Thermal laser with dynamic beam control

Abstract

A thermal laser is provided that emits narrowband, coherent infrared (IR), mid-infrared (MIR), or visible radiation with an adjustable emission angle. The laser includes a metasurface that produces a narrowband, coherent thermal radiation lobe at a desired frequency. The thermal radiation is coupled with an oscillating Fabry-Perot (FP) resonant mode of a Fabry-Perot cavity having an electrostatically or thermally tunable reflective phase shift at the metasurface. This thermal laser design enables the emission of radiation lobes to be controlled continuously through a range of angles by modulating the phase shift at the metasurface.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to U.S. Non - Provisional Patent Application No. 18 / 146,600, filed on December 27, 2022, the entire content of which is incorporated herein by reference. Background of the Invention

[0003] The mid - infrared (MIR) region is an important optical band for applications ranging from free - space laser communication to chemical sensing applications. These applications require narrow - band light sources with high - speed direction control of the emitted light. Generally, beam control can be achieved with mechanical devices (such as gimbal mirrors), optical phased - array antennas that can control the relative phase of each element, or more recently, liquid - crystal - based geometries. While each technique has its own advantages and disadvantages, they all share a common limitation: they require external light sources, such as bulky and expensive quantum - cascade lasers.

[0004] Alternative sources of MIR light are ubiquitous: thermal radiation. Due to the motion of charged particles, any material at a non - zero temperature will emit radiation over a broad frequency range. Thermal radiation from common materials will be incoherent, isotropic, and broadband, making it a poor choice for narrow - band, direction - controlled applications. However, recent advances in nano - engineering suggest that it may be possible to engineer the emissivity of structural materials to produce narrow - band, anisotropic, or coherent thermal radiation sources. (See, e.g., C.W. Hsu et al., Nature Reviews Materials 1, 16048 (2016); J.-J. Greffet et al., Nature 416, 61 - 64 (2002).) All that is necessary to generate the desired light is to heat the sample, thus providing an efficient MIR radiation source. However, the above examples are static and cannot be dynamically adjusted.

[0005] Recently, graphene has been considered a candidate material for thermal - engineering devices to achieve active control of thermal emission. (V.W. Brar et al., Nature Communications 6, 7032 (2015).) Graphene, a two - dimensional lattice of carbon atoms, can undergo a significant change in its optical permittivity in the MIR by changing its charge - carrier density via electronic control. (V.W. Brar et al., Nano Letters 13, 2541 - 2547 (2013).) Theoretical predictions and experimental demonstrations show that graphene can dynamically regulate black - body emission; however, angle regulation at a constant amplitude has not been demonstrated. (Brar et al., 2015.) Summary of the Invention

[0006] Thermal lasers are provided that emit narrowband, coherent radiation (including infrared (IR), mid-infrared (MIR), and / or visible light radiation) with an adjustable emission angle. Methods of using the lasers are also provided.

[0007] One embodiment of a thermal laser includes: a back reflector; a metasurface; and a dielectric spacer disposed between the back reflector and the metasurface. The metasurface includes: a phase-shifting dielectric layer having an electrically or thermally tunable Fermi level or refractive index; and a planar array of metal elements disposed on the phase-shifting dielectric layer in a periodic arrangement. The laser further includes: conductive contacts configured to apply a voltage across the phase-shifting dielectric or a phase-shifting dielectric heater in thermal communication with the phase-shifting dielectric; and a dielectric spacer heater in thermal communication with the dielectric spacer.

[0008] One embodiment of a method for using a thermal laser of the type described herein to generate a steerable thermal laser beam includes the steps of: heating the dielectric spacer to generate thermal radiation, wherein the thermal radiation couples to an oscillating Fabry-Perot resonance mode in the dielectric spacer to generate a coherent radiation lobe at the emission angle; and applying a voltage across the phase-shifting dielectric layer or changing the temperature of the phase-shifting dielectric, thereby changing the emission angle of the coherent radiation lobe.

[0009] After reading the following drawings, detailed description, and appended claims, other principal features and advantages of the invention will become apparent to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Exemplary embodiments of the invention will be described hereinafter with reference to the drawings, wherein like numerals represent like elements.

[0011] Figure 1A is a schematic diagram of one embodiment of a thermal laser. Figure 1B Shows Figure 1A a detailed schematic diagram of the design parameters of the thermal laser. Figure 1C Shows Figure 1A and Figure 1B a cross-section of the thermal laser. Figure 1D Shows an illustration of the operation of a thermal laser. Thermal dipole emission in the material couples to radiation that resonates in a Fabry-Perot resonator, which is confined by the metasurface on at least one side. The metasurface has a variable reflection phase that is controlled by gap plasmons coupled to an embedded graphene sheet. Figure 1E Shows a scanning electron microscope (SEM) image (top view) of the thermal laser.

[0012] Figures 2A - 2B Shows the experimental emission results of a thermal laser based on a graphene / gold nanoresonator metasurface as described in the examples. Figure 2AShows the emissivity tunability as a function of the applied voltage, the thermal radiation emitted from the carbon nanotubes, and the blackbody (BB) reference. Figure 2B Shows the angular dependence of the emissivity at a constant applied voltage. All measurements were carried out at 250 °C, and assuming that the blackbody reference has an emissivity equal to one.

[0013] Figure 3 Shows the experimental tuning of the emission lobes of an exemplary thermal laser for three applied voltages.

[0014] Figure 4A Shows the calculated values of φ as a function of the Fermi level. m of. Figure 4B and Figure 4C Shows the calculated absorption ( Figure 4B ) frequency and angular spectrum ( Figure 4C ) corresponding to Fermi levels of 0.48 eV and 0.622 eV for 0 V and -560 V, respectively. In the angular absorption spectrum calculation, the operating frequency was fixed at 1498 cm -1 . The carrier mobility of graphene was assumed to be 300 cm 2 / V·s. DETAILED DESCRIPTION

[0015] Provided is a thermal laser that emits narrowband, coherent infrared (IR) (including mid-infrared (MIR)) and / or visible radiation with a tunable emission angle. The laser includes a dielectric medium that generates thermal radiation and a metasurface that enables the thermal radiation to be emitted in the form of narrowband, coherent radiation lobes at desired frequencies and angles. The thermal radiation in the dielectric medium couples with the oscillating Fabry - Perot (FP) resonance modes in the dielectric medium, where the FP resonance conditions can be electrically or thermally tuned by changing the reflection phase shift at the metasurface. This FP cavity design enables the control of the emitted radiation lobes to continuously pass through an angular range by modulating the phase shift at the metasurface. As a result, the thermal laser provides an efficient, tunable, and narrowband radiation source that can operate without the need for mechanical devices, optical phased arrays of antennas, or liquid crystals.

[0016] Figures 1A - 1C is a schematic diagram of an exemplary embodiment of a thermal laser. The laser includes a dielectric spacer 102 sandwiched between a back reflector 104 and an upper metasurface 106 that provides a top reflector. The metasurface 106 includes a planar array of metal elements 107 arranged in a periodic pattern, and a thin layer of phase - shift medium 108 disposed between the metal elements 107 and the dielectric spacer 102. The metasurface 106, the dielectric spacer 102, and the back reflector 104 together form an FP cavity. The phase - shift medium 108 is characterized in that it has an electrically and / or thermally tunable Fermi level and / or refractive index, which can be modulated to change the phase of the radiation reflected at the metasurface 106. For illustrative purposes,Figure 1A and Figure 1B Suitable materials for each layer of the thermal laser are shown. However, it should be understood that these materials are only used as examples, and according to the operating principle of the device described below, other materials can be used for each layer.

[0017] In Figure 1A and Figure 1B the illustrated embodiment of the thermal laser, the phase shift medium 108 has an electrically tunable Fermi level. Thus, Figure 1A and Figure 1B the thermal lasers in include conductive contacts 110 and 112 that are configured to apply a voltage across the phase shift medium. In the exemplary embodiment of the device shown here, the back reflector 104 also serves as a back gate electrode (i.e., a conductive contact). Embodiments of devices utilizing a phase shift medium with a thermally tunable refractive index will include a heat source, such as a heating element (not shown), in thermal communication with the phase shift medium 108 to modulate the temperature of the phase shift medium. For the purposes of this disclosure, a heat source in thermal communication with the phase shift medium is referred to as a phase shift medium heater. The thermal laser also includes a heat source (not shown) in thermal communication with the dielectric spacer 102 to heat the dielectric spacer and generate thermal radiation. For the purposes of this disclosure, a heat source in thermal communication with the dielectric spacer is referred to as a dielectric spacer heater.

[0018] The metal elements 107 of the metasurface 106 are nanoresonators that enable constructive interference between thermal dipole radiations that are in phase with the oscillating FP resonance modes at variable angles supported by the FP cavity. This constructive interference enables the directional emission of narrowband, coherent radiation from the laser in angular lobes 119. The size and spacing of the metal elements 107 determine the emission wavelength of the emission lobes, while the FP cavity resonance conditions determine the emission angle of the emission lobes. In Figure 1A and Figure 1B the lasers, the metal elements include parallel metal strip gratings separated by subwavelength gaps. However, other metal element geometries that produce coherent directional angular radiation emission can be used. Gold is a non-limiting example of a metal that can be used to form the metal elements 107.

[0019] Based on the operating principle of the device described in more detail below, the phase-shifting medium can be selected from a variety of two-dimensional (2D) or three-dimensional (3D) materials, as long as the material can produce an electrically or thermally tunable reflection phase shift at the metasurface. Graphene is an example of a 2D material with an electrically tunable Fermi level that can be used as the phase-shifting medium. Other materials that experience a change in Fermi level and / or refractive index in response to a change in applied bias or temperature include, but are not limited to, indium tin oxide (ITO), indium zinc oxide (IZO), titanium nitride, vanadium dioxide (VO2), germanium antimony telluride (GST), and titanium nitride. Electro-optic polymers (organic polymers that change their refractive index when an external voltage is applied) can also be used. The 2D phase-shifting medium can include a single layer of 2D material, such as single-layer graphene. However, multilayer 2D materials can also be used.

[0020] The dielectric spacer 102 acts as a thermal emitter and can include a variety of dielectric materials, since according to Planck's law, all objects at a non-zero temperature will emit thermal radiation in the electromagnetic spectrum, where the exact wavelength and intensity depend on the temperature of the object. Silicon nitride, aluminum oxide, and diamond are several non-limiting examples of dielectric materials that can be used. The temperature to which the dielectric material is heated will depend on the material and the intended application of the thermal laser. Typically, the spacer will be heated to a temperature above room temperature (e.g., greater than 25 °C), and more typically, a temperature of at least 200 °C. By way of example only, temperatures in the range from about 250 °C to about 500 °C are generally suitable. However, temperatures outside this range can also be used.

[0021] For device simplicity, the dielectric spacer can act as both an FP cavity and a dielectric gate for electrostatic gating of the phase-shifting medium. However, this device design may require the use of undesirably high voltages to gate the phase-shifting medium and unnecessarily limits the achievable tuning range. Therefore, a separate dielectric gate material, such as a doped semiconductor, can be used to achieve electrostatic gating with significantly smaller voltages, allowing for a larger doping range and thus a larger angular emission tuning range.

[0022] The layers constituting the thermal laser can be in direct contact or can be separated by one or more additional layers of material that protect the thermal laser layers from mechanical damage and / or chemical degradation and / or facilitate the fabrication of the device, as long as the additional layer does not interfere with the operation of the laser. For example, thin metal oxide layers such as aluminum oxide (Al2O3) 114 and / or hafnium oxide (HfO2) 116 can be present above and / or below the phase-shifting medium layer to provide mechanical support and / or protect the phase-shifting medium from contamination during processing.

[0023] The Fermi level or refractive index of the phase-shifting medium 108 determines the angle of the emission lobe 119. Thus, by adjusting the Fermi level or refractive index of the phase-shifting medium 108, the emission angle can be changed. This modulation is enhanced by the sub-wavelength dielectric gap (g) between the metal element 107 that supports the gap plasmon mode and enhances the electric field. (As used herein, the term "sub-wavelength" refers to a wavelength less than the emission wavelength.) These enhanced electric fields enhance the light-matter interaction between the FP cavity resonance mode and the phase-shifting medium 108.

[0024] The operation of the thermal laser is schematically shown in Figure 1D Heating the dielectric spacer 102 enhances its thermal emission due to the oscillation of dipoles in the material. The thermal radiation couples to the oscillating Fabry - Perot resonance mode of the FP cavity and forms a gap plasmon when interacting with the nanoresonator 107, which strongly couples the thermal radiation to the phase-shifting medium 108. As a result, the metasurface emits thermal radiation in an emission lobe centered at a given emission angle. The emission angle depends on the reflection phase at the metasurface 106, which can be modulated by modulating the Fermi level and / or refractive index of the phase-shifting medium 108.

[0025] In the case of an electronically tunable phase-shifting medium such as graphene, the emission angle of the lobe is controlled by electronically modulating the carrier density and thus the Fermi level of the phase-shifting medium incorporated into the metasurface. When the Fermi level is adjusted by applying an external voltage, the phase shift of the reflection at the metasurface 106 is adjusted. This changes the resonance condition of the FP cavity, thus selecting a resonance mode with a transverse component and providing an adjustable emission angle for the laser. The angular dependence of the emissivity will depend on the specific materials used to construct the metasurface and the FP cavity. However, generally, for a given bias voltage or temperature, the angular emission lobe will be centered about the normal direction and deviate from the normal direction by changing the bias voltage or temperature of the phase-shifting medium.

[0026] Starting from Kirchhoff's law, the emission angle regulation can be explained in more detail. Kirchhoff's law states that changing the absorptivity of a thermal emitter by designing the optical structure is equivalent to changing the emissivity to obtain the desired spatial and temporal thermal emission spectra. Directional thermal absorption / emission can be obtained through constructive interference between adjacent antenna elements in the device, which correspond to thermally excited dipoles in the thermal emitter. However, the thermal dipoles are excited at random spatial and temporal positions and are not as well-ordered as required for constructive interference. To obtain directional thermal emission, the main emission path should be a spatially non-localized resonant optical mode such that only the emitted thermal dipoles couple to this resonant optical mode and are in phase with it. All other dipoles will not emit radiation outside the device, thus keeping the energy within the thermal emitter. In the thermal laser disclosed herein, the spatially non-localized resonant optical mode is provided by the oscillation of the FP resonance in the thick dielectric spacer 102 sandwiched between the back reflector 104 and the metasurface 106. The emission pattern of the emission angle lobe is modulated by shifting the phase of the reflection at the interface of the phase-shifting medium, which modulates the resonance condition of the FP cavity.

[0027] Resonant emission of the FP mode occurs when the out-of-plane wavevector k out satisfies the constructive interference condition,

[0028] 2k out h + φ m + φ b = 2π * m (1)

[0029] where φ m and φ b are the phase shifts of the propagating optical mode upon reflection at the metasurface and the back reflector, respectively, h is the thickness of the dielectric spacer, and m is an integer. In the infrared range, the metal thin film back reflector acts as a perfect electrical conductor, resulting in φ b ≈ π. The momentum matching condition for free space light can be derived from Snell's law as:

[0030] k out = cos(θ)nk free (2)

[0031] where θ is the emission angle, n is the refractive index of the dielectric, and k free is the free space wavevector. A larger free space momentum requires a larger θ to satisfy the momentum matching condition. Both φ b and k out h are independent of the Fermi level of the phase-shifting medium. In contrast, the reflection phase shift φ m of the metasurface depends on the optical conductivity of the phase-shifting medium.

[0032] In Figures 1A - 1CIn a thermal laser, the metal element 107 forms gap plasmon polaritons, which strongly enhance the light-matter interaction at the phase-shifting dielectric interface, leading to a deviation from the reflection of a perfect metal mirror. As the Fermi level of the phase-shifting dielectric increases, the optical loss of the gap plasmon polaritons also increases, resulting in a further deviation of the reflection phase from that of a perfect mirror, thus changing φ m . It should be noted that the source of the reflection phase shift at the metasurface is not due to plasmon polaritons in the phase-shifting dielectric; rather, this is due to the strong interaction of the FP resonance modes at the phase-shifting dielectric. Thus φ m 's change leads to a change in the emission angle θ to maintain the constructive interference condition (Equation 1).

[0033] The operation of a thermal laser using a thermally tunable phase-shifting dielectric is similar to that of a thermal laser using an electronically tunable phase-shifting dielectric. In the case of a thermally tunable phase-shifting dielectric, the emission angle of the lobe is controlled by the thermal modulation of the refractive index of the phase-shifting dielectric incorporated into the metasurface. When the refractive index is adjusted by changing the temperature of the phase-shifting dielectric, the phase shift of the reflection at the metasurface 106 is adjusted. This changes the resonance condition of the FP cavity and provides a tunable emission angle for the laser.

[0034] In the following example, a thermal laser with a tunable emission angle is described using graphene as an exemplary phase-shifting dielectric. The thermal laser of this example is designed to maximize the signal tuning around 1500 / cm. However, the design principle of the thermal lasers described herein is not limited to this specific wave number and can be applied across the mid-infrared spectrum and beyond. This includes but is not limited to thermal lasers having emission wavelengths in the range from 3 μm to 40 μm. Similarly, although the exemplary thermal laser exhibits a tunable emission angle of ±16° with respect to the normal, the thermal laser is not limited to this specific angular range. Larger angular ranges can be achieved, including ±20°, ±25°, ±30° or larger ranges.

[0035] Example

[0036] This example experimentally demonstrates the active control of angular thermal emission in a continuous range of up to + / -16° at 1500 cm Figures 1A - 1E using the device of -1 .

[0037] Device fabrication.

[0038] The device used in this example consists of 30 nm thick, 1 micron wide gold nanoresonators (metal elements) separated by 40 nm on top of a material stack consisting of 5 nm of HfO2, graphene flakes (phase-shifting medium), 30 nm of Al2O3, and a 2 micron thick SiN film (dielectric spacer). The SiN film has a gold back reflector which also serves as a back gate electrode. A bilayer of 100 nm thick silicon oxide (SiO x ) and 30 nm of gold is within the gap between the gold nanoresonators, which is the result of a negative resist used via electron beam lithography to pattern the structure. The patterned area size is 4 mm x 4 mm, and the film is supported on a 200 micron thick Si frame. Electrical contact to the graphene is achieved by wire bonding to gold electrodes separated by a thin HfO2 protective layer.

[0039] To measure the thermal emission of the active region, the sample (with electrical connections for gating) is placed on a heating stage with positioning and rotation control. The acceptance angle of the emitted light is 3°, and a polarizer is used since the signal is polarized along the gold nanoresonators.

[0040] Tunable emission measurement of the thermal emitter.

[0041] Figure 2A (Left axis) shows the emissivity at normal incidence at 250 °C of a device with two doped graphene Fermi levels doped via the back gate geometry (see Figure 1A ). The emissivity of the structure is calculated by normalizing the emitted radiation of the sample to the emitted radiation of a reference carbon nanotube blackbody (see right axis). By electrostatically tuning the Fermi level of the graphene from 0.3 eV to 0.6 eV, the emission peak redshifts from 1500 cm -1 to 1450 cm -1 , thus showing that the thermal emission peak is widely tunable with little change in intensity.

[0042] To study the angular dependence characteristics of these emission peaks, the sample is rotated and the variation of the emissivity as a function of the emission angle is measured, as seen in Figure 2B . These measurements are for a constant doping value and temperature. By increasing the emission measurement angle from 0° to 30°, a blueshift is observed in the thermal radiation characteristics, thus confirming the directional thermal emission of the structure. The slight decrease in the peak intensity at larger angles (30°) is partly due to the extension of the measurement region at larger angles to include some low-emissivity, unpatterned gold regions, thus reducing the apparent emission peak amplitude.

[0043] Figure 3 Plotted for the wave number 1508 -1Angular-dependent emissivity of the hybrid graphene-gold metasurface as a function of the doping value. The emission peaks form lobes in the range from 0° at high doping to 16° at low doping. For a doping value of 0.6 eV, the emission peak is observed to be strongest at normal incidence and decreases in intensity with increasing angle. As the Fermi level decreases, the lobes shift from normal incidence to increasing angles up to 16°, allowing for continuous tuning in this range.

[0044] Figure 4A Shows the phase of the reflection coefficient of the graphene-based metasurface for transversely magnetic polarized light as a function of the Fermi level. As the Fermi level increases, the optical losses of the gap plasmon increase, further shifting the reflection phase away from that of a perfect mirror, thus changing φ. m It should be emphasized that the origin of the reflection phase shift is not due to graphene plasmons, but due to the strong interaction of the FP resonance at the graphene surface. Thus, the change in φ m will change the emission angle θ to maintain the constructive interference condition (Equation 1).

[0045] Figure 4B and Figure 4C Shows the calculated total absorption spectrum of the device. The overall behavior is consistent with experimental observations (see Figure 3 ), although the tuning range of the emitter in the experimental setup is smaller and the emission lobes are wider compared to the theoretical prediction. This discrepancy may be due to variations in the superstructure geometric parameters across the entire 4x4 mm 2 device and changes in the carrier density during the heating process.

[0046] The power of the thermal emitter can be calculated according to Planck's law for the spectral emissivity of a gray body. Using the measured emissivity (∼0.9) and the bandwidth of the constant angle tuning (4 cm -1 ), the emission power over the entire 4 mm x 4 mm area is calculated to be ≈0.14 mW. Considering convection and radiation losses, this emission power requires 200 mW to maintain the temperature of the device, resulting in an extremely efficient MIR source.

[0047] Materials and Methods

[0048] A 2-μm-thick, 5 mm x 5 mm SiN film on a 200-μm-thick Si frame was purchased from Norcada. The metal deposition for the back reflector included a 2.5-nm-thick chromium adhesion layer and 100 nm of gold. A 30-nm-thick Al2O3 thin film was grown on top of the SiN film using atomic layer deposition (FijiG2 ALD). Once the Al2O3 was grown, the prepared graphene sheets were transferred on top of the Al2O3 thin film. The graphene was purchased from Grolltex and grown on a copper foil. To remove the foil, a PMMA protective layer (950k, A4, MicroChem Corp.) was first added on top of the graphene. The copper foil was etched away with FeCl3 (CE-100, Transene), and then the graphene / PMMA stack was cleaned in a series of deionized water baths until transferred onto the prepared film. Once transferred, the PMMA was removed by soaking in acetone at 60 °C for 1 h. After the graphene transfer, a 5-nm-thick HfO2 film was grown via atomic layer deposition. To prepare the SiN film for the following steps, the Si frame of the sample was bonded to a carrier Si chip with PMMA (950k, A8, MicroChem Corp.). Then the prepared substrate was coated with a negative hydrogen silsesquioxane resist (HSiQ, 6%, DisChem Inc.) at 100 nm. Then the sample was exposed and patterned using an electron beam lithography tool, Elionix ELS G-100. After exposure, the sample was developed in MF-321 for 90 s, rinsed in DI water for 30 s, and then rinsed in IPA for 30 s. For the top metal deposition, a metal mask was placed above the substrate to create the electrically disconnected regions. The deposition included a 2.5-nm-thick chromium adhesion layer and 30 nm of gold.

[0049] FTIR measurements.

[0050] Emission measurements were performed using a Bruker Vertex 70 FTIR attached to a Hyperion 2000 microscope, which has a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector and a potassium bromide (KBr) beam splitter. A carbon nanotube source was used as a blackbody reference measurement.

[0051] As used herein, the term "exemplary" means serving as an example, instance, or illustration. Any aspect or design described herein as "exemplary" is not necessarily to be construed as preferred or superior to other aspects or designs. Additionally, for the purposes of this disclosure and unless otherwise specified, "a / an" may mean only one or may mean "one or more". Embodiments of the invention covering any construction consistent therewith are contemplated.

[0052] The foregoing description of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and modifications and variations are possible in light of the above teachings, or may be obtained from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is intended to be defined by the appended claims and their equivalents.

Claims

1. A thermal laser, comprising: A back reflector; A metasurface, the metasurface comprising: A phase - shift dielectric layer having an electrically - tunable or thermally - tunable Fermi level or refractive index, and A planar array of metal elements arranged periodically on the phase - shift dielectric layer; A dielectric spacer disposed between the back reflector and the metasurface; and A conductive contact configured to apply a voltage across the phase - shift dielectric or across a phase - shift dielectric heater in thermal communication with the phase - shift dielectric; and A dielectric spacer heater in thermal communication with the dielectric spacer.

2. The thermal laser according to claim 1, comprising a conductive contact configured to apply a voltage across the phase - shift dielectric.

3. The thermal laser according to claim 2, wherein the phase - shift dielectric is graphene.

4. The thermal laser according to claim 3, wherein the planar array of metal elements is a planar array of parallel metal strips separated by sub - wavelength gaps.

5. The thermal laser according to claim 4, wherein the metal strips are gold strips.

6. The thermal laser according to claim 5, wherein the dielectric spacer is a silicon nitride spacer.

7. The thermal laser according to claim 1, wherein the planar array of metal elements is a planar array of parallel metal strips separated by sub - wavelength gaps.

8. The thermal laser according to claim 6, wherein the metal strips are gold strips.

9. The thermal laser according to claim 1, wherein the phase - shift dielectric comprises indium tin oxide (ITO), indium zinc oxide (IZO), titanium nitride, vanadium dioxide (VO2), germanium antimony telluride (GST), titanium nitride, or an electro - optic polymer.

10. The thermal laser according to claim 1, wherein the dielectric spacer is an alumina spacer or a diamond spacer.

11. The thermal laser according to claim 1, comprising the phase - shift dielectric heater.

12. A method for generating a steerable thermal laser beam using the thermal laser according to claim 1, the method comprising: Heating the dielectric spacer to generate thermal radiation, wherein the thermal radiation couples to an oscillating Fabry - Perot resonance mode in the dielectric spacer to generate a coherent radiation lobe at an emission angle; And Applying a voltage across the phase - shift dielectric layer or changing the temperature of the phase - shift dielectric, thereby changing the emission angle of the coherent radiation lobe.

13. The method according to claim 12, comprising applying the voltage across the phase - shift dielectric layer.

14. The method according to claim 12, comprising changing the temperature of the phase - shift dielectric.

15. The method according to claim 12, wherein the phase - shift dielectric is graphene.

16. The method according to claim 15, wherein the planar array of metal elements is a planar array of parallel metal strips separated by sub - wavelength gaps.

17. The method according to claim 16, wherein the metal strips are gold strips.

18. The method according to claim 17, wherein the dielectric spacer is a silicon nitride spacer.

19. The method according to claim 12, wherein the planar array of metal elements is a planar array of parallel metal strips separated by sub-wavelength gaps.

20. The thermal laser according to claim 12, wherein the phase-shifting medium comprises graphene, indium tin oxide (ITO), indium zinc oxide (IZO), titanium nitride, and vanadium dioxide (VO2), germanium antimony telluride (GST), titanium nitride, or an electro-optic polymer, and the dielectric spacer is a silicon nitride spacer, an alumina spacer, or a diamond spacer.

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