Generation of electromagnetic radiation from a photonic crystal maser.

Abstract

In a general aspect, a photonic crystal maser includes a dielectric having an array of cavities periodically aligned to define a photonic crystal structure within the dielectric. The dielectric also includes a region within the array of cavities that defines defects within the photonic crystal structure. An elongated slot extends through the region from a slot opening in a surface of the dielectric at least partially through the dielectric. The elongated slot and the cavity array define a waveguide in the dielectric. The dielectric further includes an input coupler aligned with an end of the elongated slot and configured to couple a reference radio frequency (RF) electromagnetic radiation into the waveguide. The photonic crystal maser also includes a vapor or source of vapor within the elongated slot, and an optical window covering the elongated slot.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 17 / 667,296, filed February 8, 2022, the disclosure of which is incorporated herein by reference. [Background technology]

[0002] The following description relates to generating electromagnetic radiation from a photonic crystal maser.

[0003] A maser produces coherent electromagnetic radiation by stimulated emission of atoms or molecules that build up in a resonator. The stimulated emission produces electromagnetic radiation at the emission frequency of the atoms or molecules that build up in the resonator. The resonator can be used to capture some of the stimulated emission from the atoms or molecules so that photons in the resonator can stimulate more emission from the excited atoms or molecules into the resonator mode. To do so, the resonator can have a mode of vibration that corresponds to the emission frequency of the atoms or molecules, thereby allowing the resonator to "resonate" with the atoms or molecules. The resonator can thus store energy and amplify the emission of electromagnetic energy from the maser by coherent stimulated emission. [Brief description of the drawings]

[0004] [Figure 1] 1 is a schematic diagram of an example system showing a photonic crystal maser injection locked to a seed source. [Figure 2A] 1 is a perspective schematic diagram of an example photonic crystal maser that includes a dielectric bonded to two optical windows. [Figure 2B] 2B is a top schematic view of the example photonic crystal receiver of FIG. 2A, in which a dielectric includes a ring resonator structure. [Figure 3A] 1A and 1B are schematic top and perspective views of an example photonic crystal maser including an input coupler and an output coupler. [Figure 3B]FIG. 3B is a perspective schematic diagram of the example photonic crystal maser of FIG. 3A, in which an antenna serves as a reference RF oscillator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] In a general aspect, a photonic crystal maser is disclosed that includes a photonic crystal structure and an input coupler for coupling reference radio frequency (RF) electromagnetic radiation into a waveguide of the photonic crystal maser. For example, the photonic crystal maser can include a dielectric having an array of cavities arranged periodically to define a photonic crystal structure in the dielectric. The dielectric can also include a region in the cavity array that defines defects in the photonic crystal structure. An elongated slot that passes through the region extends at least partially through the dielectric from a slot opening in a surface of the dielectric. The elongated slot and the cavity array can define a waveguide in the dielectric. The dielectric can further include an input coupler aligned with an end of the elongated slot and configured to couple reference RF electromagnetic radiation into the waveguide. The reference RF electromagnetic radiation can have one or both of a controlled frequency and a controlled phase.

[0006] The photonic crystal maser also includes a vapor (or a source of vapor) within the elongated slot, and an optical window covering the elongated slot. The optical window has a window surface bonded to a surface of the dielectric to form a seal around the slot opening. In doing so, the optical window can define a vapor cell with the dielectric. In many variations, the vapor includes one or more input electronic transitions and an output electronic transition coupled to the one or more input transitions. The output electronic transitions emit a target radio frequency (RF) electromagnetic radiation and resonate with a guided mode of the waveguide. Examples of possible electronic transitions include Rydberg transitions, atomic transitions, molecular transitions, etc. In some variations, an optical mirror is disposed at one or both ends of the elongated slot. In some variations, a photonic crystal mirror is disposed at one or both ends of the elongated slot. Other configurations of the photonic crystal maser are possible as described below.

[0007] Photonic crystal masers can be used in communications and other applications where a highly directional, low noise, frequency stable, coherent radio frequency source is required. For signaling, controlling the frequency and phase of the photonic crystal maser output can be useful to encode information in either or both the phase and the frequency. In applications where efficient generation of RF radiation is required, the frequency and phase can be controlled by injection seeding a high power photonic crystal maser with a low power seed oscillator, such as a clock stabilized oscillator. The output of the low power seed oscillator is injected into the photonic crystal maser by coupling the output of this oscillator into an antenna or a waveguide, which can then be fed into the photonic crystal maser from another coupling element (e.g., an input coupler). In this way, a small amount of seed RF radiation is efficiently injected into the cavity of the photonic crystal maser. Because the threshold for photonic crystal masers is small, only a small amount of seed radiation is needed to injection lock the photonic crystal maser to the low power seed oscillator.

[0008] In many variations, the seed radiation is stronger than the spontaneous or blackbody processes and zero distribution of the maser modes to lock the photonic crystal maser. The phase and frequency stability of a low-power seed oscillator is transferred to the photonic crystal maser by injection seeding. In these variations, the photonic crystal maser can be used as an extremely low noise amplifier that produces a highly directional coherent output from the seed radiation. Photonic crystal masers can also be optically pumped, so the size, weight, and power (SWaP) of any resulting system can be smaller than a conventional stabilized maser. Photonic crystal masers can be configured to operate around room temperature and emit light at any frequency near the Rydberg transition of the photonic crystal maser. In this case, the photonic crystal maser can be a Rydberg atomic maser. The frequency range of the target RF electromagnetic radiation from the photonic crystal maser can range from MHz to THz, with a spectral bandwidth of about 1 Hz to about 100 MHz. A low power seed oscillator can tune the photonic crystal maser over the bandwidth of the maser cavity and gain medium, while locking the output phase to an external clock. The frequency can also be locked to the clock output by preventing the seed frequency from drifting.

[0009] 1, a schematic diagram of an example system 100 is presented showing a photonic crystal maser 102 being injection locked to a seed source 104. The seed source 104 can operate at a lower power than the photonic crystal maser 102. The photonic crystal maser 102 includes a gain medium 106 (e.g., steam) operable to generate a target RF electromagnetic radiation 108 in response to receiving an optical signal (not shown) and a reference RF electromagnetic radiation 110 (or seed output) from the seed source 104. The gain medium 106 may be disposed between two mirrors 112a, 112b, which facilitate the build-up of the RF electromagnetic radiation in the gain medium 106 until a threshold intensity is reached. The two mirrors 112a, 112b can help define a maser cavity 114 of the photonic crystal maser 102.

[0010] The photonic crystal maser 102 also includes an input coupler 116 that couples the reference RF electromagnetic radiation 110 into the waveguide of the photonic crystal maser 102. The input coupler 116 can reduce coupling losses between the seed source 104 and the maser cavity. During operation, the intensity of the reference RF electromagnetic radiation 110 is strong enough to overcome noise (e.g., spontaneous emission, blackbody radiation, etc.) in the maser cavity 114. Such intensity allows the seed source 104 to control the output of the photonic crystal maser 102. The seed source 104 can be a tunable radio frequency source whose tuning range covers (or nearly covers) the operating range of the photonic crystal maser 102. In some examples, such as shown in FIG. 1, the seed source 104 is referenced to a phase reference 118, such as a temperature stabilized crystal oscillator or other type of clock. The reference RF electromagnetic radiation 110 is coupled through a partially transparent window 112b. Input coupler 116 matches the seed output mode to a cavity mode (or guided mode) of photonic crystal maser 102 .

[0011] In some applications, there may be a need to control the frequency and phase of a photonic crystal maser. Controlling the frequency and phase (and possibly the amplification) may allow for signal encoding. Encoding a signal onto the output of a photonic crystal maser may be advantageous for use cases such as communications and radar. In these cases, the coherence and directionality of the radio frequency emission from a photonic crystal maser may also be useful, making the photonic crystal maser an ideal device for such applications. Controlling the phase and frequency is advantageous for timing applications, as the photonic crystal maser may be locked to a clock such as a GPS timing signal and then used as a local oscillator. A collection of ground stations for radar or communications may then use phase-sensitive detection, such as heterodyning, to improve signal detection within the radar or communications network. Phase- and frequency-locked photonic crystal masers may also be used to efficiently transfer timing signals elsewhere.

[0012] One method of controlling the frequency and phase of a photonic crystal maser is by injection locking. In this method, a clock, or timing with a clock signal, can be used to frequency and phase lock a radio frequency source emitting a low-intensity signal. The low-intensity output of a radio frequency oscillator can be coupled into the maser cavity to bias the emission of the low-intensity signal to a specific frequency and phase of the low-power oscillator. By "seeding" the photonic crystal maser with a specific radio frequency wave of selected frequency and phase, the stimulated emission that produces the maser output is preferentially promoted to occur at the selected phase and frequency. When this occurs, the exponential cascade of the inverted population into the promoted modes by the stimulated emission suppresses masing at other frequencies and phases. The photonic crystal maser can then mimic the output of the seed to produce a directional coherent output of fixed phase and frequency at higher power.

[0013] The advantage of injection seeding is that a low power injection seed can be effectively converted into a highly coherent directional beam with great intensity and total power. Injection seeds are straightforward to build and configure with clocks such as chip-scale atomic clocks and quartz crystal oscillators, as well as low power radio frequency equipment. Full control of emission frequency and phase may be possible in certain cases and may be easier than with photonic crystal masers alone due to the relatively long emission wavelength compared to lasers. The phase uncertainty of a photonic crystal maser can be expressed as ΔΦ ≥ 2πλ / l, where l is the cavity length and λ is the wavelength, if the photonic crystal maser is initiated by a random process such as spontaneous emission or blackbody radiation. Typically, λ / l ≈ 0.1 results in ΔΦ ≈ π / 5, which is large for many applications. The spectral bandwidth of a photonic crystal maser can be around 1-10 MHz, which is large for many timing applications. Frequency stability can be as low as 10 MHz by locking the seed source to a GPS clock and using this to control the output of the photonic crystal maser. 12 This kind of stability is suitable for time transfer applications, and can be achieved by injection seeding even with low-quality oscillators locked to a GPS clock.

[0014] Photonic crystal masers may have a low masing threshold, so the seed power can be small. In these cases, the reference RF electromagnetic radiation from the seed source only needs to overcome the oscillator noise level. For example, the seed strength after overcoming coupling losses into and propagation losses within the maser cavity may need to overcome spontaneous emission, blackbody radiation within the cavity, and zero-point energy in the field modes. Spontaneous emission may be around 1 kHz or less, but values ​​<100 Hz are more typical. The zero-point energy of the field modes is small compared to the spontaneous emission for blackbody radiation and the Rydberg transition. In many cases, the zero-point energy can be effectively ignored. In the transition itself, the seed only needs to be around a few tens of Hz above the Rabi frequency in some cases.

[0015] In the absence of a seed, a natural seed for a photonic crystal maser can be produced by spontaneous emission or blackbody radiation. The phase can be random when masing is initiated, and the frequency can be such that the emission rate combined with the phase is maximum and the losses in the maser cavity are minimum. As the power of the injection seeding increases compared to the power of the noise source effectively coupled into the cavity, a larger portion of the maser output reflects the nature of the seed. When the seed is powerful compared to the noise source, the maser frequency locks to the seed. During such locking, the seeded mode of the photonic crystal maser (e.g., a guided mode of the photonic crystal maser) extracts energy from the gain medium. In many instances, the seed is resonant or near-resonant with a transition inversion. Usually, the spontaneously emitted photons extract energy from the gain medium and begin to amplify the number of photons in a particular mode. Injection locking occurs when the seed signal frequency is (approximately) matched to a resonant mode of the maser cavity (or waveguide) sufficiently to overcome the potential for spontaneous emission and blackbody radiation to steal significant amounts of energy from the gain medium.

[0016] Referring now to FIG. 2A, a perspective schematic diagram of an example photonic crystal maser 200 includes a dielectric 202 bonded to two optical windows 204, 206. The photonic crystal maser 200 can be similar to the photonic crystal maser 102 described with respect to FIG. 1. The dielectric 202 includes an array of cavities 208 arranged periodically to define a photonic crystal structure 210 within the dielectric 202. For example, the cavities 208 of the array can be arranged on respective sites of a two-dimensional lattice, such as an oblique lattice, a square lattice, a rectangular lattice, a hexagonal lattice, an oblique lattice, etc. In FIG. 2A, each cavity 208 is defined by a through hole. However, other shapes of the cavities 208 (e.g., blind holes, internal voids, etc.) are possible, including combinations of shapes. The dielectric 202 also includes regions 212 (or defect regions) within the array of cavities 208 that define defects within the photonic crystal structure 210. A region 212 may be defined by the absence of cavities 208 on two or more adjacent sites of the two-dimensional lattice. In Figure 2A, the region 212 is a linear region with no rows of cavities 208. However, other geometric shapes are possible, such as curved, circular, elliptical, serpentine, square, rectangular, hexagonal, etc.

[0017] The dielectric 202 may be formed of a material that is substantially transparent to RF electromagnetic radiation. This material may have a high resistivity, e.g., ρ>10 3 The dielectric 202 may be an insulating material having a resistivity of Ω·cm and may correspond to a single crystal material, a polycrystalline material, or an amorphous (or glass) material. For example, the dielectric 202 may be formed of silicon. In another example, the dielectric 202 may be a silicon oxide (e.g., SiO2, SiO), such as quartz glass, borosilicate glass, or aluminosilicate glass. x In some examples, the material of the dielectric 202 is an oxide material, such as magnesium oxide (e.g., MgO), aluminum oxide (e.g., Al2O3), silicon dioxide (e.g., SiO2), titanium dioxide (TiO2), zirconium dioxide (e.g., ZrO2), yttrium oxide (e.g., Y2O3), lanthanum oxide (e.g., La2O3), etc. The oxide material may be a non-stoichiometric oxide (e.g., SiO x) or a combination of one or more binary oxides (e.g., Y:ZrO2, LaAlO3, etc.). In a particular variation, the combination is BaLn2Ti4O 12 where Ln refers to one or more elements from the lanthanide group of the periodic table of elements. In other examples, the material of the dielectric 202 is a non-oxide material, such as silicon (Si), diamond (C), gallium nitride (GaN), calcium fluoride (CaF), etc.

[0018] The dielectric 202 further includes an elongated slot 214 through the region 212, which extends from a slot opening in a surface of the dielectric 202 at least partially through the dielectric 202. In FIG. 2A, the elongated slot 214 extends completely through the dielectric 202 to a second slot opening. The elongated slot 214 and the array of cavities 208 define a waveguide in the dielectric 202. In operation, the waveguide can direct RF electromagnetic radiation (or waves of RF electromagnetic radiation) along an axis of the region 212, e.g., toward an end of the dielectric 202. The dielectric 202 also includes an input coupler 216 aligned with an end of the elongated slot 214 and configured to couple a reference radio frequency (RF) electromagnetic radiation (e.g., from a seed source) into the waveguide. The reference RF electromagnetic radiation has one or both of a controlled frequency and a controlled phase. In some examples, the reference RF electromagnetic radiation resonates with a guided mode of the waveguide. In some examples, such as that shown in Figure 2A, the input coupler 216 extends from a side of the dielectric and terminates in a tapered end. However, other locations or geometries are possible. The input coupler 216 is further described with respect to Figures 3A and 3B.

[0019] The example photonic crystal maser 200 also includes a vapor (or source of vapor) within the elongated slot 214. The vapor can include components such as a gas of alkali metal atoms (e.g., Rydberg atoms), a noble gas, a gas of diatomic halogen molecules, or a gas of organic molecules. For example, the vapor can include a gas of alkali metal atoms (e.g., K, Rb, Cs, etc.), a noble gas (e.g., He, Ne, Ar, Kr, etc.), or both. In another example, the vapor can include a gas of diatomic halogen molecules (e.g., F2, Cl2, Br2, etc.), a noble gas, or both. In another example, the vapor can include a gas of organic molecules (e.g., acetylene), a noble gas, or both. Other combinations for vapors with other components are also possible. The source of vapor can generate the vapor in response to an energy stimulus such as heat, exposure to ultraviolet radiation, irradiation with laser light, etc. For example, the vapor may correspond to a gas of alkali metal atoms, and the source of the vapor may correspond to a mass of alkali metal that has been cooled sufficiently to be in a solid or liquid phase when placed within the elongated slot 214.

[0020] In many implementations, the vapor has electronic transitions (e.g., Rydberg transitions, atomic transitions, molecular transitions, etc.) defined between pairs of electronic energy levels. In particular, the vapor includes one or more input electronic transitions and an output electronic transition coupled to the one or more input electronic transitions. The output electronic transitions are operable to emit target RF electromagnetic radiation and resonate with one or more guided modes of the waveguide. In some implementations, the output electronic transitions and the reference RF electromagnetic radiation resonate with the same guided mode of the waveguide. In some implementations, the example photonic crystal structure 200 includes a laser (e.g., a pump laser) configured to generate an optical signal capable of exciting one or more input electronic transitions of the vapor. However, other energy sources are possible (e.g., a source of radio frequency photons). In some implementations, the output electronic transitions are operable to emit target RF electromagnetic radiation having a frequency in the range of 100 MHz to 1 THz.

[0021] Photonic crystal structure 210 can define a photonic bandgap for the target RF electromagnetic radiation within the waveguide. The photonic bandgap can be for a transverse magnetic (TM) mode, a transverse electric (TE) mode, or both, of the target RF electromagnetic radiation within the waveguide. The photonic bandgap can enable photonic crystal structure 210 to affect properties of the target RF electromagnetic radiation. For example, photonic crystal structure 210 can be configured to concentrate the target RF electromagnetic radiation within elongated slot 214. Photonic crystal structure 210 can also be configured to reduce the group velocity of the target RF electromagnetic radiation (or waves of the target RF electromagnetic radiation) along the direction of elongated slot 214 (e.g., along the axis of elongated slot 214). Such an effect can enable photonic crystal structure 210 to control the absorption and emission of photons by the vapor.

[0022] In some implementations, the elongated slot 214 extends partially through the dielectric 202, which includes a surface that defines a slot opening of the elongated slot 214. In these implementations, the example photonic crystal receiver 200 includes an optical window (e.g., optical window 204) having a window surface that covers the elongated slot 214 and is bonded to the surface to form a seal around the slot opening. Such sealing can assist the optical window and dielectric 202 in sealing the vapor (or a source of vapor) within the elongated slot 214, thereby defining a vapor cell within the region 212. The optical window can be bonded to the dielectric 202 using contact bonds, anodic bonds, glass frit bonds, or the like. Such bonds can be formed using techniques described in U.S. Pat. No. 10,859,981, entitled "Vapor Cells Having One or More Optical Windows Bonded to a Dielectric Body," the disclosure of which is incorporated herein by reference in its entirety.

[0023] The optical window may be formed of a material that is transparent to the electromagnetic radiation (e.g., laser light) used to stimulate the vapor to emit the targeted RF electromagnetic radiation. For example, the optical window may be transparent to the infrared wavelength range of electromagnetic radiation (e.g., 700-5000 nm), the visible wavelength range of electromagnetic radiation (e.g., 400-700 nm), or the ultraviolet wavelength range of electromagnetic radiation (e.g., 10-400 nm). Additionally, the material of the optical window may have a high resistivity, e.g., ρ>10 3 The material may be an insulating material having a resistivity of Ω·cm and may correspond to a single crystal material, a polycrystalline material, or an amorphous (or glass) material. For example, the material of the optical window may be a silicon oxide (e.g., SiO2, SiO) such as found in quartz, fused silica, or borosilicate glass. x In another example, the optical window material can include aluminum oxide (e.g., Al2O3, Al), such as found in sapphire or aluminosilicate glasses. x O y In some examples, the optical window material is an oxide material, such as magnesium oxide (e.g., MgO), aluminum oxide (e.g., Al2O3), silicon dioxide (e.g., SiO2), titanium dioxide (e.g., TiO2), zirconium dioxide (e.g., ZrO2), yttrium oxide (e.g., YO3), lanthanum oxide (e.g., La2O3), etc. The oxide material may be a non-stoichiometric oxide (e.g., SiO x ), and one or more binary oxides (e.g., Y: ZrO2, LaAlO3, BaLn2Ti4O 12 In another example, the optical window material is a non-oxide material, such as diamond (C) or calcium fluoride (CaF).

[0024] In some implementations, the surface of the dielectric 202 defines a cavity opening for each of the array of cavities 208. The optical window may or may not cover each of the cavity openings. In implementations in which the optical window does not cover the cavity openings, the window surface of the optical window may form a seal around each of the cavity openings.

[0025] In some implementations, the elongated slot 214 extends completely through the dielectric 202. For example, as shown in FIG. 2A , the dielectric 202 can include a first surface 218 opposite a second surface 220, and the elongated slot 214 extends from the first surface 218 through the dielectric 202 to the second surface 220. The first surface 218 can define a first slot opening 222 of the elongated slot 214, and the second surface 220 can define a second slot opening (not shown) of the elongated slot 214. In these implementations, the example photonic crystal maser 200 includes a first optical window 204 and a second optical window 206 that cover the first slot opening and the second slot opening of the elongated slot 214, respectively. The first optical window 204 and the second optical window 206 each have a window surface bonded to a surface of the dielectric 202 and can seal the vapor (or a source of vapor) in the elongated slot 214 to define a vapor cell. In such a case, the first optical window 204 can have a first window surface 224 that can cover the first slot opening 222 and that is bonded to the first surface 218 of the dielectric 202 to form a seal around the first slot opening 222. Similarly, the second optical window 206 can have a second window surface 226 that can cover the second slot opening and that is bonded to the second surface 220 of the dielectric 202 to form a seal around the second slot opening.

[0026] In implementations in which the example photonic crystal maser 200 includes first and second optical windows 204 and 206, the first and second surfaces 218 and 220 of the dielectric 202 can define respective first and second cavity openings of the array of cavities 208. In these implementations, the first and second optical windows 204 and 206 can cover or not cover respective first and second cavity openings. In these optical window implementations, the first window surface 224 can form a seal around each of the first cavity openings and the second window surface 226 can form a seal around each of the second cavity openings.

[0027] In some implementations, the example photonic crystal maser 200 includes an output coupler 228 configured to impedance match the target RF electromagnetic radiation to the surrounding environment of the photonic crystal maser 200. In these implementations, the region 212 in the array of cavities 208 can extend along an axis 230, and the elongated slot 214 can be aligned parallel to (e.g., coincident with) the axis 230. In this case, the dielectric 202 includes the output coupler 228, which can extend from an end 232 (or side) of the dielectric 202 and be aligned with the axis 230. The output coupler 228 can be an integral part of the dielectric 202, but can also be a separate body. If separate, the output coupler 228 can be formed of a dielectric material. However, the output coupler 228 can also be a conventional coupler formed of a metal. FIG. 2A shows the output coupler 228 as a protrusion from the dielectric 202 that terminates in a taper. However, other geometries for output coupler 228 are possible.

[0028] In some implementations, the output coupler 228 is electromagnetically coupled to an output mirror, such as a photonic crystal mirror 234, to impedance match the output beam to the medium (e.g., air) through which the output beam is intended to propagate. The photonic crystal mirror 234 may be defined by one or more offset cavities that are spatially offset from an ideal periodic position in the array. The one or more offset cavities may be proximate an end (e.g., end 232) of the elongated slot 214 and have a respective spatial offset away from the end of the elongated slot 214. The one or more offset cavities may also be proximate a side of the elongated slot 214 and have a respective spatial offset away from the side of the elongated slot 214. Other locations are possible. A lens may be added to collimate the output beam. In some variations, the elongated slot 214 may be tapered (e.g., in width along the axis of the elongated slot 214). Such tapering may aid in the formation of the photonic crystal mirror 234.

[0029] In some implementations, a polarizer can be added to the output coupler 228 to filter the polarization of the output beam. For example, the output coupler 228 can include a narrow portion terminating in a tapered end and aligned with the tapered end. An array of coplanar segments can extend outward from the narrow portion and have a periodic spacing along the array. The array of coplanar segments can be configured to filter the polarization of the target RF electromagnetic radiation.

[0030] In some implementations, photonic crystal mirror 234 is disposed at one or both ends of elongated slot 214. FIG. 2A illustrates the case where photonic crystal mirror 234 is present at both ends of elongated slot 214. The presence of photonic crystal mirror 234 may increase the output power, may lower the gain threshold of the maser, or both. For example, photonic crystal mirror 234 may reflect electromagnetic radiation that traverses region 212 during operation (e.g., targeted RF electromagnetic radiation emitted by the vapor during operation of example photonic crystal maser 200). In this capacity, region 212 may function as part of the maser cavity, such as an interior of the maser cavity. Additionally, photonic crystal mirror 234 may assist array of cavities 208 and elongated slot 214 in defining a cavity structure (e.g., a slot waveguide) for the electromagnetic radiation emitted by the vapor.

[0031] In many variations, photonic crystal mirror 234 corresponds to a change in dimensional characteristics of photonic crystal structure 210 near the ends of elongated slot 214. For example, the transmission of the target RF electromagnetic radiation through photonic crystal structure 210 at the ends of elongated slot 214 can be varied by varying the spacing of cavities 208 in the array, the thickness of dielectric 202, the diameter of cavities 208 in the array, etc. It should be understood that a perfect photonic crystal geometry for the target RF electromagnetic radiation (or resonant wave) within dielectric 202 can act as a perfect reflector, but without the photonic crystal it can act as a perfect transmitter.

[0032] In some implementations, photonic crystal mirror 234 is tuned to a resonant frequency ω of the cavity (or waveguide) of photonic crystal structure 210. c The reflectivity may be greater than 80% for frequencies of electromagnetic radiation at or near the photonic crystal structure 210. This reflectivity may increase the quality factor Q of a cavity associated with the photonic crystal structure 210 (or region 212 therein), thereby lowering the threshold condition for imaging to occur. In some variations, the reflectivity is greater than 85%. In some variations, the reflectivity is greater than 90%. In some variations, the reflectivity is greater than 92%. In some variations, the reflectivity is greater than 94%. In some variations, the reflectivity is greater than 96%.

[0033] In some implementations, optical mirrors (not shown) are disposed at one or both ends of the elongated slot 214. The optical mirrors may be angled with respect to the optical path defined by the elongated slot 214 or may be angled perpendicular to the optical path. The optical mirrors may serve to direct an optical signal along a longitudinal axis of the elongated slot, such as axis 230. To that end, the optical mirrors may include a surface configured to reflect such an optical signal. The optical signal may include light received into the elongated slot 214 from a laser.

[0034] In some implementations, the vapor is a vapor of atoms where each atom can act as an emitter. In operation, the photonic crystal structure 210 surrounds the elongated slot 214 and can slow down and focus electromagnetic waves at the atomic transition frequencies of the atoms. The atoms are pumped with a laser such that a population inversion is established based on the u→l transition of the atoms resonating with a resonant mode (or guided mode) of the waveguide. The emission of radiation into the resonant mode of the waveguide can be enhanced due to the stronger electric field to facilitate emission. Stimulated emission dominates to create a coherent directed master beam along the waveguide that can be impedance matched and shaped for free space propagation. Photonic crystal mirrors can be implemented at the ends of the elongated slot 214 so that the radiation can propagate back and forth within the elongated slot 214 and be further amplified. The elongated slot 214 receives the energy stored in the population inversion and releases this energy into the guided mode resulting in a coherent directed beam of radiation. Similar operation is possible in implementations of photonic crystal maser example 200 in which the vapor is a molecular vapor.

[0035] The example photonic crystal maser 200 may be constructed in a manner suitable for mass production. The example photonic crystal maser 200 may be combined with a Rydberg atomic receiver, a Rydberg atomic vapor cell sensor, or an array of Rydberg atomic vapor cell sensors to create a device capable of transmitting and receiving RF radiation. The amount of power output by the example photonic crystal maser 200 may be controlled to exceptionally low levels by varying the intensity of the laser. Furthermore, the switching time may be nanoseconds, since the cavity lifetime may be on this scale. Because of the nanosecond lifetime and the ability to modulate the laser at GHz bandwidth, modulating the laser may imprint baseband modulation on a carrier frequency at the same frequency scale (e.g., GHz).

[0036] FIG. 2A illustrates the example photonic crystal maser 200 as having a linear region 212 and a linear elongated slot 214, although other geometries are possible. For example, FIG. 2B presents a top schematic view of the example photonic crystal receiver 200, but the dielectric 202 includes a ring resonator structure. In particular, the region 212 forms a loop (e.g., an elliptical loop) with an array of cavities 208. In many variations, the loop is a closed loop, such as a circular, elliptical, or oval loop. The elongated slot 214 extends along a loop axis (e.g., an elliptical axis) of the loop to form a looped slot 250 (e.g., an elliptical slot). All or a portion of the looped slot 250 may be partitioned (e.g., by a transparent wall, lens, mirror, etc.) to accommodate a vapor or vapor source. FIG. 2B illustrates the case where a portion 252 of the looped slot 214 accommodates a vapor or vapor source. 2B, the array of cavities 208 forms a loop, however, other distributions for the array of cavities 208 are possible.

[0037] The looped slot 250 can be associated with a first loop direction 254 and a second loop direction 256 along a loop axis, with the first loop direction 254 being opposite the second loop direction 256. In these configurations, the looped slot 250 includes a first RF port 258 and a second RF port 260. A first directional coupler 262 and a second directional coupler 264 are coupled to the first RF port 258 and the second RF port 260, respectively. The first directional coupler 262 is configured to receive a first portion of the target RF electromagnetic radiation traveling along the first loop direction, and the second directional coupler 264 is configured to receive a second portion of the target RF electromagnetic radiation traveling along the second loop direction. In many variations, the first directional coupler 262 is more strongly coupled to the looped slot 250 than the second directional coupler 264.

[0038] The looped configuration for the region 212 and the elongated slot 212 may provide some advantages over a linear configuration in certain cases. For example, the linear configuration allows the target RF electromagnetic radiation, when emitted, to travel primarily along two opposing directions, such as along the linear axis of the elongated slot 214. While traveling in this manner, the target RF electromagnetic radiation may be amplified by interacting with the vapor, which may act as a gain medium. However, this bidirectional traveling may also allow photons associated with the traveling target RF electromagnetic radiation (e.g., waves of photons traveling back and forth within the elongated slot 214) to interfere with one another and establish a standing wave of the target RF electromagnetic radiation within the elongated slot 214. This standing wave is associated with a series of minimum and maximum field strengths along the length of the elongated slot 214. The minimum field strengths may result in little or no stimulated emission in some portions of the vapor, whereas the maximum field strengths may saturate stimulated emission in other portions. The maximum field strengths may be particularly undesirable if the energy of the electromagnetic field in those portions may be absorbed by the vapor at the location of those portions. As a result, standing waves of targeted RF electromagnetic radiation may not efficiently utilize the vapor within elongated slot 214. The targeted RF electromagnetic radiation may saturate the output power at a lower magnitude of field strength than if all of the vapor were utilized.

[0039] In contrast, the looped configuration may, in certain cases, incorporate a directional coupler to establish a traveling wave of the target RF electromagnetic radiation to more efficiently use the vapor. For example, a high efficiency directional coupler (e.g., first directional coupler 262) may function as a unidirectional device at the first RF port 258 of the looped slot 250, and a low efficiency directional coupler (e.g., second directional coupler 264) may be used as an output coupler at the second RF port 260 of the looped slot 250. In the looped configuration, first photons circulating in the first loop direction 254 (e.g., clockwise direction) are coupled out of the looped waveguide with low efficiency, and second photons circulating in the second loop direction 256 (e.g., counterclockwise direction) are coupled out of the cavity with high efficiency. The loss of the second photons is designed such that the second photons do not reach the maging threshold when they interact with the vapor. In contrast, the first photon is so strong that when it interacts with the vapor, it extracts all of its energy from the gain medium. In this situation, the targeted RF electromagnetic radiation forms a traveling wave (particularly in the vapor), and therefore the entire vapor can be used as a gain medium to provide energy for imaging. Furthermore, the traveling wave allows the vapor to be selectively confined within a portion of the looped slot 250. It is not necessary for the entire volume of the looped slot 250 to be filled with vapor. The portion can be selected in length and position to maximize the participation of the vapor in stimulated emission.

[0040] When a photonic crystal maser transmits a signal, the amplitude, phase and frequency of the signal can be controlled so that information can be encoded in the output signal. In addition, multiple photonic crystal masers can be phase and frequency locked so that they are mutually coherent. Making photonic crystal masers mutually coherent may allow techniques such as heterodyning to be used for signal detection. Other coherent techniques are possible. Coherent masers can also be made to transmit timing signals and can be used for holography. These techniques are used, for example, in communications, time transfer, non-destructive testing, and radar. Photonic crystal masers have high directivity and are therefore advantageous for applications such as point-to-point communications. In contrast, antennas usually have low directivity.

[0041] Photonic crystal masers can also have high spectral purity compared to other radio frequency sources. Photonic crystal masers are resonant devices whose structures can resonate at single electron transitions (e.g., Rydberg transitions), so that other frequencies are not emitted (e.g., spurious or harmonics). Photonic crystal masers exhibit high spectral purity due to the narrow electronic transitions. Furthermore, the wavelengths emitted by photonic crystal masers can also pass through materials that laser emission cannot pass through, including human tissue. As a result, photonic crystal masers can be used for imaging applications in non-destructive testing and medical diagnostics. For example, photonic crystal masers can be used to image the contents of a cargo or detect people passing through a wall. In some implementations, photonic crystal masers are sources that use the same lasers and concepts as Rydberg atom-based electric potential measurements. In some implementations, photonic crystal masers are all-dielectric devices. Photonic crystal masers can function, for example, as sources or local oscillators for Rydberg atom-based receivers or vapor cell sensors without adding metal to the setup. All-dielectric technologies are more electromagnetically transparent than technologies based on metallic components. As such, photonic crystal masers can be effectively paired with Rydberg atomic receivers and sensors to build all-dielectric transceivers.

[0042] In many implementations, the photonic crystal maser (e.g., photonic crystal maser 200 of FIG. 2A or FIG. 2B) includes a reference RF oscillator (or seed source) configured to produce a reference RF electromagnetic radiation. The reference RF oscillator is electromagnetically coupled to an input coupler (e.g., input coupler 216 of dielectric 202). The reference RF oscillator can enable the photonic crystal maser to be injection seeded with the reference RF electromagnetic radiation. Injection seeding is a process that allows the output of a high-power RF oscillator (e.g., photonic crystal maser) to be controlled by a low-output power RF oscillator, so-called seeding. The phase and frequency of the high-output RF oscillator can be controlled because these quantities reflect the quantity of the seed. For example, the phase and frequency of a Rydberg atomic maser can be controlled by injection seeding from a low-power radio frequency source. This radio frequency source can have spurious or amplified noise and still fix the phase and frequency of the Rydberg atomic maser. As such, it is possible to obtain a fixed phase and frequency from the seed even with a low-quality inexpensive seed oscillator. The seed oscillator can be of low quality, since the Rydberg atomic maser acts as a filter for spurs in the seed oscillator output, and the amplitude fluctuations need only overcome the competing processes that cause masing to begin: spontaneous emission, blackbody radiation, and zero-point energy in the field modes. Conventional radio frequency equipment can allow control of the phase and frequency of the seed oscillator by clocking the output of the seed oscillator as a reference using a device such as a phase-locked loop.

[0043] In many implementations, masing in photonic crystal masers occurs by population inversion at atomic or molecular transitions. In the case of Rydberg atomic masers, population inversion occurs in a gas of alkali atoms at the Rydberg transition. Once population inversion is achieved, feedback can be used to sustain the maser operation. Feedback may be done by a maser cavity built around the gain medium (or vapor). In the case of photonic crystal masers, the maser cavity is formed by a photonic crystal structure. Reflecting mirrors are designed into the photonic crystal at the ends of an elongated slot (see, for example, mirrors 112a, 112b in FIG. 1). The gain medium fills the channel and is pumped via a laser by reflecting light to optical mirrors placed at the ends of the channel. Holes in the structure can slow down the radio frequency wave to increase its interaction with the gain medium. Other geometries are also possible. For example, the maser cavity can be a traveling wave cavity built by making a radio frequency waveguide circular (e.g., a loop). Almost any shape can be created using waveguides, as long as the bend radii of the various parts of the structure are not too lossy. In the case of a circular maser cavity, the gain medium or slots can run along one of the edges, such as a flattened section along the edge of the circular waveguide. A directional coupler can be designed into the loop so that radio frequency photons traveling in a particular direction are coupled out of the maser cavity. The directional coupler acts as a unidirectional device so that masing occurs primarily (or entirely) in one direction, since radio frequency photons coupled into the directional coupler can be emitted from the cavity causing losses. The implementation of this unidirectional configuration can prevent interference between forward and reverse traveling waves that would otherwise lead to a reduction in the output power of the maser. In some instances, the unidirectional cavity has no "hot spots" in it, leading to an increase in output power.

[0044] Masing can occur when photons build up inside the maser cavity by stimulated emission. Stimulated emission produces coherent photons whose number inside the cavity grows exponentially. As the photons make a round trip between the end mirrors of the cavity, they trigger more coherent emission into the masing mode. The output of the maser leaks out of one of the cavity end mirrors (e.g., mirror 112a in FIG. 1), which is designed to be partially transparent. This mirror transmission usually limits the maximum number of round trips that a photon can make inside the maser cavity.

[0045] During the initial stage of maser maser, the frequency and phase of the maser output can be determined by the first photon. Usually, the first photon is generated by spontaneous emission in the gain medium, so the phase is random, and the frequency is statistically determined by the properties of the maser cavity (e.g., how much feedback there is at a particular frequency, the spectral profile of the gain medium, etc.). Since spontaneous emission can occur in all directions, only photons that can couple into the maser cavity are "useful" to initiate maser maser. Useful photons can be those that are emitted within the solid angle that the maser cavity defines. In the case of photonic crystal masers at room temperature, blackbody radiation from the walls of the structure (e.g., or dielectric) can also generate photons that can initiate maser maser. In general, the number of photons generated by blackbody radiation and spontaneous emission is much larger than the zero-point energy of the electromagnetic field modes of the maser cavity, but zero-point energy photons can also initiate maser maser. In the case of lasers, the frequency of light is much larger than the radio frequency emitted by the maser, so blackbody radiation can be neglected. In other words, the modes of the electromagnetic field at the laser frequency are not thermally populated.

[0046] To control the frequency and phase of the maser, the photonic crystal maser can be biased towards photons that initiate masing while suppressing competition with photons that do not have the desired characteristics (e.g., different frequency, random phase, etc.). Injection seeding can be used to bias the photonic crystal maser. In this approach, injection seeding is used to provide photons that seed the maser. The number of photons required, or power, can depend on the design of the maser cavity and the operating frequency, depending on the inverse transition emission rate or spectral characteristics of the gain medium.

[0047] In some implementations, the strength of the external seeding electromagnetic field within the maser cavity is large compared to the useful spontaneously emitted, blackbody generated, and zero-point distributed electromagnetic fields. The seed electromagnetic field (or reference RF electromagnetic radiation) must be coupled into the maser cavity (or waveguide) since it is generated outside the photonic crystal maser. The coupling of the seed electromagnetic field into the maser cavity can be characterized by the transmittance of the end mirror through which the seed electromagnetic field is injected and the mode matching factor, which characterizes how much of the seed electromagnetic field is efficiently matched to the maser mode (or waveguide mode). The electric field approach can be useful because the phase is preserved. In that case, the seed electromagnetic field E inside the maser cavity sc is the seed E delivered by conventional electronic devices, as shown in Eq. (1). s can be related to.

[0048]

number

[0049] At least two additional factors can complicate injection seeding. First, the spectral purity of the output is affected by the E sc E * sc E n E * n The amount of unwanted medging can vary with an amount exceeding . Noise can steal some of the energy stored in the inverted population and cause medging at unwanted frequencies and phases despite the presence of the primary seed. This behavior can be attributed to the fact that injection seeding is not a threshold phenomenon. The amount of unwanted medging can be quantified by the spectral purity P, which is the ratio of the energy in the desired modes to the energy in the undesired modes, as shown in equation (3).

[0050]

number

[0051] In some implementations, the output of the photonic crystal maser can be phase and frequency modulated by varying the characteristics of the seed oscillator. Standard techniques for phase and frequency modulation of radio frequency oscillators can be used. The time it takes for the photonic crystal maser to respond and change its phase and frequency follows the general principles described above. The change can be made on a time scale set by the lifetime of the maser cavity. In some examples, the time scale is nanoseconds, set by the quality factor Q of the cavity and the speed of light in the medium, which can be slowed down by slow wave structures designed into the maser cavity.

[0052] 3A, top and perspective schematic views of an example photonic crystal maser 300 are presented, including an input coupler 302 and an output coupler 304. The example photonic crystal maser 300 may be similar to the photonic crystal maser 200 described with respect to FIGS. 2A and 2B. The maser output passes through the output coupler 304. A second mode-matching structure 306 and a coupler (e.g., input coupler 302) are disposed at the other end of the dielectric of the photonic crystal maser 300. As shown, the input coupler 302 and the mode-matching structure 306 are designed to efficiently couple in a seed electromagnetic field (or reference RF electromagnetic radiation) from a waveguide.

[0053] Referring now to Figure 3B, a perspective schematic diagram of the example photonic crystal maser 300 of Figure 3A is presented, but with antenna 308 acting as the reference RF oscillator. In the illustrated case, input coupler 302 and mode matching structure are designed to efficiently couple in a seed electromagnetic field (or reference RF electromagnetic radiation) from antenna 308 in close proximity. Compared to Figure 3A, the configuration shown in Figure 3B allows the seed source (e.g., antenna 308) to be remote from example photonic crystal maser 300. Example photonic crystal maser 300 is designed to couple in a plane wave or a specific antenna gain pattern from antenna 308.

[0054] 3A and 3B contrast two different methods of seeding the example photonic crystal maser 300. In particular, the example photonic crystal maser 300 can be fed or mode-matched as a seed source to a waveguide (FIG. 3A) or a free-space antenna (FIG. 3B). However, other types of reference RF oscillators are also possible. For example, it is also possible to couple between the masers via free space or a waveguide. A conventional RF source can also be coupled to the antenna or waveguide. The reference RF oscillator can be referenced to or include a clock, such as a thermally stabilized quartz oscillator, a chip-scale atomic clock, a GPS signal, or other type of timing reference. The power of the reference RF oscillator can be adjusted until the seeding is sufficient to injection seed the example photonic crystal maser 300. In some examples, the example photonic crystal maser 300 produces output powers of nW to μW. In these examples, the injection power from the reference RF oscillator can be on the order of nW, or even less. The small amount of seed power, and the fact that the seed RF field does not need to be spectrally pure, allows a wide range of economical sources to be used to seed the photonic crystal maser example 300. A timing reference can generate a specific starting point for the DSP wave that can be used for the seed. Analog seeds can be generated in a similar manner.

[0055] In some implementations, the photonic crystal masers described herein may be constructed by creating a photonic crystal frame (or dielectric) and then bonding one or more optical windows to the frame. These components may be bonded together using techniques described in U.S. Pat. No. 10,859,981, entitled "Vapor Cells Having One or More Optical Windows Bonded to a Dielectric Body," and U.S. Pat. No. 11,137,432, entitled "Photonic Crystal Receivers," the disclosures of which are incorporated herein by reference in their entireties. For example, contact bonding methods may be used to bond the photonic crystal frame to one or both optical windows. Additionally, adhesive layers (e.g., adhesive layers formed of silicon or silicon dioxide) may be used to facilitate bonding the photonic crystal frame to one or both optical windows.

[0056] The photonic crystal frame and the optical window may be formed of a dielectric material. For example, the photonic crystal frame may be formed of silicon and the optical window may be formed of glass (e.g., borosilicate glass, fused silica glass, etc.). As another example, the photonic crystal frame, the optical window, or both may be formed of BaLn2Ti4O 12 (BLT), where Ln corresponds to one or more elements selected from the lanthanide elements. In many variations, the optical window is transparent to the laser light used to pump the maser. This transparency may be present throughout the entire optical window or may be limited to a portion of the optical window, such as the portion covering the elongated slot.

[0057] In some implementations, the vapor includes gas atoms, gas molecules, or both. The vapor, or generally the emitter, is located in an elongated slot centered on a defect (e.g., a line defect) in the photonic crystal. This location allows the vapor to experience a different electromagnetic mode environment than in free space. In many variations, the mode structure of the electromagnetic field is modified by the presence of the photonic crystal frame. The photonic crystal frame can also be designed to slow down the electromagnetic wave by increasing the group velocity index at and around a specific design frequency. Slowing down the electromagnetic wave can increase the stimulated emission rate of the emitter into that mode by increasing the energy density in the cavity relative to free space. The slowed down electromagnetic wave can be largely confined to the elongated slot, so that the electromagnetic field can be further enhanced (e.g., by modifying the resonant mode).

[0058] One advantage of this configuration is that the slowing and focusing of the electromagnetic field can lower the masing threshold by increasing the emission rate into the resonant modes. This lowering allows for maser operation at lower gain since the gain threshold is lowered. The local field modes at the resonant frequency can be designed to correspond to specific transition frequencies of vapors present within the elongated slot. In some examples, color centers or other materials can be placed within the elongated slot. The emission rate into these modes can be significantly increased relative to emission into other electromagnetic field modes, e.g., modes corresponding to directions perpendicular to the elongated slot or other frequencies.

[0059] In some examples, the maser is initiated by creating an inversion on a high-lying transition of an emitter placed inside an elongated slot, such as by laser excitation of a high-lying Rydberg state. For example, a pump laser can be coupled into the elongated slot using a mirror and an optical fiber. Spontaneous emission into the slot photonic crystal mode triggers an avalanche of stimulated emission into this mode to create the maser. The masing process can yield a coherent directed electromagnetic wave source at the design frequency. The ends of the elongated slot and the photonic crystal frame can be constructed as mirrors for the maser by varying the geometry of the photonic crystal. Furthermore, multiple passages of the wave through the elongated slot can be designed into a structure that allows for greater amplification of the electromagnetic wave. In some variations, a taper placed adjacent to the output mirror can be used to couple the output electromagnetic wave into free space and shape the output beam. Other structures for shaping the beam at the output, such as lenses, are also possible. The device can be amplitude modulated by modulating the pumping (e.g., modulating the intensity of the laser light).

[0060] In some implementations, low temperature contact bonding as described in U.S. Pat. Nos. 10,859,981 and 11,137,432 is used for vacuum sealing. One of the optical windows (or a fill hole in one of the optical windows) can be contact bonded such that the atomic sample remains pure. Other methods of bonding may require applying high temperatures and / or voltages to the vapor cell to cause significant outgassing that may impair the performance of the maser through collisions. In certain cases, a small stem may be used to fill the vapor cell. In these cases, the optical window can be anodically bonded to the frame. The structure is made of all dielectric materials.

[0061] In some implementations, photonic crystal masers can produce coherent radiation in the radio frequency (RF) regime (e.g., 100 MHz to 1 THz) and as such can function similarly to lasers emitting radio frequency (RF) electromagnetic radiation. In some variations, the regime ranges from 20 kHz to 1 THz. In other variations, the regime ranges from 20 kHz to 300 GHz. Photonic crystal masers can include a monolithic photonic crystal frame and a vapor cell integrated into the frame to build a maser capable of producing up to about 100 nW of power. Although the power is low, the device may be fabricated from dielectric materials and can be used to produce directed RF signals for wireless testing and communication. The radiation beam is directional and coherent, allowing long distance propagation without the spatial spread normally produced by antennas. Such spatial spread reduces the R of the beam intensity. -2 where R is the propagation distance. The dielectric structure of the photonic crystal maser allows it to be located in an anechoic chamber and produce modulated RF signals for testing without significantly disturbing the environment. Photonic crystal masers can also be used for timing and frequency references, for RF spectroscopy, or as local oscillators in combination with Rydberg atom-based receivers or other types of conventional receivers.

[0062] In some examples, a photonic crystal maser includes a photonic crystal structure (or frame) and fiber optic circuitry for channel pumping laser light into elongated slots of the photonic crystal structure. The photonic crystal maser may further include one or more pump lasers and electronics to control these lasers. The electronics may also control output couplers, or other structures, to shape (e.g., collimate) the output beam from the pump laser and impedance match the output beam to the propagation medium.

[0063] In some aspects of what is described, the photonic crystal maser can be described by the following example. Example 1. A photonic crystal maser comprising: A dielectric material comprising: an array of cavities arranged periodically to define a photonic crystal structure within a dielectric; a region within the cavity array that defines a defect within the photonic crystal structure; an elongated slot through region extending from a slot opening in a surface of the dielectric at least partially through the dielectric, the elongated slot and the cavity array defining a waveguide in the dielectric; and an input coupler aligned with an end of the elongated slot and configured to couple a reference radio frequency (RF) electromagnetic radiation into the waveguide, the reference RF electromagnetic radiation having one or both of a controlled frequency and a controlled phase; a dielectric material comprising: an optical window having a window surface covering the elongated slot and adhered to a surface of the dielectric to form a seal around the slot opening; A steam or steam source in an elongated slot, the steam being: one or more input electronic transitions, and an output electronic transition coupled to one or more input electronic transitions and operable to emit a targeted RF electromagnetic radiation, the output electronic transition resonating with a guided mode of the waveguide; a source of steam or vapor in an elongated slot, A photonic crystal maser comprising: Example 2. A photonic crystal maser as described in Example 1, wherein the reference RF electromagnetic radiation resonates with a guided mode of the waveguide. Example 3. The photonic crystal maser of example 1 or example 2, wherein the input coupler extends from a side of the dielectric and terminates at a tapered end. Example 4. The photonic crystal maser of any one of Example 1 or Examples 2-3, comprising a reference RF oscillator configured to produce reference RF electromagnetic radiation, the reference RF oscillator being electromagnetically coupled to a dielectric input coupler. Example 5. A photonic crystal maser as described in Example 1 or any one of Examples 2-4, comprising a laser configured to generate an optical signal capable of exciting one or more input electronic transitions of the vapor. Example 6. The photonic crystal maser of Example 1 or any one of Examples 2-5, wherein the photonic crystal structure is configured to concentrate the target RF electromagnetic radiation within the elongated slot. Example 7. The regions in the cavity array extend along an axis and the elongated slots are aligned parallel to the axis; the photonic crystal structure is configured to reduce a group velocity of the target RF electromagnetic radiation along a direction parallel to the axis; The photonic crystal maser according to any one of Example 1 or Examples 2 to 6. Example 8. The photonic crystal maser of example 1 or any one of examples 2-7, wherein the cavity array comprises one or more offset cavities that are spatially offset from an ideal periodic position within the array. Example 9. The photonic crystal maser of example 1 or any one of examples 2-8, comprising an optical mirror disposed at an end of the elongated slot. Example 10. The photonic crystal maser of Example 1 or any one of Examples 2-9, comprising an output coupler, the dielectric material aligned at a second end of the elongated slot and configured to impedance match the target RF electromagnetic radiation to an ambient environment of the photonic crystal maser. Example 11. The photonic crystal maser of example 10, wherein the output coupler extends from a side of the dielectric and terminates at a tapered end. Example 12. The photonic crystal maser of example 1 or any one of examples 2-11, wherein the photonic crystal structure defines a photonic bandgap associated with a transverse magnetic (TM) mode of the target RF electromagnetic radiation within the waveguide. Example 13. The photonic crystal maser of example 1 or any one of examples 2-12, wherein the photonic crystal structure defines a photonic bandgap associated with a transverse electric (TE) mode of the target RF electromagnetic radiation within the waveguide. Example 14. The photonic crystal maser of example 1 or any one of examples 2-13, wherein the vapor comprises a gas of alkali metal atoms. Example 15. A region forms a loop in the cavity array, and the elongated slot extends along the loop axis of the loop to form a looped slot; a steam or steam source is disposed within at least a portion of the annular slot; The photonic crystal maser according to any one of Example 1 or Examples 2 to 14. Example 16. A looped slot is associated with a first loop direction and a second loop direction along a loop axis, the first loop direction being opposite to the second loop direction; the looped slot includes a first RF port and a second RF port; Photonic crystal maser a first directional coupler and a second directional coupler coupled to the first RF port and the second RF port, respectively, the first directional coupler configured to receive a first portion of the target RF electromagnetic radiation traveling along a first loop direction, and the second directional coupler configured to receive a second portion of the target RF electromagnetic radiation traveling along a second loop direction; The photonic crystal maser according to Example 15.

[0064] In some aspects of what is described, a method of generating radio frequency (RF) electromagnetic radiation may be described by the following example. Example 17. A method for generating radio frequency (RF) electromagnetic radiation, the method comprising: receiving a reference RF electromagnetic radiation at a dielectric input coupler; Dielectrics are part of the photonic crystal maser an array of cavities arranged periodically to define a photonic crystal structure within a dielectric; a region within the cavity array that defines a defect within the photonic crystal structure; an elongated slot through a region extending from a slot opening in a surface of the dielectric to at least partially through the dielectric, the elongated slot and the cavity array defining a waveguide in the dielectric; and an input coupler aligned with an end of the elongated slot and configured to couple reference RF electromagnetic radiation into the waveguide; Equipped with Photonic crystal maser an optical window having a window surface covering the elongated slot in the dielectric and adhered to a surface of the dielectric to form a seal around the slot opening; sealed within an elongated slot in the dielectric; one or more input electronic transitions excited by the optical signal; and an output electronic transition coupled to one or more input electronic transitions and operable to emit a targeted RF electromagnetic radiation, the output electronic transition resonating with a guided mode of the waveguide; Steam containing and receiving an optical signal into an elongated slot of a dielectric material; emitting a target RF electromagnetic radiation from the vapor in response to receiving the optical signal, the target RF electromagnetic radiation being based on a reference RF electromagnetic radiation; A method comprising: Example 18. The method of example 17, wherein the step of receiving the reference RF electromagnetic radiation includes coupling the reference RF electromagnetic radiation to the dielectric waveguide by operation of an input coupler. Example 19. The method of example 17 or example 18, wherein the target RF electromagnetic radiation has one or both of a frequency and a phase set by one or both of a frequency and a phase of a reference RF electromagnetic radiation, respectively. Example 20. The method of any one of Example 17 or Examples 18-19, wherein the reference RF electromagnetic radiation resonates with a guided mode of the waveguide. Example 21. Generating Reference RF Electromagnetic Radiation by Operation of a Reference RF Oscillator The method according to any one of Example 17 or Examples 18 to 20, comprising: Example 22. The method of example 21, wherein the step of generating the reference RF electromagnetic radiation includes controlling one or both of a frequency and a phase of the reference RF electromagnetic radiation. Example 23. Amplifying targeted RF electromagnetic radiation by resonating at least a portion of the targeted RF electromagnetic radiation between an output electron transition and a guided wave mode. The method according to any one of Example 17 or Examples 18 to 22, comprising: Example 24. The method of example 23, wherein the step of amplifying the target RF electromagnetic radiation includes concentrating the target RF electromagnetic radiation within an elongated slot. Example 25. The regions in the cavity array extend along an axis and the elongated slots are aligned parallel to the axis; the step of amplifying the targeted RF electromagnetic radiation includes reducing a group velocity of the targeted RF electromagnetic radiation along a direction parallel to the axis; The method according to Example 23 or Example 24. Example 26. A cavity array comprising one or more offset cavities spatially offset from an ideal periodic position within the array; the step of amplifying the target RF electromagnetic radiation includes reflecting the target RF electromagnetic radiation into an offset cavity; The method according to any one of Example 23 or Examples 24-25. Example 27. The method of any one of Example 17 or Examples 18-26, wherein the step of receiving an optical signal into the elongated slot includes interacting the optical signal with vapor in the elongated slot. Example 28. The method of any one of Example 17 or Examples 18-27, wherein the step of receiving an optical signal into the elongated slot includes reflecting the optical signal off a mirror disposed at an end of the elongated slot. Example 29. Directing target RF electromagnetic radiation toward an end of a dielectric body by operation of a waveguide. The method according to any one of Example 17 or Examples 18 to 28, comprising: Example 30. A dielectric material is aligned with a second end of the elongated slot and includes an output coupler configured to impedance match the target RF electromagnetic radiation to an ambient environment of the photonic crystal maser; The method further comprising: coupling the emitted or amplified target RF electromagnetic radiation to an output coupler; impedance matching the coupled target RF electromagnetic radiation to a dielectric surrounding by operation of the output coupler; Including, The method according to Example 17 or any one of Examples 18-29.

[0065] While this specification contains many details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular examples. Certain features described herein or shown in the drawings in the context of separate implementations can be combined. Conversely, various features that are described or shown in the context of a single implementation can also be implemented separately in many embodiments or in any suitable subcombination.

[0066] Similarly, although operations are shown in a particular order in the figures, this should not be understood as a requirement that such operations be performed in the particular order or sequence shown, or that all of the operations shown be performed, to achieve desired results. In certain situations, multitasking and parallel processing may be advantageous. Furthermore, it should be understood that the separation of various system components in the implementations described above does not require such separation in all implementations, and that the program components and systems described may generally be integrated together in a single product or packaged into multiple products.

[0067] Although several embodiments have been described, it should be understood that various modifications may be made and, therefore, other embodiments are within the scope of the following claims.

Claims

1. A photonic crystal maser comprising: A dielectric material comprising: an array of cavities arranged periodically to define a photonic crystal structure within said dielectric; a region within the cavity array that defines a defect in the photonic crystal structure; an elongated slot through the region extending from a slot opening in a surface of the dielectric at least partially through the dielectric, the elongated slot and the cavity array defining a waveguide in the dielectric; and an input coupler aligned with an end of the elongated slot and configured to couple a reference radio frequency (RF) electromagnetic radiation into the waveguide, the reference RF electromagnetic radiation having one or both of a controlled frequency and a controlled phase; A dielectric material comprising: an optical window having a window surface covering the elongated slot and adhered to the surface of the dielectric to form a seal around the slot opening; A steam or a source of steam within the elongated slot, the steam comprising: one or more input electronic transitions; and an output electronic transition coupled to the one or more input electronic transitions and operable to emit targeted RF electromagnetic radiation, the output electronic transition resonating with a guided mode of the waveguide; a source of steam or said steam in said elongated slot, A photonic crystal maser comprising:

2. The photonic crystal maser of claim 1 , wherein said reference RF electromagnetic radiation is resonant with said guided mode of said waveguide.

3. The photonic crystal maser of claim 1 , wherein said input coupler extends from a side of said dielectric body and terminates in a tapered end.

4. 4. The photonic crystal maser of claim 1, further comprising a reference RF oscillator configured to produce the reference RF electromagnetic radiation, the reference RF oscillator being electromagnetically coupled to the dielectric input coupler.

5. A photonic crystal maser according to any one of claims 1 to 3, comprising a laser configured to generate an optical signal capable of exciting the one or more input electronic transitions of the vapour.

6. The photonic crystal maser of any one of claims 1 to 3, wherein the photonic crystal structure is configured to concentrate the target RF electromagnetic radiation within the elongated slot.

7. the regions in the cavity array extend along an axis and the elongated slots are aligned parallel to the axis; the photonic crystal structure is configured to reduce a group velocity of the target RF electromagnetic radiation along a direction parallel to the axis. The photonic crystal maser according to any one of claims 1 to 3.

8. A photonic crystal maser according to any one of claims 1 to 3, wherein the cavity array comprises one or more offset cavities that are spatially offset from an ideal periodic position within the array.

9. A photonic crystal maser according to any one of claims 1 to 3, comprising optical mirrors disposed at the ends of said elongated slot.

10. 4. The photonic crystal maser of claim 1, further comprising an output coupler aligned with a second end of the elongated slot and configured to impedance match the target RF electromagnetic radiation to an ambient environment of the photonic crystal maser.

11. The photonic crystal maser of claim 10 , wherein the output coupler extends from a side of the dielectric body and terminates in a tapered end.

12. The photonic crystal maser of any one of claims 1 to 3, wherein the photonic crystal structure defines a photonic bandgap within the waveguide associated with a transverse magnetic (TM) mode of the target RF electromagnetic radiation.

13. The photonic crystal maser of any one of claims 1 to 3, wherein the photonic crystal structure defines a photonic bandgap within the waveguide associated with a transverse electric (TE) mode of the target RF electromagnetic radiation.

14. The photonic crystal maser of any one of claims 1 to 3, wherein the vapor comprises a gas of alkali metal atoms.

15. the regions form loops within the cavity array, the elongated slots extending along loop axes of the loops to form looped slots; the steam or the source of steam is disposed within at least a portion of the annular slot; The photonic crystal maser according to any one of claims 1 to 3.

16. the looped slots are associated with a first loop direction and a second loop direction along the loop axis, the first loop direction being opposite the second loop direction; the looped slot includes a first RF port and a second RF port; The photonic crystal maser comprises: a first directional coupler and a second directional coupler coupled to the first RF port and the second RF port, respectively, the first directional coupler configured to receive a first portion of the target RF electromagnetic radiation traveling along the first loop direction, and the second directional coupler configured to receive a second portion of the target RF electromagnetic radiation traveling along the second loop direction.

16. The photonic crystal maser of claim 15.

17. 1. A method for generating radio frequency (RF) electromagnetic radiation, the method comprising: receiving a reference RF electromagnetic radiation at a dielectric input coupler; the dielectric is part of a photonic crystal maser; an array of cavities arranged periodically to define a photonic crystal structure within said dielectric; a region within the cavity array that defines a defect in the photonic crystal structure; an elongated slot through the region extending from a slot opening in a surface of the dielectric to at least partially through the dielectric, the elongated slot and the cavity array defining a waveguide in the dielectric; and the input coupler aligned with an end of the elongated slot and configured to couple the reference RF electromagnetic radiation into the waveguide; Equipped with The photonic crystal maser comprises: an optical window having a window surface covering the elongated slot in the dielectric and adhered to a surface of the dielectric to form a seal around the slot opening; sealed within the elongated slot of the dielectric; one or more input electronic transitions excited by the optical signal; and an output electronic transition coupled to the one or more input electronic transitions and operable to emit targeted RF electromagnetic radiation, the output electronic transition resonating with a guided mode of the waveguide; Steam containing and receiving the optical signal into the elongated slot of a dielectric material; emitting the target RF electromagnetic radiation from the vapor in response to receiving the optical signal, the target RF electromagnetic radiation being based on the reference RF electromagnetic radiation; A method comprising:

18. 20. The method of claim 17, wherein receiving the reference RF electromagnetic radiation comprises coupling the reference RF electromagnetic radiation to the dielectric waveguide by operation of the input coupler.

19. 20. The method of claim 17, wherein the target RF electromagnetic radiation has one or both of a frequency and a phase set by one or both of a frequency and a phase, respectively, of the reference RF electromagnetic radiation.

20. The method of any one of claims 17 to 19, wherein the reference RF electromagnetic radiation is resonant with the guided mode of the waveguide.

21. generating said reference RF electromagnetic radiation by operation of a reference RF oscillator; The method according to any one of claims 17 to 19, comprising:

22. 22. The method of claim 21, wherein generating the reference RF electromagnetic radiation comprises controlling one or both of a frequency and a phase of the reference RF electromagnetic radiation.

23. amplifying the targeted RF electromagnetic radiation by resonating at least a portion of the targeted RF electromagnetic radiation between the output electron transition and the waveguide mode. The method according to any one of claims 17 to 19, comprising:

24. 24. The method of claim 23, wherein the step of amplifying the target RF electromagnetic radiation comprises concentrating the target RF electromagnetic radiation within the elongated slot.

25. the regions in the cavity array extend along an axis and the elongated slots are aligned parallel to the axis; the step of amplifying the target RF electromagnetic radiation includes slowing a group velocity of the target RF electromagnetic radiation along a direction parallel to the axis.

24. The method of claim 23.

26. the cavity array comprising one or more offset cavities spatially offset from an ideal periodic position within the array; the step of amplifying the target RF electromagnetic radiation includes reflecting the target RF electromagnetic radiation into the offset cavity.

24. The method of claim 23.

27. The method of any one of claims 17 to 19, wherein receiving the optical signal into the elongated slot comprises allowing the optical signal to interact with the vapor in the elongated slot.

28. 20. The method of claim 17, wherein the step of receiving the optical signal into the elongated slot comprises reflecting the optical signal off a mirror located at an end of the elongated slot.

29. directing the target RF electromagnetic radiation toward an end of the dielectric body by operation of the waveguide. The method according to any one of claims 17 to 19, comprising:

30. an output coupler, the dielectric member aligned with a second end of the elongated slot and configured to impedance match the target RF electromagnetic radiation to an ambient environment of the photonic crystal maser; The method further comprising: coupling the emitted or amplified target RF electromagnetic radiation to the output coupler; impedance matching the coupled target RF electromagnetic radiation to the dielectric surroundings by operation of the output coupler. Including, The method according to any one of claims 17 to 19.

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