Metasurface-enhanced pyroelectric photodetector with sub-nanosecond electrical response

Abstract

A thermal photodetector is disclosed that converts incident electromagnetic radiation into an electrical signal with sub nanosecond response. The photodetector includes a pyroelectric layer, a continuous metallic layer in direct thermal and electrical communication with the pyroelectric layer, a dielectric spacer, and a metasurface of subwavelength metallic structures forming a nanogap cavity. Absorbed radiation is dissipated as heat primarily in the continuous metallic layer and transferred directly into the pyroelectric layer without an intervening thermal isolation structure. Device dimensions are selected such that an electrical RC time constant is less than 500 picoseconds, shifting the speed limitation from thermal diffusion to electrical response and enabling gigahertz bandwidth operation.

Description

METASURFACE-ENHANCED PYROELECTRIC PHOTODETECTOR WITH SUBNANOSECOND ELECTRICAL RESPONSESTATEMENT OF GOVERNMENT INTEREST

[0001] This invention was made with Government support under Federal Grant no. FA9550-21-1-0312 awarded by the Air Force Office of Scientific Research. The Federal Government has certain rights to this invention.CROSS-REFERENCE TO RELATED APPLICATION(S)

[0002] This application claims the benefit of US Provisional Application No. 63 / 749,926, filed on January 27, 2025, and which is incorporated herein for all purposes.TECHNICAL FIELD

[0003] The present disclosure relates to optical and infrared photodetection. More particularly, the disclosure relates to thermal photodetectors employing pyroelectric transduction and metasurface-based optical absorption architectures enabling electrical response at gigahertz frequencies.BACKGROUND

[0004] Thermal photodetectors convert incident electromagnetic radiation into heat and subsequently into an electrical signal. Pyroelectric detectors constitute one class of thermal detectors in which a temperature change induces a change in spontaneous polarization in a pyroelectric material, resulting in a measurable electrical current or voltage.[0005| Historically, conventional pyroelectric photodetectors have been designed under the assumption that thermal time constants dominate device response. As a result, such detectors commonly incorporate thermally isolating membranes, air gaps, or undercut structures intended to reduce heat loss and increase temperature rise. While these approaches improve responsivity for slowly varying signals, they impose fundamental limits on temporal response.

[0006] Separately, plasmonic metasurfaces have been developed to enhance optical absorption in subwavelength volumes. These structures enable strong electromagnetic confinement and efficient conversion of optical energy into heat within metallic elements. However, integration of such absorbers with pyroelectric transduction layers has generally followed conventional thermal-detector design paradigms, leaving speed constrained by thermal diffusion rather than electrical readout.

[0007] As optical communication systems continue to expand into higher data rates and broader wavelength bands, including the O band and C band, there is an unmet need for photodetectors capable of responding to intensity-modulated optical signals at gigahertz frequencies without reliance on semiconductor junctions, optical cavities, or external fdtering elements.

[0008] What is needed, therefore, are thermal photodetector architectures that accommodate the fact that, once thermal diffusion is no longer rate-limiting, response speed is governed by intentionally engineered electrical RC time constants, while preserving efficient optical absorption and direct electrical readout.BRIEF SUMMARY

[0009] Metasurface-enhanced pyroelectric photodetectors with sub-nanosecond electrical response are described.

[0010] One embodiment provides a thermal photodetector comprising a pyroelectric layer having a spontaneous polarization that varies in response to a temperature change, a continuous metallic layer disposed on the pyroelectric layer in both electrical and thermal communication with the pyroelectric layer, a dielectric spacer layer disposed on the continuous metallic layer, and a metasurface comprising a plurality of subwavelength metallic structures disposed on the dielectric spacer layer.

[0011] In this embodiment, planar dimensions of the pyroelectric layer and the continuous metallic layer define an electrical capacitance such that an electrical RC time constant of the photodetector is less than 500 picoseconds, whereby the electrical response bandwidth reaches at least the gigahertz regime.[0012| Another embodiment provides such a photodetector in which the continuous metallic layer functions as a primary thermal injection interface, dominating ohmic dissipation of electromagnetic energy coupled through the metasurface and transferring heat directly into the pyroelectric layer without an intervening thermally isolating structure.

[0013] A further embodiment provides such a photodetector configured for direct electrical readout without an external optical cavity or optical filter, enabling detection of intensity-modulated optical radiation in an optical communications band.[0014| Yet another embodiment provides a method of detecting optical radiation including coupling incident optical radiation into plasmonic modes using a metasurface, dissipating electromagnetic energy as heat in a continuous metallic layer, transferring the heat into a pyroelectric layer, generating an electrical signal from a change in spontaneous polarization,and reading out the electrical signal with an electrical RC time constant less than 500 picoseconds.[0015| Still another embodiment provides an optical communication system incorporating the thermal photodetector, wherein temporal modulation of an optical signal in an O band or C band is recovered electrically by the photodetector.

[0016] This Summary’ is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The features and advantages described herein are not exhaustive. Additional features and advantages will be apparent to those of ordinary skill in the art in view of the drawings, specification, and claims. The language used herein is selected for clarify and instructional purposes and does not limit the scope of the claimed subject matter.BRIEF DESCRIPTION OF THE DRAWINGS[0017| FIG. 1 is an illustration showing a layered thermal photodetector structure including a metasurface, a dielectric spacer, a continuous metallic layer, a pyroelectric layer, and a substrate in accordance with embodiments of the present disclosure.

[0018] FIG. 2 is an illustration showing optical absorption and thermal injection pathways within the photodetector of FIG. 1.

[0019] FIG. 3 is an illustration showing electrical readout and equivalent circuit representation of the photodetector, including capacitance and resistance defining an RC time constant.[0020| FIG. 4 is a flow chart illustrating a method of detecting intensity-modulated optical radiation using a thermal photodetector, in accordance with embodiments of the present disclosure.

[0021] FIG. 5 illustrates an optical communication system, in accordance with embodiments of the present disclosure, incorporating the thermal photodetector described herein, in which intensity modulated optical radiation is transmitted to the photodetector and converted into an electrical signal for data recovery.[00221 FIG. 6 depicts a test device in accordance with the present disclosure that was used for laboratory' verification of performance, together with a graph of bandwidth versus device diameter found during experimentation by the applicant.[0023| FIG. 7 is a schematic representation of metasurface-enabled photodetectors illustrating key dimensions (top), showing also a SEM image of the metasurface absorber (lower left), and finite element simulations of a single plasmonic nanostructure showing a cross section of thermal diffusion to the pyroelectric layer 30 ps after resonant excitation of the metasurface (lower right).

[0024] FIG. 8 illustrates a white light reflectance spectrum of a photodetector having an active area of 1.3 x 1 () ' mm2(40 pm diameter), together with photocurrent responsivity spectra measured under pulsed 100 nW optical excitation, including a comparison to a detector employing a gold film absorber instead of a metasurface, and further illustrates photocurrent measured for the illustrated photodetector under pulsed 790 nm excitation at the indicated power with a beam diameter maintained smaller than the device diameter.

[0025] FIG. 9 schematically illustrates distributed feedback laser operation in which two optical frequencies interfere to form beat nodes and shows the reduction in detector response as the beat frequency approaches the photodetector 3 dB bandwidth (top), further illustrating normalized integrated photocurrent responses versus excitation frequency for devices with different active area diameters used to extract bandwidth values (lower left), and comparing the extracted bandwidths to RC limits derived from measured capacitance and from an ideal parallel plate capacitor model as a function of active area (lower right).

[0026] FIG. 10 illustrates finite element simulation results showing a time derivative of temperature and an absolute temperature change at an Au AIN interface of a photodetector under resonant optical excitation.

[0027] FIG. 11 illustrates white light reflectance spectra for photodetectors having different active area dimensions.[0028| FIG. 12 illustrates measured detector performance metrics, including responsivity spectra under pulsed optical excitation, responsivity under resonant excitation, noise equivalent power derived from the responsivity spectra, and measured noise equivalent power under pulsed optical excitation for photodetectors having different active area dimensions.

[0029] FIG. 13 illustrates measured capacitance values for photodetectors having different active area dimensions as determined using an impedance analyzer.[0030| FIG. 14 illustrates a schematic representation of an optical and electrical measurement setup for characterizing photodetectors, including multiple optical excitation sources, reflected light analysis components, and electrical signal measurement instrumentation.DETAILED DESCRIPTION[0031 | Metasurface-enhanced pyroelectric photodetectors with sub-nanosecond electrical response are described.

[0032] Embodiments described herein relate to thermal photodetectors and systems incorporating such thermal photodetectors in which the dominant limitation on response speed is the electrical RC time constant of the device rather than opto-thermal or thermos-electric conversion. Metasurface enhanced optical absorption provided by a metasurface, together with rapid heat injection through a continuous metallic layer into a pyroelectric layer, enables optical-to-electrical conversion on timescales faster than thermal diffusion. Consequently, detector response speed is governed by electrical RC characteristics arising from the planar dimensions of the pyroelectric layer and the continuous metallic layer.

[0033] FIG. 1 is an illustration showing a layered thermal photodetector structure including a metasurface, a dielectric spacer, a continuous metallic layer, a pyroelectric layer, and a substrate in accordance with embodiments of the present disclosure. Referring to FIG. 1, one embodiment of a thermal photodetector 10 includes a pyroelectric layer 30 disposed on a supporting substrate 20. In embodiments, the supporting substrate 20 may be formed from silicon, sapphire, silicon carbide, glass, or other materials compatible with thin film deposition and microfabrication processes. In some embodiments, the supporting substrate 20 may be electrically conductive or may include an electrically conductive region or layer, and may serve as an electrical contact, for example as a second electrical contact 80 as depicted in FIG. 1, electrically coupled to a surface of the pyroelectric layer 30. In embodiments the supporting substrate 20 may comprise a semiconductor material.[0034 | The pyroelectric layer 30 may comprise aluminum nitride, although other pyroelectric materials may be employed in embodiments. The pyroelectric layer 30 exhibits a spontaneous polarization that varies as a function of temperature. In embodiments, the pyroelectric layer 30 has a thickness adequate to support the spontaneous polarization that is necessary for pyroelectric functionality.

[0035] The pyroelectric layer 30 may be formed through thin film deposition techniques including reactive sputtering, pulsed laser deposition, metal organic chemical vapor deposition, or atomic layer deposition, selected to produce a highly oriented crystalline film with a defined polar axis. In embodiments in which aluminum nitride is used, the deposition process may be controlled to promote c axis orientation substantially normal to the plane of the supporting substrate 20 in order to maximize the effective pyroelectric coefficient. Following deposition, or following deposition of the continuous metallic layer 40, described below, the pyroelectriclayer 30 may be paterned through conventional microfabrication techniques including photolithography and dry or wet etching, for example reactive ion etching or inductively coupled plasma etching, to define the lateral dimensions of the thermoelectric portion of the device. The patterning process may be selected to produce well defined edges and uniform thickness across the active area, thereby enabling predictable electrical capacitance and thermal response characteristics. In embodiments, the lateral dimensions of the pyroelectric layer 30 are selected in coordination with the thickness of the layer and the electrical properties of the associated electrodes to achieve a target electrical RC time constant, as described in further detail below. In embodiments, the pyroelectric layer 30 is formed with a thickness between approximately 20 nanometers and 300 nanometers.

[0036] A continuous metallic layer 40 is disposed directly on the pyroelectric layer 30. The continuous metallic layer 40 may serve simultaneously as an electrical electrode and as a thermal injection interface. In embodiments, the metallic layer 40 comprises gold and has a thickness selected to support efficient ohmic dissipation of plasmonically coupled electromagnetic energy. For example, the metallic layer 40 may have a thickness at least on the order of several optical skin depths at the operating wavelength, so that (i) incident electromagnetic energy coupled into the metal is predominantly dissipated within the metal volume (ohmic loss) rather than transmited through the film, while (ii) the film remains sufficiently thin to limit unnecessary thermal mass. The optical absorption coefficient a of a metal is related to the extinction coefficient k by a = 4rtk / X, so the corresponding 1 / e field penetration depth (optical skin depth) is 5 = 1 / a = X / (4a:k). Using tabulated near infrared optical constants for gold (Au), representative k values are approximately k ~ 8.65 at X = 1310 nm and k « 10.35 at = 1550 nm. These values correspond to optical skin depths of approximately:At 1310 nm: 5 ~ 1310 nm / (47i-8.65) ~ 12.1 nmAt 1550 nm: 5 ~ 1550 nm / (47i- 10.35) » 11.9 nm

[0037] Accordingly, in embodiments, the gold thickness may be selected to be at least about three skin depths (about 36 nm) at the operating wavelength, and more particularly about five to eight skin depths (about 60 nm to about 95 nm). In one nonlimiting example, the continuous gold layer 40 has a thickness in a range from about 30 nmto about 150 nm. for example about 50 nm, about 60 nm, about 80 nm, or about 100 nm, to support efficient ohmic dissipation of plasmonically coupled electromagnetic energy' while maintaining low electrical sheet resistance and a low thermal time penalty.

[0038] As used herein, the term '’continuous layer’" with respect to a metallic film refers to a metallic film that is substantially laterally continuous across the thermoelectric portion of thedevice, including across the active area of the pyroelectric layer 30. In this context, continuous means that the metallic layer 40 forms an unbroken, electrically and thermally conductive sheet over the pyroelectric layer 30, in contrast to the metasurface 60, which is intentionally discontinuous and comprises spatially separated subwavelength metallic structures.

[0039] The continuity of the metallic layer 40 is defined with respect to the thermoelectric function of the device, such that the metallic layer 40 provides an electrical electrode and a thermal injection interface into the pyroelectric layer 30 that are substantially laterally uniform. This definition does not require that the metallic layer 40 extend substantially beyond the lateral dimensions of the pyroelectric layer 30, except to the extent necessary to provide electrical connection to external circuitry via one or more electrical contact leads 75 (indicated schematically in FIG. 5).

[0040] Accordingly, the term continuous metallic layer 40 excludes patterned, segmented, or resonant metallic structures designed to support localized electromagnetic modes and instead denotes a fdm that is functionally continuous across the pyroelectric layer 30 for purposes of charge collection, electrical capacitance definition, and thermal energy’ transfer into the pyroelectric layer 30.

[0041] The continuous metallic layer 40 may be formed through physical vapor deposition techniques including electron beam evaporation or sputter deposition, or through chemical deposition techniques selected to produce a dense, low resistivity metallic film with good adhesion to the underlying pyroelectric layer 30. In embodiments in which gold is employed, an adhesion promoting interlayer having a thickness sufficiently small to avoid degrading thermal and electrical coupling may optionally be used, or the deposition process may be controlled to promote direct adhesion to the pyroelectric material.[00421 Following deposition, the continuous metallic layer 40 may be patterned through photolithography and metal etching processes, including wet chemical etching or dry etching, or through a lift off process in which the metallic layer is deposited over a patterned resist and subsequently defined by resist removal. The patterning of the continuous metallic layer 40 may define the lateral extent of the electrical electrode over the pyroelectric layer 30 and may further define one or more electrical contact leads 75 extending from the active region to external circuitry 100 (indicated schematically in FIG. 5), while maintaining lateral continuity of the metallic layer 40 across the thermoelectric portion of the device.

[0043] In embodiments, an opaque layer 43, depicted in FIG. 1, is provided to block transmission of incident or scattered photons toward the supporting substrate 20 and thereby reduce parasitic optical absorption and unwanted photoelectric effects. The opaque layer 43may comprise a highly absorbing and reflective metal, such as gold, selected to provide effective optical blocking at the operating wavelengths with minimal added thermal mass. In other embodiments, the opaque layer 43 may comprise other electrically conductive or semi conductive materials selected to provide optical opacity at one or more target wavelength ranges, including aluminum, chromium, titanium, platinum, tungsten, or multilayer metal stacks. In other embodiments, the opaque layer 43 may comprise nonmetallic materials commonly used in semiconductor device fabrication that are substantially opaque at near infrared wavelengths. Such materials may include doped or undoped semiconductor layers, amorphous or polycrystalline silicon, silicon rich silicon nitride, silicon carbide, germanium, chalcogenide materials, or other absorptive dielectric films. These materials may be selected to provide optical attenuation or absorption without requiring high electrical conductivity, and may be advantageous where electrical isolation, reduced parasitic capacitance, or compatibility with complementary metal oxide semiconductor processing is desired. The thickness and composition of such opaque layers may be selected to achieve sufficient optical blocking while limiting added thermal mass and preserving the temporal response of the photodetector. The opaque layer 43 may be positioned laterally adjacent to or surrounding the thermoelectric portion of the device, or may underlie selected regions, such that photons not absorbed by the metasurface are prevented from reaching the substrate.

[0044] In some embodiments, the opaque layer 43 is deposited and patterned simultaneously with deposition and patterning of the continuous metallic layer 40, for example using a common metallization step and lithographic definition, such that both layers are formed from the same metallic material and thickness. In other embodiments, the opaque layer 43 comprises a compound or multilayer structure and is deposited and formed in coordination with deposition and patterning of the pyroelectric layer 30 and the continuous metallic layer 40. for example as part of a stacked thin film process. In such embodiments, the opaque layer 43 may be engineered to provide enhanced optical attenuation, tailored thermal properties, or improved process integration, while remaining functionally distinct from the continuous metallic layer 40 that serves as the primary electrical electrode and thermal injection interface. In other embodiments, the opaque layer 43 may be formed from an electrically conductive material, such as gold, and may serve as part of the second electrical contact 80, thereby enabling electrical coupling between the supporting substrate 20 and a top surface of the thermal photodetector 10, as depicted in FIG. 1.

[0045] In embodiments, as depicted in FIG. 1, a dielectric spacer layer 45 is disposed on the continuous metallic layer 40. The dielectric spacer layer 45 electrically isolates ametasurface 50 from the continuous metallic layer 40 while enabling strong near field coupling. In embodiments, the dielectric spacer layer 45 has a thickness selected to be substantially smaller than the free space wavelength of incident electromagnetic radiation, such that evanescent near field components generated by the metasurface 50 extend across the dielectric spacer layer 45 and couple efficiently into the continuous metallic layer 40. By way of nonlimiting example, the dielectric spacer layer 45 may have a thickness in a range from about 5 nanometers to about 80 nanometers, for example about 10 nanometers, about 20 nanometers, about 30 nanometers, or about 50 nanometers, depending on the target optical coupling strength and electrical isolation requirements.

[0046] The dielectric spacer layer 45 may comprise one or more dielectric materials selected from aluminum oxide, silicon dioxide, silicon nitride, hafnium oxide, titanium dioxide, zirconium oxide, or combinations thereof, and may be deposited by atomic layer deposition, chemical vapor deposition, sputtering, or other thin film deposition techniques that provide uniform thickness control and low defect density. The dielectric spacer layer 45 may be configured to suppress direct electrical conduction between the metasurface 50 and the continuous metallic layer 40 while permitting strong capacitive coupling and electromagnetic field overlap.

[0047] As a result, the electromagnetic energy coupled into the metasurface 50 is converted into localized oscillating currents and associated ohmic losses within the metasurface 50 and the continuous metallic layer 40, with a substantial fraction of the resulting thermal energy transferred across the dielectric spacer layer 45 into the continuous metallic layer 40 via near field coupling and subsequent thermal diffusion, and from there injected into the pyroelectric layer 30. The dielectric spacer layer 45 therefore functions as an electrical isolation layer without acting as a thermal isolation barrier at the relevant time scales of operation.[0048J A metasurface can be in the form of an array of metallic nanostructures having lateral dimensions selected to define a resonant wavelength. As depicted in FIG. 1, a metasurface 50 comprising a plurality of subwavelength metallic structures 55 is disposed on the dielectric spacer layer 45. The subwavelength metallic structures 55 may comprise discrete metallic elements such as nanoantennas, nanorods, patches, or other plasmonic geometries, arranged in aperiodic or aperiodic array. Each subwavelength metallic structure 55 has lateral dimensions smaller than the free space wavelength of incident electromagnetic radiation, such that collective interaction with the incident field gives rise to localized surface plasmon resonances and strong electromagnetic field confinement in the near field region adjacent to the metasurface 50.

[0049] The geometry, lateral dimensions, thickness, and spacing of the subwavelength metallic structures 55 are selected to define one or more resonant absorption wavelengths. These parameters control the effective inductive and capacitive response of the metasurface 50, thereby setting the resonance condition and the spectral position of peak absorption. In embodiments, the resonant absorption arises from excitation of plasmonic modes supported jointly by the subwavelength metallic structures 55 and the continuous metallic layer 40 across the dielectric spacer layer 45, wherein electromagnetic energy is concentrated within the dielectric spacer layer 45 and at opposing metal interfaces, resulting in enhanced local field intensity and efficient conversion of electromagnetic energy' into ohmic loss within the subwavelength metallic structures 55 and the continuous metallic layer 40.

[0050] In embodiments, the metasurface 50 is engineered to achieve near unity absorption at the resonant wavelength through impedance matching to free space, destructive interference of reflected waves, or cooperative coupling between the metasurface 50 and the underlying continuous metallic layer 40. In embodiments, the resonant wavelength defined by the metasurface 50 lies in a near infrared spectral range between approximately 700 nanometers and 1600 nanometers, and in further embodiments lies within an optical communications band selected from an O band or a C band. In embodiments, the wavelength selectivity afforded by the resonance provided by metasurface 50, the continuous metallic layer 40, and the dielectric spacer layer 45 enables the photodetector 10 to read out an impinging optical signal of a specific wavelength directly, without an external optical filter or optical cavity. The dielectric spacer layer 45 plays a role in setting the coupling strength and phase relationship between the metasurface 50 and the continuous metallic layer 40, thereby influencing both the absorption efficiency and the spatial distribution of dissipated power.[00511 In embodiments, the resonant wavelength produced by the geometry, lateral dimensions, thickness, and spacing of the subwavelength metallic structures 55 of the metasurface 50 lies within a near infrared spectral range, including wavelength ranges used for optical communications. In such embodiments, the metasurface 50 may be designed to selectively absorb radiation at one or more target wavelengths while exhibiting reduced absorption at other wavelengths, thereby providing intrinsic spectral selectivity. The metasurface 50 may further be configured to support narrowband or broadband absorption profiles depending on the geometry' and arrangement of the subwavelength metallic structures 55. The resulting absorbed or coupled electromagnetic energy is converted into lattice heat within the metallic elements on picosecond time scales, substantially shorter than the thermalresponse time of the pyroelectric layer 30, enabling high speed thermal excitation of the pyroelectric material.[0052| FIG. 2 is an illustration showing optical absorption and thermal injection pathways within the photodetector of FIG. 1. As depicted in FIG.2, upon illumination by incident optical radiation 60, the incident optical radiation 60 is coupled into plasmonic modes supported by the metasurface 50. Electromagnetic energy associated with these plasmonic modes is dissipated as heat 65 within a region of resistive heating 62 that is predominantly within the continuous metallic layer 40. Because the continuous metallic layer 40 is in direct thermal contact with the pyroelectric layer 30, the heat 65 is injected efficiently into the pyroelectric layer 30. Note that although FIG. 2 schematically depicts heat 65 as flowing in a single direction toward the pyroelectric layer 30, in practice the generated heat propagates from the absorption region in multiple directions in accordance with isotropic thermal diffusion. However, due to the intimate thermal contact between the continuous metallic layer 40 and the pyroelectric layer 30, and the relatively low thermal conductivity of the surrounding gaps (not numbered) above and lateral to the continuous metallic layer 40. a substantial fraction of the generated heat 65 is preferentially conducted into the pyroelectric layer 30.[0053J The resulting temperature change in the pyroelectric layer 30 induces a change in spontaneous polarization within the pyroelectric layer 30, thereby generating an electrical signal between a first electrical contact 70 and a second electrical contact 80 that are coupled to opposite sides of the pyroelectric layer 30, as depicted in FIG. 1. In embodiments, the first electrical contact 70 and the second electrical contact 80 may be formed from electrically conductive materials including metals or conductive compounds selected to provide low electrical resistance and stable interfaces with the pyroelectric material. In specific embodiments, the continuous metallic layer 40 may serve as the first electrical contact 70, and the supporting substrate 20, or a conductive layer associated with the supporting substrate 20, may serve as the second electrical contact 80, while in embodiments the opaque layer 43 may serve as an electrical contact to the supporting substrate 20.

[0054] FIG. 3 is an illustration showing electrical readout and equivalent circuit representation of the photodetector, including capacitance and resistance defining an RC time constant. FIG. 3 illustrates an equivalent electrical representation 90 of the photodetector 10, showing a pyroelectric capacitance 92 and a series resistance 94. Readout circuitry 100 may be impedance matched to the photodetector 10 to avoid introducing dominant external RC limitations. In embodiments the readout circuitry 100 comprises an impedance matched transmission line or waveguide structure coupled to the continuous metallic layer, where insuch embodiments one more of the leads 75 is considered to be a component within the readout circuitry 100.[00551 In embodiments, such as depicted in FIG. 1, the photodetector 10 is configured such that planar dimensions of the pyroelectric layer 30 and the continuous metallic layer 40 define an electrical capacitance between the continuous metallic layer 40 and the supporting substrate 20, across the pyroelectric layer 30 acting as the capacitive dielectric. In such embodiments, this geometrically defined capacitance is the primary contributor to the total pyroelectric capacitance 92 shown in the equivalent electrical representation of FIG. 3.[0056| Assuming aluminum nitride is employed as the pyroelectric layer 30, the total capacitance of the photodetector includes a pyroelectric capacitance 92 associated with the pyroelectric layer 30, as well as parasitic capacitances associated with the first and second electrical contacts 70 and 80 and any electrical lead traces (not shown) connected thereto. For devices having relatively large lateral dimensions, the parasitic capacitances of the electrical contacts 70 and 80 and associated interconnects may be small relative to the pyroelectric capacitance 92 and may be neglected to first order. Under such conditions, the capacitance C that is the pyroelectric capacitance 92 may be approximated by the parallel plate relation C = (F0£rA) / t,where is the permittivity' of free space, Eris the relative permittivity7of aluminum nitride, A is the lateral area of overlap between the continuous metallic layer 40 and the pyroelectric layer 30, and t is the thickness of the pyroelectric layer 30. For aluminum nitride, eris approximately 8 to 10 in thin film form.

[0057] This structure further exhibits an associated electrical resistance defined by current flow through the first electrical contact 70 and the second electrical contact 80 to readout circuitry 100, including series resistances associated with metallization, contact interfaces, and interconnects. In embodiments where the second electrical contact 80 comprises an opaque conductive layer in electrical communication with the supporting substrate 20, additional contact resistance may arise at the interface between the opaque layer and the substrate 20. In embodiments wherein the continuous metallic layer 40 itself serves as the first electrical contact 70, contact resistance at the upper electrode interface is substantially eliminated.

[0058] An electrical RC time constant r may therefore be approximated asT = R x c,where R represents the total series resistance between the first and second electrical contacts 70 and 80 and C represents an effective capacitance of the photodetector. In embodiments, theeffective capacitance includes a pyroelectric capacitance 92 determined by the geometry' of the continuous metallic layer 40 and the pyroelectric layer 30, and may further include additional capacitance contributions associated with the first and second electrical contacts 70 and 80 and any electrical interconnections coupled thereto. For purposes of analysis, the pyroelectric capacitance 92 may provide a primary contribution to the effective capacitance, while in other embodiments the effective capacitance reflects a combined contribution of the pyroelectric capacitance 92 and one or more parasitic capacitances.

[0059] In embodiments, the lateral dimensions of the continuous metallic layer 40 within the thermally active region, which dominate the capacitance, are reduced through lithographic patterning of both the continuous metallic layer 40 and the pyroelectric layer 30 using high resolution photolithography or electron beam lithography, followed by anisotropic etching or lift off. In particular, the pyroelectric layer 30 may be patterned using inductively coupled plasma etching with chlorine based or fluorine based chemistries selected for aluminum nitride, including Ch, BCh, or combinations thereof, to achieve vertical sidewalls and lateral feature sizes on the order of 200 nanometers or less. In embodiments employing electron beam lithography, lateral dimensions below 100 nanometers may be achieved, enabling aggressive reduction of the effective capacitor area.

[0060] The continuous metallic layer 40 may be patterned either by lift off or by dry or wet etching, for example using ion milling or iodine-based or potassium iodide-based gold etchants, with minimum line widths limited primarily by lithographic resolution and film thickness. In embodiments, the patterned metallic features defining the thermally active area have lateral dimensions constrained to a few micrometers or less, while electrical lead portions extending away from the active region are formed with substantially larger widths to reduce series resistance.

[0061] Conversely, the total resistance through the first and second electrical contacts 70 and 80 is minimized through the use of low resistivity metals such as gold, short current paths between the thermally active region and external circuitry, increased cross sectional area of contact traces outside the active region, and direct electrical coupling to impedance matched readout circuitry, such that the electrical RC time constant is dominated by the intentionally engineered pyroelectric capacitance rather than by contact or interconnect resistance.[00621 By way of example, limiting the thermally active area of the continuous metallic layer 40 to less than approximately 10 square micrometers, when combined with an aluminum nitride pyroelectric layer 30 having a thickness of at least approximately 50 nanometers, yields a pyroelectric capacitance 92 on the order of 10 femtofarads. When combined with a total seriesresistance on the order of hundreds of ohms, such a structure defines an RC time constant less than 500 picoseconds. In embodiments, the RC time constant is less than 200 picoseconds. As a result, the electrical response bandwidth of the photodetector 10 reaches or exceeds one gigahertz or exceeds two gigahertz.

[0063] Unlike conventional pyroelectric detectors, embodiments described herein do not rely on thermally isolating membranes, air gaps, undercut structures, or layers inserted for the purpose of intentionally increasing thermal isolation. Instead, thermal conduction between the continuous metallic layer 40, the pyroelectric layer 30, and the supporting substrate 20 is intentionally efficient, and speed is governed by electrical design.

[0064] FIG. 4 is a flow chart illustrating a method of detecting intensity-modulated optical radiation using a thermal photodetector, in accordance with embodiments of the present disclosure. Referring to FIG. 4, in embodiments, incident optical radiation 60 having O-band or C-band intensity modulation is coupled 110 into plasmonic modes, dissipated 120 as heat in the continuous metallic layer 40, which is transferred into the pyroelectric layer 30, where it is converted 130 into an electrical signal via pyroelectric transduction in the pyroelectric layer 30, and read out 140 with a sub-nanosecond RC time constant, where, in embodiments, this sub-nanosecond RC time constant is less than 500 picoseconds.

[0065] FIG. 5 illustrates an optical communication system, in accordance with embodiments of the present disclosure, incorporating the thermal photodetector described herein, in which intensity modulated optical radiation is transmitted to the photodetector and converted into an electrical signal for data recovery. Referring to FIG.5, embodiments include an optical communication system 200 comprising an optical source 210 configured to emit optical radiation within an optical communications band. The optical radiation is intensity modulated by an intensity modulator 220 according to a data signal, such that information is encoded in a temporal modulation pattern of the optical intensity.

[0066] The optical communication system 200 further comprises a transmission path 230 configured to convey the intensity modulated optical radiation from the optical source 210 toward a receiver location. In embodiments, the transmission path 230 may comprise free space propagation, an optical waveguide, or an optical fiber 240. In specific embodiments, the transmission path 240 comprises a single mode or multimode optical fiber 240 configured to support transmission within standard optical communications wavelength ranges.

[0067] The thermal photodetector 10 is positioned to receive incident intensity modulated optical radiation 250 conveyed by the transmission path 230. Upon receiving the incident optical radiation 250, the thermal photodetector 10 converts intensity variations of the opticalradiation 250into corresponding temperature variations within the pyroelectric layer 30, which are in turn converted into an electrical signal 260 as described above.[0068| Readout circuitry 100 is electrically coupled to the thermal photodetector 10 through the contact leads 75 from the first and second electrical contacts 70 and 80, and is configured to receive the electrical signal generated 260 by the thermal photodetector 10. The readout circuitry 100 may comprise amplifiers, impedance matching networks, filters, and signal processing circuitry configured to recover the temporal modulation pattern of the intensity modulated optical radiation 250 from the electrical signal 260. In some cases, readout circuitry 100 is fabricated on the substrate to which the thermal photodetector is deposited or otherwise formed.[0069| In embodiments, the optical communications band of the incident optical radiation 250 is selected from an O band or a C band. In such embodiments, the metasurface 50 and associated structures of the thermal photodetector 10 are configured to efficiently couple and convert optical radiation within the selected band.[0070| In embodiments, the readout circuitry 100 has an electrical bandwidth exceeding an inverse of the electrical RC time constant of the thermal photodetector 10. As a result, the readout circuitry 100 does not impose a dominant bandwidth limitation, and the recovered electrical signal faithfully represents the temporal modulation of the incident optical radiation 250. In such embodiments, the overall system bandwidth is governed by the intentionally engineered RC response of the thermal photodetector 10 rather than by external readout limitations.

[0071] In alternative embodiments, the structures of thermal photodetectors 10 described herein are not limited to single, standalone devices and may be fabricated as part of a two-dimensional or one-dimensional array on a common substrate. In such array implementations, the present disclosure enables the arrays of individual photodetectors 10 to be laterally scaled to smaller form factors while preserving the described optothermal and electrical operating principles. Array-based configurations enable spatially resolved detection, parallel signal acquisition, and increased aggregate throughput, and may be particularly advantageous for imaging, multiplexed sensing, or wavelength-selective detection applications. Reducing the lateral dimensions of individual photodetectors 10 further decreases device capacitance, which can improve electrical bandwidth, reduce RC time constants, and enable higher-speed operation. Smaller form factors also support higher device density, reduced material usage, and improved integration with on-chip readout electronics, thereby facilitating compact, scalable, and manufacturable photodetector systems. In further alternative embodiments, themetasurfaces 50 of individual photodetectors 10 within an array may be intentionally designed with different geometries, dimensions, or material parameters so as to be tuned to different resonant wavelengths. Such wavelength-selective tuning enables multispectral or hyperspectral detection within a single integrated array, supports wavelength-division multiplexing, facilitates simultaneous detection of multiple optical channels or spectral bands, and allows system-level functionality such as spectral discrimination, parallel sensing, and enhanced information extraction without the need for external filters or moving optical components.

[0072] In additional alternative embodiments, the thermal photodetector 10 may be operated without connection to external readout circuitry 100 in the conventional sense. In some implementations, the electrical response generated by the thermal photodetector 10 is directly coupled to downstream functional circuitry that processes, transforms, or reuses the signal without producing an intermediate readout, display, or stored measurement. By way of example, the photocurrent or voltage generated by the thermal photodetector 10 may be amplified and used to drive a secondary device, such as a modulation element, actuator, oscillator, or wavelength conversion component, such that the photodetector serves as an opto-thermal-to-electrical transduction stage within a larger functional system rather than as a sensing endpoint. In other embodiments, raw electrical signals generated at two or more photodetectors 10, or at different operating frequencies of a single photodetector, may be directly compared, mixed, gated, or thresholded by analog or digital circuitry to perform functions such as spectral discrimination, logical decision-making, feedback control, or trigger generation, without ever producing an externally readable output. In such configurations, the thermal photodetector 10 operates as an embedded functional element whose output is consumed internally by the system, enabling reduced system complexity', lower latency, and more compact implementations.

[0073] The embodiments described herein and the accompanying illustrated examples are provided solely for purposes of illustration and explanation, and are not intended to be exhaustive or to limit the scope of the claimed subject matter to the specific forms disclosed. Variations, modifications, and alternative implementations will be apparent to those of ordinary skill in the art in view of this disclosure and are considered to fall within the scope of the disclosure as defined by the appended claims. Unless expressly stated otherw ise, the drawings are not drawn to scale, and relative dimensions, proportions, and thicknesses of illustrated elements may be exaggerated or simplified for clarity of presentation.

[0074] The following Example is provided by way of illustration and not by way of limitation.[0075| EXAMPLE[0076| Thermal photodetectors, including thermoelectric and pyroelectric types, are critical for detection of long-wave infrared light but are limited by slow response times due to thermal diffusion. If thermal photodetectors with high speeds and sensitivities could be realized, they w ould allow for flexible sensing at any wavelength, as all selectivity of the absorber would be reproduced in the detectivity. Disclosed here is a room-temperature, pyroelectric photodetector which couples a thermally-sensitive aluminum nitride layer with a metallic metasurface that acts as an efficient, spectrally-selective photothermal converter. These pyroelectric detectors exhibit record-breaking speeds, with 3dB bandwidths up to 2.8 GHz, which corresponds to a rise time of 125 ps (see, e.g., Figure 9). These ultrafast speeds are attained while maintaining competitive responsivities and noise equivalent powers as small as 96 pW / vHz. By varving the active area of devices, it was determined that even the smallest attainable devices are limited by their resistance-capacitance time constants, and finite element simulations suggest that thermal response times as fast as 30 ps might be realized. These metasurface-enabled pyroelectric photodetectors approach the speeds of commercial semiconductor photodiodes, contrary to the common conception of thermal detectors as slow, and present opportunities for advanced sensing such as spectrally broad multispectral imaging or polarimetry.

[0077] FIG. 6 depicts a test device in accordance with the present disclosure that was used for laboratory verification of performance, together with a graph of bandwidth versus device diameter found during experimentation by the applicant.

[0078] Thermal photodetectors, which operate through the conversion of absorbed light to heat and the subsequent conversion of this heat to an electrical response, offer wide spectral sensitivity and high damage thresholds. Because thermal detectors can utilize distinct materials for light absorption versus signal generation, they are frequently employed as broadband detectors in the far infrared and microwave regions of the electromagnetic spectrum, where alternative photodetector options are scarce. Applications of thermal photodetectors outside of the far infrared has been limited, however, due to low sensitivities and long response times, often due to thermal loss to the environment and slow thermal diffusion, respectively. State-of-the-art thermal photodetectors such as photonic cavity-coupled graphene bolometers have achieved ultrafast speeds, with response times as short as 35 ps, but these bolometers offer a trade-off between speed and sensitivity and frequently require cryogenic temperatures for high-sensitivity detection.

[0079] Alternatively, pyroelectric detectors are a prominent class of thermal photodetector which generate electrical signal via heat-induced spontaneous polarization changes within athermally responsive material. Pyroelectric photodetectors typically operate on millisecond time scales, orders of magnitude slower than the nano- to pico-second response times of commercial semiconductor photodiodes. Though a few examples of conventional pyroelectric photodetectors have demonstrated picosecond response times, this fast operation required unusually powerful laser excitation of > 50 kW, unsuitable for practical applications. Recent developments, however, suggest that replacing traditional thick blackened absorbers with nanophotonic absorbers can simultaneously increase sensitivity by imparting spectral selectivity and increase speed by minimizing thermal mass. Metallic antennas, metallic and dielectric gratings, and nanoscale hole arrays have been employed to improve the speed and sensitivity of thermal detectors generally, while arrays of film-coupled metallic nanoparticles have allowed for pyroelectric detectors which achieve microsecond-scale response times. Recently, it was shown that pyroelectric photodetectors paired with a nanogap cavity metasurface absorber exhibited RC limited response times as small as 700 ps, which corresponds to a 500 MHz bandwidth assuming the device operates as a low pass filter. The metasurface absorber consisted of colloidal silver nanocubes separated from a flat gold film by a thin (< 10 nm) polymer layer which acted as a photothermal converter. The small thermal mass of the metasurface absorber led to fast thermal response times, but the ultimate speeds which might be achieved for devices with smaller RC time constants measured with faster readout equipment remained unknown.

[0080] Demonstrated in the present example is a pyroelectric photodetector integrated with a nanogap cavity metasurface sensitive to near infrared light. This device exhibited a 2.8 GHz 3dB bandwidth, corresponding to a 125 ps rise time, the fastest response time of a pyroelectric photodetector reported to date. In addition, the photodetectors exhibited competitive responsivities and noise equivalent powers, were ultrathin, required no external power or bias, operated at room temperature, and could be readily integrated into on-chip applications. Device bandwidths increased systematically as active areas decreased, and electrical characterization revealed that even the smallest devices were limited by the RC time constants. Devices were created that have response times competitive with those of commercial semiconductor photodiodes, disproving the frequent supposition that thermal photodetectors must be slow. In addition, this demonstrated that the use of precision metasurfaces offers the opportunity to straightforwardly build advanced sensing functionalities such as multispectral imaging and polarimetry.

[0081] Results and Discussion

[0082] Photodetector Design and Fabrication[0083| Metasurface-enabled pyroelectric photodetectors were fabricated by layering a nanogap cavity metasurface structure on top of a pyroelectric thin fdm (FIG.6). In a particular embodiment fabricated for testing purposes, the metallic metasurface consists of an array of nanoscale silver square prisms (90 nm x 90 nm x 35 nm) separated from a gold fdm by a thin dielectric layer, here 10 nm AI2O3, referred to as a ‘gap’ between the metals. This structure acted as an antenna to selectively absorb resonant light nearly completely while reflecting other wavelengths. The inherent loss of the plasmonic structures then caused this absorbed energy to be dissipated as heat into the gap on picosecond time scales, which allowed the metasurface to simultaneously act as a spectral fdter and a photothermal converter. The heat generated by the metasurface upon light absorption was then transferred through the gold layer to an underlying pyroelectric thin fdm, here -180 nm AIN. The AIN layer sandwiched between the gold fdm of the metasurface and a heavily doped silicon wafer patterned with electrical contacts formed a capacitor. At room temperature, this AIN layer was spontaneously polarized, which defined the starting capacitance of the structure.[00841 Upon heating, the macroscopic dipole moment of the AIN decreased, and this change in polanzation resulted in a change in electrical charge at the AIN surface. This generated a current according to[0086| where A is the active area of the device (defined as the illuminated, metasurface-coated area), / ? is the pyroelectric coefficient, and dT / dt is the change in temperature over time.[00871 Finite element simulations illustrated the operative mechanism of the device and suggested rapid timescales for photothermal conversion and heat dissipation. Absorbed light is converted to heat via oscillatory losses within 5 ps, and the heat reaches the AIN layer within 30 ps (FIG. 7). Heat generation is localized in both the plasmonic mirror and nanostructure according to the relative lossiness of each metal. Heat dissipation through the gold to the AIN is then rapid, occurring within 30 ps with dT / dt maximized a mere 4 ps after excitation. This can be seen in FIG. 10, which shows finite element simulations of the time derivative of temperature and absolute temperature change at the Au / AIN interface upon resonant light excitation. The speed of this thermal transfer paired with the small thermal mass of the devices suggests that careful electrical design might result in ultrafast photodetection.

[0088] Photodetector Performance

[0089] The near-perfect, spectrally selective absorption of the metasurface, which initiates the photodetector response, is illustrated by white light reflectivity spectra (FIG. 8 and FIG.11). Here, the gold mirror alone efficiently reflects near-infrared light, while the metasurface exhibits a stark relative decrease (> 95%) in reflectivity centered at 790 nm. The resonance wavelength is determined by the size of the Ag nanostructures and the thickness of the AI2O3 dielectric layer, which allows the possibility of photodetectors which are spectrally selective across the visible and infrared portions of the spectrum.|0090] FIG. 8 shows the white light reflectance spectrum of a detector with a 1.3 x 103mm2active area (40 pm diameter) along with the photocurrent responsivity spectra of the detector that is illustrated, measured upon pulsed 100 nW light excitation, as compared to that of a detector in which a gold film rather than a metasurface layer acts as an absorber, and further shows a photocurrent measured for the illustrated device upon pulsed 790 nm excitation at the indicated power, with the beam size maintained to consistently have a diameter 5 pm smaller than that of the device.|0091] Photodetector responsivities were determined via a lock-in amplifier which measures the current produced by the device in response to tunable, pulsed laser excitation (100 nW) with the beam size carefully controlled to be ~5 pm smaller than the device diameter, as described in the Methods section (FIG. 8). The photocurrent responsivity closely followed the reflectance spectrum, with a maximum peak responsivity of 1.64 mA / W at 790 nm for a device with a 1.3 x |()3mm2active area (40 pm diameter). This was nearly two orders of magnitude larger than the photocurrent responsivity of gold films with no metasurface under the same excitation conditions. Noise spectral densities of the detectors were determined with the same lock-in amplifier, as described in the Methods section, and the noise equivalent power (NEP) was taken as the ratio of the photocurrent responsivity to the noise spectral density (FIG.12). For devices with a 1.3 x |()3mm2active area, photocurrent responsivities upon 790 nm excitation correspond to a NEP as small as 96 p W / Hz. This demonstrated the effectiveness of the metasurface as both a spectral filter and as a photothermal converter.(0092] The operative photodetector speeds are here quantified as the 3dB bandwidth (VBW), which corresponds to the rise time (irise) of the device assuming operation as an ideal low pass filter, as VBW = 0.35 / Trise. Values of VB were measured by probing the photodetectors in the frequency domain as described in Methods. Detectors were resonantly excited (783 nm) with a sinusoidally modulated laser generated by a pair of distributed feedback lasers, where the modulation frequency corresponds to the beat mode caused by the interference of two beams with offset frequencies. The frequency of the beat mode was swept from 45 MHz to 4 GHz by tuning the frequency offset of the two beams, and the photocurrents of the devices were measured as a function of this excitation frequency with a spectrum analyzer (FIG. 9). Thephotocurrent pulse centered at each excitation frequency was integrated to produce a curve of photocurrent power as a function of excitation frequency, which was fit as an exponential decay to determine VBW, the frequency at which the photocurrent response is attenuated by half. The VBW determined in this manner for a device with a 1.3 x I O ' mm2active area is 2.0 ± 0.7 GHz, corresponded to ruse = 175 ps.|0093] FIG. 9 is a schematic representation of the function of distributed feedback lasers where two beams with different frequencies interfere to form beat nodes, where detector response decreased as the frequency of the beat node increased to approach the photodetector 3 dB bandwidth (top). FIG. 9 also shows normalized integrated photocurrent responses measured as a function of excitation frequencies for devices with the indicated active area diameters where exponential rise and decay fits determine VBW (lower left), and also shows measured values of VBW extracted from the normalized integrated photocurrent responses (circles) compared to the RC limit derived from measured capacitance values (crosses) and the RC limit predicted for an ideal parallel plate capacitor (dashed line) with the indicated active area (lower right).[0094| Photodetectors with active areas ranging from 2.9 x 103mm2to 0.3 x 103mm2(60 pm to 20 pm diameters) were fabricated, and both the photocurrent responsivity spectra and bandwidths were measured. For small devices, optically opaque gold electrical contacts prevented competitive light absorption by the underlying silicon. For all devices, white light reflectivity spectra demonstrated resonances of ~800 nm, where a small blue shift (~40 nm) was observed between the largest and smallest detectors (FIG. 11). In addition, the percentage of light absorbed at the resonant wavelength decreased for detectors with active areas < 1 x 103mm2such that the smallest detectors measured absorbed ~60% of resonant light where the largest absorbed 98% of resonant light. For all devices, photocunent responsivity spectra closely followed the reflectivity (FIG. 8 and FIG. 12 (upper left)). As predicted by Equation 1 , average photocurrent responsivities under resonant excitation were maximized for the largest measured devices and decreased systematically with smaller active areas, albeit with device-to-device deviations that made the trend non-monotonic (FIG. 12 (lower left)). Peak responsivities of 1.23 ± 0.16 mA / W were observed for detectors with 2.9 x |()3mm2active areas and 0.31 ± 0.22 mA / W for 0.3 x 10 ' mm2active areas, which corresponds to average NEP values of 2 nW / ^Hz and 7.2 nW / ^Hz , respectively (FIG. 12 (upper and lower right)). Contrary to responsivities, values of VBW. and thus device speeds, increased for smaller photodetectors, as demonstrated by curves of photocurrent as a function of excitation frequency(FIG. 9 (lower left)). For devices with 0.3 x 103mm2active area, VBW = 2.80 ± 0.08 GHz and thus Trise = 125 ± 4 pS.[0095| Speed Limitations

[0096] Devices were electrically characterized using an impedance analyzer (Keysight KT-E4990A) by measuring their impedance as a function of voltage modulation frequency to determine device capacitance (Methods). These capacitance values were used to quantify the RC time constants of each device. Capacitance values increased systematically with detector active area from 1.1 to 2.4 pF (FIG. 13), which predicts RC limited bandwidths of to 2.8 ± 0.4 GHz for the smallest devices. Values of the RC-limited bandwidths align closely with the measured VBW values, which suggests that even the smallest detectors are RC-limited.

[0097] The 2.8 GHz 3dB bandwidth shown here represents the fastest pyroelectric photodetector reported to date. These ultrafast speeds are attributable to the low thermal mass and small capacitances of the detectors. In a typical case, pyroelectric detectors, and in fact thermal detectors generally, rely on thick blackened absorbers which exhibit spectrally flat responses in the visible and infrared portions of the spectrum. Though this allows for a broad spectral response, the increased volume results in a large thermal capacitance. This slows the thermal response of the device, in many cases to the point that the thermal response is rate limiting, resulting in the millisecond response times frequently reported for pyroelectric photodetectors. For the metasurface-enabled detectors reported herein, absorption is accomplished by nanoscale plasmonic structures, which both lend spectral selectivity and contribute near negligible thermal mass. This decreases the thermal response time of the devices and imbues spectral selectivity which results in increased device sensitivity and allows for thinner pyroelectric layers. In total, this results in a device with remarkably small thermal masses and thermal response times significantly shorter than the electrical response times, suggested by finite element simulations to be as short as 30 ps.

[0098] Further, high speeds in the devices reported here are in part due to small device capacitances, which are minimized by decreasing the active area. In this way, pyroelectric detectors present a tradeoff between rise time and responsivity, as predicted by the area dependence of the photocurrent and the RC time constant. Low sensitivity of previous pyroelectric detectors has hindered photodetection using small, low capacitance devices. The photodetectors reported here, however, exhibit measurable responsivities for active areas as small as 0.3 x | ()3mm2(20 pm diameter). While responsivities do follow detector size, even the smallest detectors herein exhibit NEP values < 10 nW / ^Hz, which is competitive with much larger detectors in previous generations of analogous devices. In testing by the applicant, theminimum achievable detector size, and thus the smallest achievable capacitance, was limited by fabrication considerations rather than by responsivity’ or NEP.[0099| Despite the achieved small capacitances, however, it should be noted that for photodetectors with active area diameters smaller than ~1 x ] O3mm2, the measured RC time constants deviated from those predicted for an ideal parallel plate capacitor of equivalent dimensions (FIG. 9 (lower right)). Predicted RC time constants for ideal devices would allow for 3dB bandwidths up to 3.7 GHz, which would correspond to a rise time of 95 ps. This suggests that deviations from ideal capacitance limits the attainable bandwidths of these metasurface-enabled pyroelectric photodetectors, which likely reflects fabrication defects for the smallest devices. Improved fabrication techniques might present the opportunity to further decrease the electrical response times of these photodetectors and thus increase photodetection speeds further still. Progressively decreased electrical response times within small devices may allow for devices entering the thermally limited regime, where simulations suggest response times could be as small as 30 ps, which would surpass the carrier-limited response times of commercial semiconductor photodiodes. Further, this would enable studies of the fundamental kinetic limits of both thermal conversion within plasmonic nanostructures and depolarization of pyroelectric materials.|0100] Conclusions

[0101] Metasurface-enabled pyroelectric photodetectors fabricated by the applicant achieved record-breaking 3dB device bandwidths of 2.8 GHz, corresponding to a rise time of a mere 125 ps. By coupling a nanoscale plasmonic metasurface absorber with a thin pyroelectric AIN film, thermal mass is kept small, and thermal response time is simulated to be only 30 ps. As such, photodetector speeds are found to be limited by the electrical response time, as measured bandwidths closely follow the RC time constants of the devices. These record-breaking speeds are augmented by spectral selectivity conferred by the metasurface absorber / photothermal converter, which can be straightforwardly altered to operate at alternative wavelengths or to add in sensitivity to additional light properties such as polarization state. These photodetectors are ultra-thin, with active layers only 305 pm thick, and small, with active areas as small as 0.3 x I ()3mm2, and require no external power or bias to operate. In total, these properties make these devices promising candidates for on-chip photodetection and imaging applications.

[0102] Methods

[0103] Pyroelectric Photodetector Fabrication: Photodetectors were fabricated by coupling an electrically active pyroelectric layer with a metallic metasurface acting as a photothermalconverter. The pyroelectric layer consisted of -150 nm aluminum nitride (AIN) deposited onto highly doped p-type silicon (0-0-1) using vacuum RF sputtering. The silicon wafer was stripped of oxide through submersion in 0.1 M hydrofluoric acid for 1 minute prior to deposition.

[0104] Sputtering was performed with a 5N Al target in an Ar / N2 atmosphere with the substrate heated to 1000 °C. This process has previously been shown to produce poly crystalline AIN with preferential c-axis orientation, which was verified by x-ray diffraction (Anton Paar XRDynamic 500), and ~ 1 nm surface roughness. The thickness of AIN was determined through imaging ellipsometry (Accurion EP3).

[0105] Devices were patterned from the AIN thin films through a series of metal deposition and lithography steps. First, 5 nm titanium (acting as an adhesion layer) and 75 nm gold were deposited using electron beam evaporation (CHA Industries). Here the Au layer served as both a mirror within the nano-gap cavity metasurface and an electrical contact for the device. The Au surface was then spin coated with a positive photoresist (Microposit Shipley 1813), which was patterned into arrays of devices by covering the device shapes with an opaque photomask and irradiating the exposed polymer with UV light. The sample was developed (Microposit MF 319) to remove the UV-exposed polymer to create a chemical-resistant mask. The exposed gold w as wet etched into the device shape via potassium iodide and iodine chemistry (GE-8110, Transene Company, Inc.), and the remaining resist was removed using Microposit Remover 1165. With the remaining Au acting as a mask, the exposed AIN was etched away in a room temperature aqueous solution of 25% tetramethylammonium hydroxide alternated with submersion in buffered oxide etch solution. Final electrical contacts were then deposited via electron beam evaporation following patterning to exclude evaporation onto the AIN / Au pillars. Patterning was accomplished with NFR 016D2, which w as exposed to UV light and developed in MF 319. A 160 nm layer of Au with a 5 nm Ti adhesion layer was evaporated, and the remaining photoresist was lifted off using Microposit Remover 1165.

[0106] Metasurfaces w ere then patterned onto the surface of the device. The Au plane atop the AIN served as a plasmonic mirror. Atomic layer deposition (Cambridge Nanotech Savannah S200) was used to deposit 10 nm aluminum oxide, which served as a dielectric spacer. Metallic nanostructures were then patterned with subsequent electron beam lithography and metal evaporation steps. Electron beam lithography w as performed using PMMA A2 resist spun coated to produce a ~75 nm film into which arrays of 90 nm x 90 nm squares spaced 100 nm apart were patterned using a 50 pA electron beam (Elonix ELS-7500 EX). After the polymer was developed in a solution of 25% methyl isobutyl ketone in isopropanol, 33 nm Ag with a 3 nm Ti adhesion layer was deposited. The remaining PMMA A2 was dissolved inMicroposit Remover 1165 to leave a metallic metasurface with an optical mode at ~ 780 nm localized in the AI2O3 beneath the Ag nanostructures.

[0107] Optical and Electrical Device Characterization: FIG. 14 illustrates schematically the optical and electrical setup used for device characterization. The setup had three optical sources: a pulsed laser, a distribute feedback (DFB) laser, and a white light source which are coupled to the sample. The reflected light was coupled to a spectrometer and an imaging camera. The photodetector was then coupled to a spectrum analyzer and a lock-in amplifier referencing to the frequency of a chopper placed in the light path.

[0108] The resonance of the metasurface was determined using reflectivity measurements performed within a home-built microscope as depicted in FIG. 14 coupled to a spectrometer (Ocean Optics Maya 2000 Pro). White-light reflectivity spectra exhibited characteristic dips corresponding to the near-complete near-infrared absorption by the metasurface.

[0109] The responsivity of the devices was measured as a photoinitiated current. The devices were excited using a pulsed supercontinuum laser (NKT Photonics SuperK Extreme) coupled with a variable ~10 nm full width half max filter head (SuperK Select). The beam was carefully shrunk or expanded to ensure that its diameter (defined as the width of the beam at which intensity7decreases to 1 / e peak intensity ) was ~5 nm smaller than that of the device. The excitation beam was chopped at a frequency of 3.9 kHz, which was coupled to a lock-in amplifier (Signal Recovery 7265). The resulting current photoresponse of the devices was measured as a function of incident wavelength by the lock-in amplifier to produce photocurrent spectra.[OHO] The current response of the devices upon on-resonance excitation (783 nm) was further quantified in the frequency domain to determine the device 3dB bandwidths (VBW), defined here as the frequency at which device response is attenuated by half. Assuming the photodetectors studied here act as an ideal RC low-pass filter, the device rise time (trise) can be quantified as ruse = 0.35 / VBW. TO measure VBW, devices were excited by a sinusoidally modulated 780 nm laser, where rapid modulation was accomplished through the interference of two sinusoidal beams frequency shifted from each other to produce a beat node. The frequency of the beat was swept by shifting the frequency offset between the interfering beams, and the photocurrent response was measured as a function of this beat frequency from 45 MHz to 4 GHz. To measure photocurrents, devices were coupled to a high-speed, low-noise readout probe (67 GHz Infinity Probe, GSG), which was coupled to a spectrum analyzer (TektronixRSA607A). Responsivity peaks at each frequency were integrated to produce curves of response vs. excitation frequency and thus determine 3dB bandwidths.[0111 | Device capacitance values were measured using a high-performance impedance analyzer (Keysight E4990A-020). This impedance analyzer used a built-in equivalent circuit model for parallel and serial resistivity (RP, Rs), inductance (L), and capacitance (C) calculations based on the measured impedance and phase of the device under test (DUT). This measurement showed the detector to have a high parallel resistivity (RP» 1 / tnC) and a low inductance (L), satisfying coL « 1 / coC, where co is the frequency. Thus, the photodetector was represented using a series C-R-L (Capacitance-Resistance-Inductance) circuit model, and the serial capacitance (Cs) was assumed to be equivalent to the capacitance (C). Based on these measured capacitances, the predicted 3dB bandwidth of each detector was calculated according toThe noise spectrum density (NSD) of the photodetectors was calculated according to[Oil 2 | Here, idark is the photodetector dark current, iSh.ort is the short circuit current, N is 1000, and B is the low pass filter bandwidth of the lock-in amplifier. Current values were measured by the same lock in amplifier that was used for measuring responsivity. Values of NSD were used to calculate the photodetector noise equivalent power according towhere R(k) is the photodetector responsivity at each incident wavelength.[0113| Finite Element Simulations: Multiphysics simulations of the nanostructures upon illumination were performed using COMSOL Multiphysics with the Wave Optics and Heat Transfer modules. The Wave Optics module was used to calculate the absorption of the single nanostructure upon on-resonance excitation with 780 nm light. This simulated the electromagnetic field distribution within the nanostructure and the absorption magnitude. The incident light was modelled as a Gaussian pulse emulating a picosecond pulsed laser.[0114| Nanostructures were then treated as heat sources undergoing resistive power loss within the Heat Transfer module, and temperature changes in the femtosecond and picosecond time regimes w ere simulated via thermal diffusion. Both modules used the periodic boundary condition applied along the horizontal axes to simulate an array of nanostructures. Along thevertical axis, perfectly matched layer boundary conditions were used to mimic the environment surrounding the nanostructure.[0115| DEFINITIONS

[0116] As used herein, the term “thermal photodetector” shall refer to a photodetection device that converts incident optical radiation into an electrical signal through an intermediate thermal process, in which absorbed optical energy is first converted to heat and the resulting temperature change is transduced into an electrical response.

[0117] As used herein, the phrase “electrical communication” shall mean a conductive coupling that permits the flow of electrical charge carriers between components sufficient to support signal generation or readout.101181 As used herein, the phrase “thermal communication” shall mean a physical coupling that permits transfer of thermal energy between components by conduction at a rate sufficient to produce a measurable temperature change.

[0119] As used herein, the term “primary thermal injection interface” shall refer to an interface through which a majority of thermally generated energy is transferred from one component into another during operation, relative to other available thermal paths.

[0120] As used herein, the term “subwavelength” shall refer to a structural dimension that is smaller than a free space wavelength of incident optical radiation at a wavelength of interest.

[0121] As used herein, the phrase “planar dimensions define an electrical capacitance” shall mean that lateral dimensions of one or more layers contribute to a capacitance value through geometric area and separation, in accordance with electrostatic principles.

[0122] As used herein, the phrase “external optical filter or optical cavity” shall refer to optical elements that are physically separate from the photodetector structure and that perform wavelength selection or resonant confinement external to the photodetector structure.

[0123] As used herein, the phrase “lateral dimensions selected to define a resonant wavelength” shall mean that in-plane geometric features are chosen such that electromagnetic resonance occurs at a target wavelength due to plasmonic or metasurface effects.

[0124] As used herein, the term “near infrared band” shall refer to optical wavelengths spanning approximately 700 nanometers to 1600 nanometers.[0125| As used herein, the term “optical communications band” shall refer to standardized wavelength bands used for optical data transmission, including at least the O band and the C band.

[0126] As used herein, the term “O band” shall refer to wavelengths between 1260 nanometers and 1360 nanometers.[0I27| As used herein, the term “C band” shall refer to wavelengths between 1530 nanometers and 1565 nanometers.[0128| As used herein, the phrase “free of a membrane, undercut, air gap, or thermally isolating layer” shall mean that no structure intentionally formed for the purpose of thermal isolation is present between the referenced components.

[0129] As used herein, the term “impedance matched” shall mean that an electrical impedance of a first component is selected to substantially correspond to an impedance of a second component over an operating frequency range so as to reduce signal reflection or loss.[0130| As used herein, the phrase “not dominated by the readout circuitry” shall mean that an electrical RC time constant associated with the photodetector structure contributes more significantly to overall temporal response than any RC contribution introduced by readout circuitry.

[0131] As used herein, the phrase “blocks optical transmission” shall mean substantially prevents transmission of incident optical radiation through a layer, such that transmitted power is negligible relative to absorbed or reflected power.[0132| As used herein, the term “intensity modulation” shall mean temporal variation in optical power corresponding to an information bearing signal.

[0133] As used herein, the phrase “configured to recover a temporal modulation pattern” shall mean arranged to convert a time varying optical intensity into a corresponding time varying electrical signal representing the modulation.

[0134] As used herein, the phrase “bandwidth exceeding an inverse of an electrical RC time constant” shall mean an electrical frequency response sufficient to resolve signal variations occurring on timescales shorter than the RC time constant.[0135| REFERENCE NUMERALS10: Thermal Photodetector20: Supporting Substrate30: Pyroelectric Layer40: Continuous Metallic Layer43: Opaque Layer45: Dielectric Spacer Layer50: Metasurface55: Sub wav elength Metallic Structures60: Incident Optical Radiation62: Region of Resistive Heating: Heat: First Electrical Contact: Contact Lead: Second Electrical Contact: Equivalent Electrical Representation: Pyroelectric Capacitance: Series Resistance0: Readout Circuitry0: Optical Communication System0: Optical Source0: Intensity’ Modulator (or Modulated Optical Output of the Source) 0: Transmission Path0: Optical Fiber0: Incident Optical Radiation0: Electrical Output Signal

Claims

1. CLAIMSWhat is claimed is:

1. A thermal photodetector comprising:a pyroelectric layer having a spontaneous polarization that varies in response to a temperature change;a continuous metallic layer disposed on the pyroelectric layer and in both electrical and thermal communication with the pyroelectric layer, the continuous metallic layer forming a primary thermal injection interface into the pyroelectric layer;a dielectric spacer layer disposed on the continuous metallic layer; anda metasurface comprising a plurality of subwavelength metallic structures disposed on the dielectric spacer layer;wherein planar dimensions of the pyroelectric layer and the continuous metallic layer define an electrical capacitance such that an electrical RC time constant of the photodetector is less than 500 picoseconds.

2. The photodetector of claim 1, wherein the electrical RC time constant is less than 200 picoseconds.

3. The photodetector of claim 1. wherein the electrical RC time constant corresponds to an electrical bandwidth of at least 1 gigahertz.

4. The photodetector of claim 1 , wherein the electrical RC time constant corresponds to an electrical bandwidth of at least 2 gigahertz.

5. The photodetector of claim 1, wherein the photodetector is configured for direct electrical readout without an external optical filter or optical cavity.

6. The photodetector of claim 1, wherein the pyroelectric layer has a thickness between 20 nanometers and 300 nanometers.

7. The photodetector of claim 1, wherein the continuous metallic layer comprises gold and has a thickness between 30 nanometers and 150 nanometers.

8. The photodetector of claim 1, wherein the dielectric spacer layer comprises aluminum oxide and has a thickness between 5 nanometers and 50 nanometers.

9. The photodetector of claim 1, wherein the metasurface comprises an array of metallic nanostructures having lateral dimensions selected to define a resonant wavelength.

10. The photodetector of claim 9, wherein the resonant wavelength lies in a near infrared band between 700 nanometers and 1600 nanometers.

11. The photodetector of claim 9, wherein the resonant wavelength lies in an optical communications band selected from an O band or a C band.

12. The photodetector of claim 1, wherein the pyroelectric layer is free of a membrane, undercut, air gap, or thermally isolating layer between the continuous metallic layer and a supporting substrate.

13. The photodetector of claim 1, further comprising first and second electrical contacts coupled to opposite sides of the pyroelectric layer.

14. The photodetector of claim 13, wherein the first electrical contact is formed by a conductive substrate and the second electrical contact is formed by the continuous metallic layer.

15. The photodetector of claim 13. further comprising readout circuitry electrically coupled to the first and second electrical contacts, the readout circuitry being impedance-matched with the photodetector such that the electrical RC time constant of the photodetector is not dominated by the readout circuitry.

16. The photodetector of claim 15, wherein the readout circuitry comprises an impedance matched transmission line or waveguide structure coupled to the continuous metallic layer.

17. The photodetector of claim 1, wherein an opaque layer blocks optical transmission into an underlying semiconductor substrate.

18. A method of detecting optical radiation comprising:coupling incident optical radiation into plasmonic modes using a metasurface; dissipating electromagnetic energy associated with the plasmonic modes as heat in a continuous metallic layer;transferring the heat from the continuous metallic layer into a pyroelectric layer; converting a resulting temperature change into an electrical signal via pyroelectric transduction; andreading out the electrical signal with an electrical RC time constant less than 500 picoseconds.

19. The method of claim 18, wherein the electrical signal corresponds to intensity modulation imposed on optical radiation in an optical communications band selected from an O band or a C band.

20. An optical communication system comprising:an optical source configured to emit optical radiation in an optical communications band, the optical radiation being intensity modulated according to a data signal;a transmission path configured to convey the intensity modulated optical radiation; the thermal photodetector of claim 1 positioned to receive the intensity modulated optical radiation; andreadout circuitry electrically coupled to the thermal photodetector,wherein the readout circuitry is configured to recover a temporal modulation pattern of the intensity modulated optical radiation from an electrical signal generated by the thermal photodetector.

21. The optical communication system of claim 20, wherein the optical communications band is selected from an O band or a C band.

22. The optical communication system of claim 20, wherein the transmission path comprises an optical fiber.

23. The optical communication system of claim 20, wherein the readout circuitry has a bandwidth exceeding an inverse of an electrical RC time constant of the thermal photodetector.

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