Fig. 1. Wavelength-selective thermal emission. (a) Emission spectra of Au/Si/Au metasurfaces consisting of one or two resonators in a unit cell^[47]; (b) comparison of the normalized emission spectra of 1D and 2D structures at T=700 K^[49]; (c) measured emissivity (absorptivity) of the tapered metamaterial structures of different sizes^[50]

Fig. 2. Polarization control of thermal emission. (a) Experimentally measured polarized emission spectra of the C-shape resonator arrays with different d^[61]; (b) measured emission spectra of thermal antenna array without a polarizer (black line), with right-handed circular polarizer (RCP, blue line), and with left-handed circular polarizer (LCP,red line)^[67]; (c) obtained DOP and DOCP of incandescent chiral metasurface from measured circularly polarized thermal emission^[70]; (d) measured emission spectra of designed F-shape chiral metasurface under left- and right- handed circular polarizations^[71]

Fig. 3. Tuning of angular properties of metasurfaces. (a) Calculated emissivity spectra at various angles from the surface normal and angular dependence of emissivity at the peak maximum wavelength for a tungsten bull’s eye structure^[83]; (b) calculated absorption of a 2D MIM square grating as a function of the SiN thickness, and measured angle-dependent thermal emission of the metasurface^[85]; (c) perspective view of thermal metasurface and its profiles of emitted light for the band-edge wavelength and nonresonant wavelength^[86]

Fig. 4. Mechanism of thermal radiation angle response and angle-resolved thermal emission spectroscopy. (a) Designed Al/SiO₂/Al metasurface and measured angle-dependent thermal emission under TE and TM polarizations^[93]; (b) sketch plot of Al/SiN/Al metasurface and measured angle-resolved thermal emission varying with increasing grating width d (white line and circle indicate the dark magnetic resonance mode and the Fano resonance point)^[94]; (c) the top image is ARTES measurement setup, and bottom images from left to right are SEM picture of the fabricated sample, the schematic of designed meta-crystal, and measured and calculated thermal emission dispersions^[95]; (d) sketch and SEM picture of the designed superlattice, and measured and calculated thermal emission dispersions, indicating the existence of interface states^[96]

Fig. 5. Dynamic control of thermal emission. (a) Schematic of electrically controllable GaN/AlGaN MQW photonic crystals based thermal emitters, and difference between the thermal emission intensity spectra at 0 V and nonzero applied voltages^[107]; (b) measured spectral absorptivity of the metamaterial emitter in both on (red curve) and off (blue curve) states, and imaging measurement result of the single-pixel metamaterial emitter^[123]; (c) demonstration of spatial emissivity control using UV projected images^[124]; (d) left image shows the emissivity modulation by controlling the density of cGST points, SEM image shows the submicron-sized bumps fabricated by ns laser pulses, and right image shows binary anticounterfeiting label in different wavelength ranges^[126]

Fig. 6. Nonreciprocal thermal emission control. (a) Sketch of the nonreciprocal thermal emitter composed of InAs grating and metal layer and calculated absorptivity and emissivity spectra of the nonreciprocal thermal emitter at the condition of incident angle θ=61.28° and B=3 T^[133]; (b) schematic of the Weyl semimetal grating (the angle dependent absorptivity, showing clear asymmetric properties)^[135]; (c) left image shows the change of refractive index caused by the modulation of the guided mode resonance grating over time, and right image shows resonance modes of area-of-interest^[138]

Fig. 7. Near-field thermal emission control. (a) Schematic of near-field thermal emission between 1D and 2D metasurfaces and radiative heat flux as a function of volume filling ratio^[151]; (b) schematic of two doped-Si metasurfaces for near-field heat transfer and room-temperature heat transfer coefficients of Si metasurfaces, and Si and SiO₂ parallel plates as functions of gap size^[152]; (c) schematic of infinite graphene-hBN heterostructure. Monocell, sandwich, double-cell structures, and corresponding photon tunneling probability^[155]. (d) stacked graphene sheets offer improved heat conductance contrast between “ON” and “OFF” switching states^[158]

Fig. 8. Infrared applications based on thermal emitters. (a) Schematic of the daytime radiative cooler and calculated emissivity at normal incidence^[165]; (b) schematic of the polymer-based hybrid radiative cooler with randomly distributed SiO₂ microsphere inclusions and measured emission spectra of hybrid radiative cooler compared with theoretical results^[167]; (c) schematic of a metafabric for daytime radiative cooling (blue, green, and red dashed boxes highlight the three-level hierarchical structure responding to the UV, VIS-NIR, and MIR bands, respectively) and temperature difference of skin simulator under different fabric samples in the same location^[171]; (d) thermal stability and emittance of tungsten metasurface solar absorbers before annealing and after 10 heating cycles^[174]; (e) multilayer graphene based metasurface design, working principle of active thermal surface, and thermal images of the camouflage design placed on the author’s hand under the voltage biases of 0 and 3 V, respectively^[194]

Fig. 9. Integrated thermal emission chip based on metasurface array. (a) Schematic of designed thermal emission chip composed of 3×3 meta-cavity array^[207]; (b) measured absorption spectra and (c) SEM picture of meta-cavity array^[207]; (d) thermal image of meta-cavity array under x polarization^[207]; (e) SEM picture of fabricated nanohole patterns of “NJU” and “PHY” and thermal images of nanohole patterns^[207]; (f) measurement setup for measurement of thermal images of meta-cavity array with and without PDMS layer, and measured emission temperature of meta-cavity pixels and measured equivalent absorption spectra of PDMS layer^[208]