 1700 (e.g., 110', 1701, 1750) may or may not be integrated within the same housing. For example, it will be understood that one or more sensors 1750 of the sensor suite and the GUV light source 110' do not need to be contained within the same physical housing and / or even be collocated. More generally, embodiments of the present disclosure can include any configuration that can communicate sensor information with a GUV light source and control operation of the GUV light source based on the sensor information. It will be understood that the GUV light source can be a UV light source described herein (e.g., 100, etc.) or can be another light source (e.g., a non-solid-state light source such as an excimer lamp or other conventional UV light source). That is, the operations and components of FIG. 17 can be used with any UV light source, including (but not limited to) the UV light sources described herein.

[0205] Advantages of embodiments of the present disclosure can include overall optimization of the operation of the GUV illumination system, including maximizing pathogen disinfection per unit cost. For example, costs can be reduced by operating the illumination device more efficiently (e.g., operating the GUV light source at a higher intensity for a shorter period of time), operating the GUV light source when a sensor indicates that the value has been maximized, and / or utilizing fewer units to cover a given space (thus reducing costs). Costs can also be reduced by reducing or minimizing the overall time a given GUV light source is on, i.e., effectively reducing the duty factor. This can extend the life of the GUV light source, thus reducing the overall operating costs. Beneficiaries of such improved operation and / or optimization can include both customers, system operators, and anyone who comes into contact with the disinfection technology.

[0206] Commercial applications of far-UVC lighting according to embodiments of the present disclosure can include the removal of pathogens from air and / or surfaces in any indoor space where humans congregate (e.g., airports, schools, hospitals, inpatient medical centers, workplaces, etc.), as well as in transportation vehicles (e.g., subway cars, trains, taxis, airplanes) and agricultural environments (e.g., animal production facilities, meat packing facilities, indoor greenhouses, etc.).

[0207] Additionally, the generation of far-UVC light 121' can have numerous applications outside of germicidal applications, which may include (but are not limited to) spectroscopy, optical sensing, detection, and the like. In particular, UV light sources configured to provide far-UVC illumination according to embodiments of the present disclosure can be used for the detection of trace amounts of chemical or biological species in various field environments (air, water, etc.) where UV fluorescence and Raman spectroscopy are widely used and developed. The use of extremely short wavelength (e.g., in the far-UVC wavelength range) excitation for such applications can each be beneficial in different ways. For example, the efficiency of Raman scattering scales as the fourth power of the excitation wavelength (1 / λ 4For fluorescence applications, moving the excitation wavelength further into the UV range can open up a wider spectral range of possible emissions while reducing or minimizing background from the excitation wavelength or background light.

[0208] Some existing light sources used to generate these (far) UV wavelengths are expensive, large, may not achieve the required wavelengths, and / or may not be safe for humans. In contrast, UV light sources according to embodiments of the present disclosure can offer several attributes that may be particularly useful for Raman and / or UV spectroscopy applications, including (but not limited to) (a) small size per unit light output, (b) low cost, (c) ability to operate in the solar blind region of the visible spectrum (i.e., having emission wavelengths in a spectral range without background noise from the sun), and (d) emission in a wavelength range that is safe for humans. The embodiments described herein can thereby provide a new way to deploy fluorescence and Raman spectroscopy in low-cost handheld or low-cost wall-mountable devices that monitor environments in which people are persistently present.

[0209] UV light sources according to embodiments of the present disclosure can further generate output light 131 (e.g., SHG / SFG light 121') over a very narrow bandwidth (e.g., having an emission linewidth or bandwidth of less than about 1 nm, e.g., less than about 0.5 nm, or less than about 0.1 nm). In some embodiments, the output light 131 can be emitted from an edge of the output coupling element 130, for example, as a coherent beam. That is, in addition to providing output light 131 in the far UVC wavelength range (about 200 nm to 240 nm), the linewidth of the emission from some embodiments of the present invention can be, for example, less than about 0.1 nm, which is much narrower than some conventional light sources. Raman spectroscopy applications in particular can benefit from the very narrow spectral width of the light source.

[0210] Various embodiments have been described herein with reference to the accompanying drawings in which exemplary embodiments are shown. However, these embodiments may be embodied in different forms and should not be construed as being limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Various modifications to the exemplary embodiments and general principles and features described herein will be readily apparent. In the drawings, sizes and relative sizes of layers and regions are not shown to scale and may, in some cases, be exaggerated for clarity.

[0211] Exemplary embodiments are described primarily with respect to specific methods and devices provided in specific implementations. However, the methods and devices may operate effectively in other implementations. Phrases such as "exemplary embodiment," "one embodiment," and "another embodiment" may refer to the same or different embodiments and to multiple embodiments. The embodiments are described with respect to systems and / or devices having certain components. However, the systems and / or devices may include fewer or additional components than those shown, and changes in the arrangement and type of components may be made without departing from the scope of the inventive concept.

[0212] Exemplary embodiments are also described in the context of a particular method having certain steps or operations. However, the methods and devices may effectively operate with other methods having different and / or additional steps / operations and different orders of steps / operations that are not inconsistent with the exemplary embodiments. Thus, the inventive concept is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein.

[0213] When an element is referred to or illustrated as being "on," "connected" to, or "coupled" to another element, it will be understood that the element can be directly on, connected to, or coupled to the other element, or there can be intervening elements. In contrast, when an element is referred to as being "directly on," "directly connected," or "directly coupled" to another element, there are no intervening elements present.

[0214] It will also be understood that, although terms such as first, second, etc. may be used in this specification to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. For example, a first element can be referred to as a second element, and a second element can be referred to as a first element, without departing from the scope of the present disclosure.

[0215] Additionally, relative terms such as "lower" or "bottom" and "upper" or "top" may be used herein to describe the relationship of one element to another element as depicted in the figures. It will be understood that the relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if the device in one of the figures is flipped over, an element described as being on the "lower" side of the other element would then be oriented on the "upper" side of the other element. Thus, the exemplary term "lower" can encompass both an orientation of "lower" and "upper", depending on the particular orientation of the figure. Similarly, if the device in one of the figures is flipped over, an element described as being "lower" or "under" the other element would then be oriented "above" the other element. Thus, the exemplary term "lower" or "under" can encompass both an orientation of upper and lower.

[0216] The terms used in the description of the invention herein are for the purpose of describing particular embodiments only and are not intended to be limiting of the invention. As used in the description of the invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0217] It will also be understood that the term "and / or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms "include", "including" and / or "comprise", "comprising", as used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0218] Embodiments of the present invention are described herein with reference to figures that are schematic illustrations of idealized embodiments (and intermediate structures) of the present invention. As such, variations from the shapes of the illustrations are expected as a result, for example, of manufacturing techniques and / or tolerances. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to represent the actual shape of a region of a device, and are not intended to limit the scope of the present invention.

[0219] Unless otherwise defined, all terms used in disclosing embodiments of the present invention, including technical and scientific terms, have the same meaning as commonly understood by those skilled in the art to which the present invention belongs, and are not necessarily limited to the specific definitions known at the time this disclosure is described. Thus, these terms can include equivalent terms created after such time. Furthermore, it will be understood that terms as defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the present specification and in the context of the related art, and not to be interpreted in an idealized or overly formal sense unless expressly defined as such herein. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0220] In connection with the above description and drawings, many different embodiments are disclosed herein. It will be understood that literally describing and illustrating all combinations and subcombinations of these embodiments would be overly repetitive and confusing. Therefore, this specification, including the drawings, shall be construed as constituting a complete written description of all combinations and subcombinations of the embodiments of the present disclosure described herein, and the manner and process of making and using them, and shall support claims to any such combinations or subcombinations.

[0221] While the invention has been described herein with reference to various embodiments, it will be understood that further variations and modifications can be made within the scope and spirit of the principles of the invention as set forth in the following claims.

Claims

1. a light emitting element that produces light of a first frequency; a nonlinear optical element that receives light of the first frequency from the light emitting element and generates far UVC light of a second frequency from the light of the first frequency; an output coupling element for selectively out-coupling the far UVC light from the nonlinear optical element as output light; An ultraviolet (UV) light source comprising:

2. the output coupling element selectively outcouples the far UVC light into at least one direction different from a propagation direction of light at the first frequency to provide the output light; Optionally, the output light is substantially free of light at the first frequency.

2. The UV light source of claim 1.

3. 10. The UV light source of claim 1, wherein the nonlinear optical element comprises aluminum nitride (AlN), and optionally at least one of the light emitting element and the output coupling element comprises a III-nitride based material.

4. 2. The UV light source of claim 1, wherein the nonlinear optical element is or comprises an optical cavity that is at least partially resonant at the first frequency.

5. The UV light source of claim 4 , wherein the nonlinear optical element has a ring configuration forming the optical cavity.

6. 6. The UV light source of claim 4 or 5, wherein the nonlinear optical element comprises a plurality of nonlinear optical elements that receive light of the first frequency from the light emitting element.

7. 7. The UV light source of claim 6, further comprising an input coupling element for receiving light at the first frequency from the light emitting element, the plurality of nonlinear optical elements being disposed along the input coupling element.

8. 8. The UV light source of claim 7, wherein each of the plurality of nonlinear optical elements has a different size and / or material, and the output coupling element includes a plurality of output coupling elements each for selectively outcoupling the far UVC light from each of the plurality of nonlinear optical elements.

9. The UV light source of claim 4 , wherein the optical cavity has the light emitting element and the nonlinear optical element therein.

10. The UV light source of claim 9 , wherein the optical cavity has a linear or closed curve shape.

11. The output coupling element comprises: a facet having a refractive index for selectively outcoupling the far UVC light in a first direction; a grating having a diffraction order for selectively outcoupling the far UVC light in a second direction different from the first direction; The UV light source according to claim 1 , comprising at least one of the following:

12. 12. The UV light source of claim 11, wherein the nonlinear optical element and the output coupling element are integrated into a same output element that outcouples the far UVC light at multiple locations or continuously along its length.

13. 13. The UV light source of claim 11 or 12, wherein the UV light source provides the output light with substantially no phase matching between the light of the first frequency and the far UVC light of the second frequency.

14. 13. The UV light source of claim 11 or 12, wherein at least one of the nonlinear optical element and the output coupling element provides phase matching between the far UVC light at the second frequency and the light at the first frequency.

15. 10. The UV light source of claim 1, wherein the light emitting element is a laser having a lasing cavity, the laser producing light at the first frequency, and optionally the laser comprising a III-nitride based material.

16. 16. The UV light source of claim 15, wherein the light emitting element further comprises one or more optical resonators disposed at first and second ends of the lasing cavity to reflect light at the first frequency.

17. 17. The UV light source of claim 15 or 16, wherein the nonlinear optical element receives light at the first frequency from an intracavity portion between a first end and a second end of the lasing cavity.

18. 17. The UV light source of claim 15 or 16, wherein the nonlinear optical element comprises a first nonlinear optical element and a second nonlinear optical element disposed at a first end and a second end of the lasing cavity, respectively.

19. 17. A UV light source as claimed in claim 15 or 16, further comprising a saturable absorber in the lasing cavity for producing light of the first frequency as a plurality of optical pulses at a predetermined pulse repetition frequency and duty factor.

20. The UV light source of claim 1 , further comprising at least one adjustment mechanism for adjusting one or more operating characteristics of the nonlinear element based on the first frequency of light.

21. 10. The UV light source of claim 1, further comprising a monitor element that measures a characteristic of the output light and generates a feedback signal to a controller that operates at least one of the light-emitting element and the adjustment mechanism.

22. a substrate having the light emitting element, the nonlinear optical element, and the output coupling element on a surface thereof; two or more of the light emitting element, the nonlinear optical element, the output coupling element, and the connecting waveguide therebetween overlap in a direction perpendicular to a surface of the substrate; 2. The UV light source of claim 1.

23. 10. The UV light source of claim 1, wherein the output coupling element comprises a plurality of output coupling elements outcoupling the far UVC light in each direction to provide the output light in a desired far-field pattern.

24. A controller; one or more sensors that detect real-time conditions in an operating environment of the UV light source and send detection signals to the controller indicative of the real-time conditions; Further comprising: the controller controls operation of the light emitting elements based on the detection signal.

2. The UV light source of claim 1.

25. 25. A UV light source according to any preceding claim, wherein the second frequency comprises a sum or a harmonic of the first frequency.

26. 26. The UV light source of claim 25, wherein the first frequency corresponds to a first wavelength in a range of approximately 400 nanometers (nm) to 480 nm, and the second frequency corresponds to a second wavelength in a range of approximately 200 nm to 240 nm.

27. 27. The UV light source of any one of claims 1 to 26, wherein the light emitting element and the nonlinear optical element comprise respective elements provided on a non-native substrate.

28. 27. The UV light source according to any one of the preceding claims, wherein the light emitting element and the nonlinear optical element are integrated in a monolithic structure.

29. 29. A UV light source according to claim 27 or 28, wherein the UV light source comprises an array including a plurality of the light emitting elements and the nonlinear optical element.

30. a light emitting element that produces light of a first frequency; a nonlinear optical element that receives light of the first frequency from the light emitting element and generates light of a second frequency from the light of the first frequency; A light source comprising a monolithic structure having:

31. 31. The light source of claim 30, wherein the monolithic structure further comprises an output coupling element for selectively outcoupling light at the second frequency from the nonlinear optical element as an output light.

32. the output coupling element selectively outcouples light at the second frequency into at least one direction different from a propagation direction of light at the first frequency to provide the output light; Optionally, the output light is substantially free of light at the first frequency.

32. The light source of claim 31.

33. 33. A light source according to any one of claims 30 to 32, wherein the second frequency of light is far UVC light.

34. 34. The light source of any one of claims 30 to 33, wherein the nonlinear optical element comprises aluminum nitride (AlN), and optionally the light emitting element and / or the output coupling element comprise a III-nitride based material.

35. A light source according to any one of claims 30 to 34, wherein the nonlinear optical element is or comprises an optical cavity that is at least partially resonant at the first frequency.

36. 35. A light source according to any one of claims 30 to 34, wherein the nonlinear optical element comprises one or more nonlinear optical elements, each having an optical cavity for receiving light of the first frequency from the light emitting element, and optionally each optical cavity being ring-shaped.

37. 37. The light source of claim 36, wherein the monolithic structure further comprises an input coupling element for receiving light at the first frequency from the light emitting element, the input coupling element having one or more nonlinear optical elements disposed along it.

38. 38. The light source of claim 37, wherein each of the plurality of nonlinear optical elements has a different dimension and / or material, and the output coupling element comprises a plurality of output coupling elements each for selectively outcoupling light of the second frequency from each of the plurality of nonlinear optical elements.

39. 36. The light source of claim 35, wherein the optical cavity has the light emitting element and the nonlinear optical element therein, the optical cavity having a linear shape or a closed curve shape.

40. 40. A light source according to any one of claims 31 to 39, wherein the nonlinear optical element and the output coupling element are integrated into an output element which outcouples light at the second frequency at multiple locations or continuously along its length.

41. 41. A light source as claimed in any one of claims 30 to 40, wherein the light emitting element is a laser including a lasing cavity, and the nonlinear optical element receives light of the first frequency from an intracavity portion between a first end and a second end of the lasing cavity.

42. a light emitting element that produces light of a first frequency; a nonlinear optical element comprising aluminum nitride (AlN) that receives the light of the first frequency from the light emitting element and generates UVC light of a second frequency from the light of the first frequency; An ultraviolet (UV) light source comprising:

43. 43. A UV light source as described in claim 42, further comprising an output coupling element for selectively out-coupling the UVC light from the nonlinear optical element as output light, optionally the UVC light being far UVC light.

44. the output coupling element selectively outcouples the UVC light into at least one direction different from a propagation direction of light at the first frequency to provide the output light; Optionally, the output light is substantially free of light at the first frequency.

44. The UV light source of claim 43.

45. 45. A UV light source according to any one of claims 42 to 44, wherein at least one of the light emitting element and the output coupling element comprises a III-nitride based material, optionally in a monolithic structure.

46. 46. ​​A UV light source according to any one of claims 42 to 45, wherein the nonlinear optical element is or comprises an optical cavity that is at least partially resonant at the first frequency.

47. 46. ​​A UV light source as claimed in any one of claims 42 to 45, wherein the nonlinear optical element comprises one or more nonlinear optical elements, each having an optical cavity for receiving light of the first frequency from the light emitting element, and optionally each of the optical cavities being ring-shaped.

48. 48. A UV light source as described in claim 47, further comprising an input coupling element that receives light of the first frequency from the light emitting element, the one or more nonlinear optical elements being disposed along the input coupling element.

49. 48. The UV light source of claim 47, wherein each of the plurality of nonlinear optical elements is of different dimensions and / or materials, and the output coupling element comprises a plurality of output coupling elements each for selectively outcoupling the UVC light from each of the plurality of nonlinear optical elements.

50. 47. The UV light source of claim 46, wherein the optical cavity has the light emitting element and the nonlinear optical element therein, the optical cavity having a linear or closed curve shape, and optionally the optical cavity at least partially resonates at the second frequency.

51. 51. A UV light source as described in any one of claims 43 to 50, wherein the nonlinear optical element and the output coupling element are integrated into an output element that outcouples the UVC light at multiple locations or continuously along its length.

52. a light emitting element that produces light of a first frequency; an optical cavity having a nonlinear optical element that receives light of the first frequency from the light emitting element and generates UVC light of a second frequency from the light of the first frequency; Equipped with the optical cavity is at least partially resonant at the first frequency; Ultraviolet (UV) light source.

53. 53. A UV light source as described in claim 52, further comprising an output coupling element for selectively out-coupling the UVC light from the nonlinear optical element as output light, optionally the UVC light being far UVC light.

54. the output coupling element selectively outcouples the UVC light into at least one direction different from a propagation direction of light at the first frequency to provide the output light; Optionally, the output light is substantially free of light at the first frequency.

54. The UV light source of claim 53.

55. 55. A UV light source according to any one of claims 52 to 54, wherein the nonlinear optical element comprises aluminium nitride (AlN) and optionally the light emitting element and / or the output coupling element comprise a III-nitride based material.

56. 56. A UV light source according to any one of claims 53 to 55, wherein the optical cavity is at least partially resonant at the first frequency and the second frequency.

57. 57. A UV light source as claimed in any one of claims 52 to 56, wherein the optical cavity comprises a plurality of optical cavities, each optical cavity having a nonlinear optical element to receive light at the first frequency from the light emitting element, optionally the optical cavity being ring-shaped.

58. 58. The UV light source of claim 57, further comprising an input coupling element for receiving light of the first frequency from the light emitting element, the plurality of optical cavities being disposed along the input coupling element.

59. 60. The UV light source of claim 58, wherein each of the plurality of optical cavities is at least one of different size and material, and the output coupling element comprises a plurality of output coupling elements each for selectively outcoupling the UVC light from each of the plurality of optical cavities.

60. 57. The UV light source of claim 56, wherein the optical cavity contains the light emitting element and the nonlinear optical element therein, the optical cavity having a linear shape or a closed curve shape.

61. 61. A UV light source as described in any one of claims 51 to 60, wherein the nonlinear optical element and the output coupling element are integrated into an output element that outcouples the UVC light at multiple locations or continuously along its length.

62. a light emitting element that produces light of a first frequency; a nonlinear optical output coupling element that receives light of the first frequency from the light emitting element, generates light of a second frequency from the light of the first frequency, and outcouples the light of the second frequency as output light at a plurality of locations or continuously along its length; A light source comprising:

63. 63. The light source of claim 62, wherein the light source provides the output light with substantially no phase matching between the light of the first frequency and the light of the second frequency.

64. the nonlinear optical output coupling element selectively outcouples light at the second frequency into at least one direction different from a propagation direction of light at the first frequency to provide the output light; Optionally, the output light is substantially free of light at the first frequency.

64. A UV light source according to claim 62 or 63.

65. 65. The light source of claim 63 or 64, wherein the nonlinear optical output coupling element comprises aluminum nitride (AlN), and optionally the light emitting element comprises a III-nitride based material.

66. A light source according to any one of claims 60 to 63, wherein the nonlinear optical output coupling element comprises or is connected to an optical cavity that is at least partially resonant at the first frequency.

67. A light source according to any one of claims 62 to 66, wherein the nonlinear optical output coupling element comprises a plurality of alternating nonlinear optical element sections and output coupling element sections along its length.

68. 67. A light source as claimed in any one of claims 62 to 66, wherein the nonlinear optical output coupling element has a first material and a second material that alter optical propagation at one of a first wavelength corresponding to the first frequency and a second wavelength corresponding to the second frequency, and that do not substantially alter optical propagation at the other of the first wavelength and the second wavelength.

69. 67. The light source of claim 66, wherein the optical cavity has therein the light emitting element and the nonlinear optical output coupling element, the optical cavity having a linear shape or a closed curve shape.

70. 70. The light source of any one of claims 62 to 69, wherein the first frequency corresponds to a first wavelength in a range of approximately 400 nanometers (nm) to 480 nm, and the second frequency corresponds to a second wavelength in a range of approximately 200 nm to 240 nm.

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