Reconfigurable and Polarization‐Dependent Grating Absorber for Large‐Area Emissivity Control Based on the Plasmonic Phase‐Change Material In3SbTe2

Metasurfaces with perfect infrared absorption promise integrated filters and compact detector elements with narrowband thermal emission. Phase‐change materials (PCMs) are prime candidates for active, non‐volatile absorption tuning. Commonly, the response of the entire metasurface is tuned, while local adaptions remain elusive. In this work, flexible encoding of different absorption/emission properties within a metasurface is shown. The plasmonic PCM In3SbTe2 (IST) is employed to obtain control over the emissivity by patterning an adaptable grating absorber metasurface. Using a commercial direct laser writing setup, the IST is locally switched from an amorphous dielectric into a crystalline metallic state, and cm‐sized stripe gratings are written above a reflecting mirror. Modification of already written patterns is demonstrated by changing the laser power and thus the IST stripe width to encode different polarization‐sensitive patterns with nearly perfect absorption into the same metasurface. Finally, an apparent local temperature pattern due to the large‐area emissivity shaping metasurface is measured with a conventional thermal camera. The results pave the way towards low‐cost, large‐area, and adaptable patterning of metasurfaces with wavelength and polarization‐selective perfect absorption, enabling applications like enhanced thermal detection, infrared camouflage, or encoding anti‐counterfeiting symbols.

DOI: 10.1002/adom.202202696 astronomy. [9] In general, thermal emission of objects depends on their corresponding non-zero temperatures and emissivities. Kirchhoff's law of thermal emission describes that the emissivity of an object is equal to the absorbance in thermal equilibrium. Consequently, the infrared emission can be artificially modulated by adapting the absorbance of a medium. Spatially resolved control of thermal emission is still highly challenging for applications like target simulations, [10] thermal management, [11] or security feature encoding. [12] Spatial control of the infrared emission is conventionally achieved by artificially varying the thermal emission properties via Fabry-Pérot cavities [13] or metasurfaces. [14][15][16] Metasurfaces are artificially nanostructured layers of subwavelength size to manipulate the properties of the incoming light like amplitude, phase, or polarization. They consist of metallic or dielectric antennas ("meta-atoms") in many different sizes and geometries and allow for functionalities, [17] such as beam steering, [18] lensing, [19] or holography. [20,21] Generally, the functionality of metasurfaces is fixed after fabrication. However, dynamic functionalities can be achieved by combining those metasurfaces with active materials. For instance, the phase transition material VO 2 offers an insulator-to-metal transition above 68 °C, [22] a gate voltage allows the injection of charge carriers in graphene [23] or doped semiconductors [24] or stressing an elastomeric substrate [24,25] leads to a resonance shift of antennas. But all these tuning mechanisms are volatile and vanish if the external stimulus is removed.
Non-volatile tuning of metasurfaces can be obtained with the well-established class of phase-change materials (PCMs). [26,27] Upon heating, these materials undergo a reversible and non-volatile phase-change from an amorphous to a crystalline phase. Both phases differ significantly in their optical and electrical properties. The reason for this is a change in bonding. While the atoms in the amorphous phase are covalently bonded, a new bonding mechanism called "metavalent" is identified in the crystalline phase for most of the PCMs. [28][29][30][31] Both phases are reversibly switchable via electrical [32] or optical pulses. [33] The pronounced change in the refractive index of PCMs is generally the basis for active metasurfaces, including polariton Metasurfaces with perfect infrared absorption promise integrated filters and compact detector elements with narrowband thermal emission. Phase-change materials (PCMs) are prime candidates for active, non-volatile absorption tuning. Commonly, the response of the entire metasurface is tuned, while local adaptions remain elusive. In this work, flexible encoding of different absorption/emission properties within a metasurface is shown. The plasmonic PCM In 3 SbTe 2 (IST) is employed to obtain control over the emissivity by patterning an adaptable grating absorber metasurface. Using a commercial direct laser writing setup, the IST is locally switched from an amorphous dielectric into a crystalline metallic state, and cm-sized stripe gratings are written above a reflecting mirror. Modification of already written patterns is demonstrated by changing the laser power and thus the IST stripe width to encode different polarization-sensitive patterns with nearly perfect absorption into the same metasurface. Finally, an apparent local temperature pattern due to the large-area emissivity shaping metasurface is measured with a conventional thermal camera. The results pave the way towards low-cost, large-area, and adaptable patterning of metasurfaces with wavelength and polarizationselective perfect absorption, enabling applications like enhanced thermal detection, infrared camouflage, or encoding anti-counterfeiting symbols.

Introduction
Control over infrared radiation has driven many innovations in various technological areas such as radiative cooling and heating, [1,2] thermal camouflage, [3][4][5][6] chemical sensing, [7,8] or optics, [34][35][36] zoom lenses, [37] holography, [38,39] active beam steerers, [40,41] metamirrors, [42] or tunable light absorbers. [43] Active infrared management has been realized by changing the amount of the crystalline PCM of GeSbTe alloys, [44,45] combining chalcogenide films with plasmonic nanoantennas, [46] or by using the hysteresis loops of the phase transition material VO 2 . [47] In addition to the previous active, spatially reconfigurable metasurfaces, it was also shown that a spatiotemporal emissivity control by inducing charge carriers is possible. [48] Recently, the new plasmonic PCM In 3 Sb 1 Te 2 (IST) has been introduced which shows a metallic behavior in the crystalline phase due to a significantly larger conductivity compared to conventional PCMs like GeSbTe-alloys. The conductivity in the crystalline phase exceeds 10-1000 times the conductivity of crystalline GST and justifies therefore the classification as a "bad metal". [49] It was possible to create arbitrary infrared plasmonic nanoantennas via direct laser writing, providing an entire new platform for fast fabrication and reconfiguration of infrared metasurfaces. [50][51][52] Heßler et al. already used IST for electric dipole resonance tuning in rod antennas, for a frequency selective perfect absorber by introducing a grating geometry, for screening metallic antennas, and for soldering two antennas together. [49] However, they were limited to small length scales up to 20 µm, which prohibits the transfer to large-area functionalized metasurfaces or employing the concept of thermal emission shaping and programming light absorbers.
Here, the plasmonic PCM IST is employed in a grating absorber structure to obtain emissivity control of a large-area reconfigurable metasurface. We take advantage of a commercial direct laser writing system with a fully automated and highly precise stage to fabricate large-area metasurfaces. A plasmonic grating structure is optically written above a gold mirror resulting in a magnetic dipole resonance caused by induced antisymmetric currents. First, the resonance wavelengths for varied bar widths are studied and compared with simulations. Afterwards, symbols are encoded by combining pixels with different emission properties and hyperspectral infrared images taken with a focal plane array (FPA) detector show different patterns depending on the operation wavelengths. We take advantage of the polarization sensitivity of gratings and encode two different symbols in the same metasurface dependent on the polarization of the light. Then, a large-area metasurface is designed and investigated with a thermal camera. Finally, we discuss our results and envision future applications for IST as a platform for active and reconfigurable nanophotonics and achieving custom designs of thermal emissions.

Main Text
The layer stack of the plasmonic metasurface is shown in Figure 1A and consists of a silicon substrate with a reflective  gold mirror on top, followed by a 450 nm thick dielectric spacer layer of ZnS:SiO 2 and a 50 nm amorphous PCM IST layer. The 70 nm capping layer on top prevents the sample from oxidation. The amorphous IST can be crystallized via laser irradiation from above; hence, its optical properties change from an amorphous dielectric state to a crystalline metallic one. The permittivity in the mid-infrared is shown as an inset for both states. A more detailed description of the permittivity can be found in Note S1, Supporting Information. In the amorphous state, IST has a nearly constant permittivity of 14 with almost zero losses. If the material is crystallized, the permittivity follows a Drude-like behavior. Hence, crystalline IST is metallic in the complete infrared spectral range. The laser switching process is performed with the commercially available direct laser writing system Photonic Professional GT from Nanoscribe (c.f. Figure 1B). Two mirror galvanometers with a fully automated stage enable highly precise addressing of the sample. Femtosecond laser pulses with a wavelength of 780 nm are focused with a 100x objective and a numerical aperture of 1.4 due to a liquid between sample and objective (see Experimental Section for more details).
We fabricated gratings of varying bar widths inside the amorphous IST. If those bars get excited with light polarized along the width of the grating bars, the charges oscillate along the width of the bars. This induces an antisymmetric current in the gold mirror due to mirror charges. These opposite currents form a magnetic dipole (MD) resonance, as indicated in Figure 1C. The resonance wavelength of the MD depends on the bar width and can be tuned to larger wavelengths by increasing the bar width. Due to the gold mirror at the bottom, the light is reflected back and the transmittance vanishes. The absorbance A of such a system only depends on the reflectance R and is therefore calculated by A = 1−R. In the case of zero reflectance, all the energy is absorbed by the electromagnetic resonance in crystalline IST. Figure 1D shows a sketch of the letters "IST" encoded in a 10×10 pixel array. Each pixel consists of a grating with a specific bar width. The MD resonance wavelength differs dependent on the width of the grating. Hence, at a certain wavelength, the letters providing a minimum reflectance can be clearly distinguished from the background in the encoded image, while at another wavelength all pixels show the same reflectance. We use this concept for image encoding and thermal emission shaping in the infrared.
We start by crystallizing bars of varied widths. The bar width of the gratings is varied from 0.4 to 2.1 µm while the period is kept constant at 2.9 µm. The increased bar width was achieved by adapting the laser power from 4 to 31 mW with a scan speed of 500 µm s −1 to crystallize the material (see Experimental Section and Note S2, Supporting Information, for further details). Light microscope images for 4 different bar widths are shown in Figure 2A. The experimentally obtained Fourier-transform infrared (FTIR) reflectance spectra and comparison with simulations are shown in Figure 2B as a colorplot. In the colorplot, a distinct minimum appears at around 6 µm for the smallest bar width. While increasing the bar width, the minimum corresponding to  the MD resonance is shifted towards larger wavelengths up to 14 µm for a maximum bar width of 2.1 µm. Numerical field simulations to confirm the assignment to a magnetic resonance are presented in Note S3, Supporting Information. Other minima around 5 µm belong to surface plasmon polariton (SPP) resonances at the gold/ZnS:SiO 2 interface. A more detailed investigation with numerical simulations of those minima can be found in Note S4, Supporting Information, which indicates that those features offer a strong dependency on the angle of incidence and the grating period. Therefore, our assignment of grating-coupled SPP resonances seems valid. The experimental data (top) are in good agreement with full-wave simulations (bottom) which confirm the shift of the MD for increased bar width (see Experimental Section for more details). Therefore, by continuously increasing the width, the MD resonance can be tuned along the complete mid-infrared spectral range. Figure 2C displays the measured and simulated reflectance spectra for bar widths of 0.8, 1.1, and 1.5 µm which will be later used for multispectral imaging. The MD resonances for those structures are at 7 µm for the width of 0.8 µm, at 9 µm for the width of 1.1 µm and at 11 µm for the width of 1.5 µm. All curves achieve a minimum reflectance around 0.3 and therefore an absorption corresponding to 0.7. The differences in the experimentally obtained spectra and the simulations can be explained by the usage of Cassegrain objectives in the experiment (c.f. Experimental Section): In addition to an angular spread from 10 to 24 degrees, also s-and p-polarized light excite the gratings simultaneously. Hence, the minima in the reflectance of the FTIR measurements are broader compared to the simulations. The differences in the absolute reflectance values between the experiment and simulations can be attributed to a finite crystallization depth, which was already shown before by Heßler et al. [49] . The broad minimum at around 9 µm can be attributed to the absorption of SiO 2 caused by the dielectric spacing layer of ZnS:SiO 2 . The enhanced fields in the MD resonance for gratings with a bar width between 0.8 and 1.5 µm around 9 µm result in an enhancement of this material characteristic absorption feature. A measured FTIR spectrum of an amorphous region of the sample can be found in Note S8, Supporting Information, which also displays this absorption feature. However, this broad feature does not affect the resonance wavelength of the MD.
The reversible switching process via reamorphization of crystalline IST has been already demonstrated by Heßler et al. for rod antennas and split-ring resonators. [49,50] The reamorphization process requires heating the PCM above the melting temperature with subsequent quenching due to intrinsic cooling rates so that the lattice is melt-quenched into the amorphous state. [33] However, the commercial system Photonic Professional GT in the Continuous Mode does not allow the control of single laser pulses. Therefore, too much energy is absorbed by the PCM and the reamorphization process, which requires melt-quenching, was not possible with this setup. Note that this is only a limitation of this commercial instrument. By using our home-build laser switching setup, allowing for single nanosecond pulses, we also show reversible switching for our grating absorber structures (see Note S8, Supporting Information).
With the results of Figure 2 and the known resonance positions for different bar widths, we now encode images in a 10 × 10 pixel array to show wavelength-dependent images. Each pixel/grating has a size of 17 × 17 µm 2 . The large size is chosen to ensure that under consideration of the diffraction limit a clear image is still visible for long wavelengths. Furthermore, we highlight the reconfigurability of the material IST by creating a sequence of four different patterns, while each subsequent pattern contains the previous ones. Light microscope images of the fabricated patterns are shown in Figure 3A. First, only the letter "I" is visible with a corresponding bar width of 1.5 µm.
i) The remaining background pixels have a bar width of 0.8 µm. Afterwards, a new pattern is created, containing the subsequent one. This means that each pattern (i-iv) is addressed and the grating widths of the pixels with a new pattern are enlarged. ii) A second "I" is added next to the previous one with different bar widths of 1.1 µm. iii) In the third pattern, the second "I" is changed to an "R", resulting in the letters "IR". iv) Finally, the grating bar width of the entire pattern is enlarged, resulting in a bar width of 1.5 µm.
Infrared images of the fabricated structures at a certain wavelength of 11 µm are shown in Figure 3B measured with an FTIR focal plane array detector (see Experimental Section). This characteristic wavelength is located in the longwave infrared (LWIR) regime (8-14 µm) and plays a crucial role in applications, such as home inspection for visualizing insulation, or heat distributions in electronics. [53] i) For the first structure, the letter "I" displays a minimum reflectivity of 0.2, while the background pixels offer a reflectivity larger than 0.6. ii) In the second image, the first "I" is still clearly visible, however, the second "I" is only weakly pronounced because the corresponding MD resonance is located at smaller wavelengths (9 µm). iii) In the third image, the same behavior is visible in the third image, the "I" is unchanged and the "R" symbol has a reflectivity of 0.4. iv) Finally, the widths of all gratings are equal to 1.5 µm resulting in minimum reflectivity for the entire image. Spectral images for the entire LWIR regime from 8.0-11.0 µm can be found in Note S5, Supporting Information.
If the infrared imaging wavelength is set to 3-5 µm in the mid-wave infrared (MWIR) range, all the features nearly disappear in each image (cf. Note S5, Supporting Information). Each pixel has almost the same reflectivity, independent of the bar width of the corresponding grating structure. Therefore, a thermal camera operating in the MWIR range will not be capable of displaying the infrared-encoded images, while a similar camera operating in the LWIR range can clearly distinguish between the different pixels.
In the next step, we combined two different polarizationencoded images by rotating the bars of the second image by 90 degrees. Hence, the second image is only excitable for horizontally polarized light. The resulting pixels are now 2d gratings with horizontally and vertically oriented bars. In addition to the previously investigated letters "IR", we added an image of a heart with a crack in the middle. A schematic sketch and the light microscope image of the fabricated metasurface are displayed in Figure 4A. The green pixels are created with a bar width of 1.5 µm, the orange pixels with a bar width of 1.1 µm, and the background (blue pixels) corresponds to a bar width of 0.8 µm. We chose background pixels with a small bar width instead of using a metallic mirror because then the infrared polarization-dependent symbols are not directly observable in the visible. A metallic gold mirror without crystallized IST would provide a stronger pronounced contrast. However, this would affect the pixel combinations which combine a background pixel in one polarization and a pixel with increased bar width in the other polarization and distort the obtained multispectral image (see Note S6, Supporting Information).
The measured FTIR spectra for both polarizations at three different positions in the image (red markers) are shown in Figure 4B. The red "x" is a pixel with a horizontal grating of 1.5 µm bar width and a vertical grating of 1.1 µm bar width, the square with horizontal bars of 1.1 µm width and 1.5 µm large vertical bars, and the star displays horizontal bars with 0.8 µm width and 1.1 µm large vertical bars. The 1d grating is now replaced by a 2d grating with bars of different widths, which influence each other. So, the previously investigated resonance positions cannot be applied here. A detailed study with numerical simulations about the resonance wavelengths is shown in Note S6, Supporting Information. The experimentally obtained resonance positions for the different combinations of bar width can be well reproduced with simulations. Therefore, we can distinguish three resonance positions, at around 6.2 µm, around 7.6 µm, and around 8.3 µm for the three corresponding bar widths.
Infrared reflection images at the resonance wavelength of 7.6 µm are shown in Figure 4C for vertically (left) and horizontally (right) polarized light. Please note that the shown images originate from the same structure as shown in Figure 4A. The letters IR are clearly visible for vertically polarized light, while for the rotated polarization the heart appears as dark blue regions with nearly perfect absorption. The pixels with the largest bar width, such as the "I" and the crack inside the heart, show less absorption. Other investigated structures which show polarization-sensitive images can be found in Note S7, Supporting Information.
Kirchhoff's law states that good thermal absorbers are also good thermal emitters. Therefore, we determined the resonance positions at each element of the 64 × 64 FPA detector. The corresponding plots are shown in Figure 4D for both polarization directions. The resonance position varies from 6 to 9 µm. Pixels with a corresponding grating bar width of 1.5 µm, in particular the letter "I" and the crack inside the heart, show a resonance wavelength of 8.6 µm. The pixels with the smallest bar width have a resonance wavelength below 6.5 µm. Transforming those images with Planck's law and the derived Wien's displacement law with 2898 m K T Peak λ = µ results in a peak emission temperature, which offers the largest absorption/emission at the corresponding temperatures. Hence, the peak emission temperature varies from 50 °C for the largest resonance wavelength to 200 °C for the pixels with the smallest bar width. Afterwards, we fabricated a large-area metasurface consisting of a heart with a bar width of 1.5 µm and a surrounding square with a bar width of 0.8 µm on a 1 × 1 cm 2 wafer. In a proof-of-principle experiment, we placed the metasurface on a heating plate to adapt to the ambient temperature. The sample was then imaged with a thermal camera and a mounted 10x objective. The measurement setup is sketched in Figure 5A. We added a photograph taken with a conventional camera, where the structure on the metasurface is barely visible.
Adv. Optical Mater. 2023, 11, 2202696   Figure 3. Reconfiguring infrared images by using pixels of different bar widths. A) Light microscope images of the fabricated images. We apply three different pixel types, one with a grating width of 1.5 µm (green), a medium grating width of 1.1 µm (orange), and a small grating width of 0.8 µm (blue). The pattern is reconfigured from i) a letter "I" in a square, ii) to a pattern with two "I", iii) to an "IR" symbol, and finally iv) to a full-sized square. The scale bar equals 30 µm. B) Infrared images at a wavelength of 11 µm located in the long-wave infrared (LWIR) regime. The different features can be clearly distinguished. A) The metasurface is placed on a heating plate and the apparent temperature distribution is measured with a thermal camera. The emissivity is set to 0.3 to match the ambient temperature with the temperature of the unpatterned sample. The inset below shows a photograph of the large area metasurface consisting of a heart. B) Thermal images at different ambient temperatures of the heating plate. For comparison, an unpatterned sample is placed next to the metasurface. C) An infrared polarizer is added between the camera and the samples. If the polarization is set to vertically polarized light, the heart is clearly visible. When the polarizer is rotated by 90 degrees, the metasurface shows the same emissivity as the unpatterned sample. Thermal images recorded by the infrared camera are shown in Figure 5B at different temperatures. We varied the temperature from 30 to 50 °C with a heating plate. Next to the prepared metasurface is an unpatterned sample with the same layer stack as a comparison. The preset emissivity assigned in the thermal camera is set to 0.3 so that the temperature of the unpatterned sample matches the ambient temperature. i) At an ambient temperature of 30 °C, the heart shows an enhanced temperature of 38 °C. Clearly, the temperature of the heart is also equal to the ambient temperature but the larger emissivity of the metasurface causes an apparent higher temperature compared to the unpatterned sample. The same behavior is visible for ambient temperatures of ii) 40 and iii) 50 °C, when the metasurface shows temperatures of 54 and 69 °C, respectively.
In Figure 5C, an infrared polarizer is placed between the camera and the metasurface. If the polarization is set to vertically polarized light (0°) perpendicular to the grating bars, the heart is still clearly observable. Again, an unpatterned sample adjacent to the MS is used for comparison. For 90° rotated polarization, the heart vanishes and both samples, the MS and the unpatterned one, show no characteristic features in the infrared image. This clearly demonstrates the polarizationdependent emissivity control of our concept.

Conclusion
In this work, we demonstrated spectral infrared emissivity shaping and spatial patterning with the plasmonic phasechange material IST by encoding different images in the metasurface via direct laser writing. We were able to control the absorption of the metasurface by continuously tuning the bar width of a grating resulting in nearly perfect absorption from 6 to 14 µm due to the MD resonance. Afterwards, we encoded an image, which is only visible in the LWIR band, in the metasurface and reconfigured already written patterns. The grating structure with polarization sensitivity was used to encode two images for different polarizations in the same metasurface. Moreover, a large-area metasurface of 1 × 1 cm 2 was fabricated and investigated. Due to its enhanced emissivity of the metasurface, an apparent local temperature pattern was observable. This pattern was also polarization-sensitive and vanished upon rotation of the incident light polarization. All in all, we demonstrated the concept of locally shaping the infrared emission in specific wavelength ranges of the LWIR and MWIR. Additionally, we reconfigured fabricated structures to enhance the emissivity of objects or to hide them via thermal camouflage. [14] This might serve as the basis to add anticounterfeiting symbols to existing structures [45,54] and adapt their pattern later for example for security reasons, or offering new opportunities such as adding invisible barcodes to objects. [55] Patterning ultrathin-film perfect absorber layers can also be used to tailor the emissivity of metasurfaces. [51,56,57] Combining all these concepts of shaping polarization and wavelength-sensitive thermal emission, paves the way towards superior control of infrared radiation in application areas such as radiative cooling, thermal camouflage, imaging, and reconfigurable encoding of anticounterfeiting symbols.

Experimental Section
Sample Fabrication: A 1 × 1 cm 2 silicon substrate was first rinsed with acetone and isopropyl alcohol for 30 s. Afterwards, a 3 nm thin chromium layer for adhesion and a 100 nm thick gold layer was thermally evaporated on the substrate. Via sputter deposition, a 450 nm thick (ZnS) 80 -(SiO 2 ) 20 spacer layer, followed by a 50 nm thick amorphous PCM layer of In 3 SbTe 2 and second (ZnS) 80 -(SiO 2 ) 20 capping layer with a corresponding thickness of 70 nm were sputtered on the gold mirror with an argon flow of 20 sccm. The capping layers were sputtered in constant power mode with 60 W and a rate of 0.02 nm s −1 , while the PCM layer was fabricated with 24 W and 0.09 nm s −1 .
Optical Switching: The optical switching of the PCM layer was done with the commercially available system Photonic Professional GT from Nanoscribe which was conventionally used for two photon absorption processes. Hence, a pulsed laser with a wavelength of 780 nm and a pulse length of 100 fs with a repetition rate of 80 MHz was focused with a 100x objective onto the sample. Between sample and objective, the liquid 3-(Trimethoxysilyl)propyl methacrylate was used to increase the Numerical aperture of the objective to 1.4. Two movable galvo mirrors allow precise movements in the nanometer range of the laser beam in a 200 µm diameter circle. A piezo stage for a range of 300 × 300 × 300 µm 3 and a motorized xy-stage enabled the fabrication of large-area metasurfaces.
The crystallization of the amorphous IST has been performed in the Continuous Mode to create grating structures. A scan speed of 500 µm s −1 was set, and the applied laser power was varied from 4 to 30.5 mW in order to achieve different bar widths.
FTIR Measurements: The shown reflectance spectra were measured by a Bruker Vertex 70 interferometer connected to a Bruker Hyperion 2000 microscope. For all measurements, a 15-fold Cassegrain objective with an angular distribution from 10 to 24 degrees and a numerical aperture of 0.4 was used. In addition to the conventional used Mercury Cadmium Telluride (MCT) detector, the IR microscope was also equipped with a focal plane array (FPA) detector of 64 × 64 pixels and a pixel resolution of 2.7 µm for hyperspectral imaging. The single spectra of the MCT detector were recorded with 2000 scans and a resolution of 8 cm −1 , while the hyperspectral images of the FPA detector were measured with 4000 scans and a resolution of 16 cm −1 . The bandwidth for integration was chosen to be 100 nm below and above the chosen imaging wavelength. Imaging with the FPA detector was still diffraction limited. Assuming a wavelength of 10 µm and with the numerical aperture of the objective, the minimum resolvable distance was calculated to be 12.5 µm. Hence, at the edges of the grating pixels, both adjacent gratings contributed to the measured spectrum. All spectra have been normalized with a gold mirror offering almost unity reflection in the complete IR spectral range.
Numerical Simulations: For all numerical simulations, the commercially available program CST Studio Suite has been used. The refractive index of the ZnS:SiO 2 spacer layer was assigned to a constant value of 2.1 without any losses and the permittivity of amorphous and crystalline IST can be found in Note S1, Supporting Information. Floquet Mode Ports were chosen for the excitation in the simulation. In lateral dimensions, unit cell boundaries were assumed. A mean angle of incidence of 17° takes the angular spread of the Cassegrain objective into account.
Thermal Camera: Here, a FLIR T335 thermal camera equipped with an additional 10× objective was deployed to image the large-area metasurface from Figure 5. The emissivity of the camera was set to 0.3 to match the ambient temperature of the unpatterned sample.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.