Fig. 1. Reciprocal directional thermal radiation design based on micro- and nano-structures. (a) First directional thermal radiation structure based on SiC grating; (b) Emissivity of SiC grating at p-polarization for different wavelengths and angles of incidence ^[20]; (c) Directional heat radiation devices consisting of equally spaced concentric circular grooves on a W or Mo metal surface ^[21]; (d) Directional and frequency-selective thermal emission using Au-SiN-Au metasurface ^[14]; (e) Thermal emission of phonons controlled by magnetic resonance modes based on Ag/SiO₂/Al-based metasurface ^[22]; (f) Control of thermal emission via a metasurface of Al/SiN/Al nanosandwich photoabsorber using modulation of parity-symmetric Fano resonance ^[23]

Fig. 2. Reciprocal directional thermal radiation design based on multilayer membrane structure. (a) Small-angle directional thermal radiation modulation achieved by a MQW layer and a two-dimensional photonic crystal (2D PC) with a lattice constant of 6.5 mm ^[24]; (b) A directional thermal emitter consisting of a one-dimensional photonic crystal (1D PC) film, a dielectric spacer layer and a continuous metal film ^[25]; (c) Narrowband radiation directional modulation by separating an Au layer from a 2D material graphene by a dielectric spacer layer ^[27];(d) Directional thermal radiation modulation by photonic crystals designed with thermal emitters and angle selectors, respectively; (e) Demonstration of the emissivity of a directional narrow-band thermal emitter by the combination of an angle selector with 20 and 70 layers, respectively, and a thermal emitter ^[26]

Fig. 3. Design of non-reciprocal directional thermal radiation based on micro-nanostructures. (a) Schematic of single- and multi-channel non-reciprocal emitters; (b) Schematic of unidirectional nonreciprocal thermal emission from SiC grating; (c) Design optimization of SiC grating in the SPhP region with respect to wavelength and emission angle and emissivity ^[35]; (d) Schematic of a nonreciprocal multiport emitter and the corresponding angular distributions of emissivity and absorptivity ^[36]; (e) Schematic of a variable gate structure prepared from Au/Ge/ZnS/Au to realize an asymmetric directional thermal emission structure ^[37]

Fig. 4. Realization of broadband directional thermal radiation modulation based on ENZ material. (a) ENZ material consisting of doped CdO and SiO₂ with Au substrate realizes narrowband directional thermal radiation modulation ^[44]; (b) Broadband directional thermal radiation modulation was achieved in the 7-12 μm and 10-14 μm wavelengths by separately designed gradient ENZ materials ^[45]; (c) A dielectric gap (Ge) is used to connect and combine two gradient ENZ materials with different wavelengths to realize broadband directional thermal emission modulation covering the full long-wave infrared (LWIR) range ^[46]; (d) Gradient ENZ metamaterials made of high-temperature-resistant oxides MgO, BZHO, and NiO, demonstrating the temperature insensitivity of broadband directional thermal emission modulation ^[47]; (e) Comparative emissivity plots of this gradient ENZ metamaterials at different temperatures (25 ℃ and 1000 ℃) ^[47]

Fig. 5. Realization of broadband directional thermal radiation modulation based on ENZ materials. (a) Gradient ENZ was characterized by growing InAs with different doping concentrations to achieve dynamic modulation of directional broadband thermal radiation ^[48]; (b) Transparent directional broadband thermal radiation structure based on Al₂O₃/Si₃N₄/ITO material designed for vertical surface radiative cooling ^[49]

Fig. 6. (a) Directional broadband thermal radiation modulation by designing photonic crystals as thermal emitter and angle selector, respectively ^[26]; (b) Structure consisting of a Si₃N₄ thermal emitter and a top transmission filter composed of BaF₂ and LiF for directional broadband thermal radiation ^[50]; (c) Schematic of an ultra-broadband directional thermal radiation structure matched to an atmospheric window, with the system consisting of a thin dielectric film on a radiating substrate; (d) Structure consisting of a flexible PEI substrate and a Ge film layer for direction-specific broadband thermal radiation modulation ^[51]; (e) Structure consisting of alternating three-phase metamaterials, including an array of ellipsoidal core-shell structures and a cylindrical array of PMMA covered with an ITO conformal coating, for realizing ultra-broadband directional thermal radiation ^[52]; (f) Thermal radiation modulation for high directivity and broadband characteristics based on a PDME structure consisting of SU-8 photoresist substrate, polymer structure, and silver reflective layer ^[53]

Fig. 7. Directed broadband non-reciprocal thermal radiation modulation. (a) Gradient magnetized ENZ multilayers composed of InAs films with different doping concentrations are utilized for non-reciprocal thermal radiation modulation ^[66]; (b) Variation of emissivity and absorptivity of nonreciprocal emitters of single-layer magneto-optical materials in the presence of different external magnetic fields; (c) A structure consisting of a magnetic Wely metal (MWS) layer with a chemical potential gradient above Au enables directional broadband non-reciprocal thermal emission in the absence of an external magnetic field ^[67]; (d) A structure consisting of two WSM layers with different Fermi energy levels and a dielectric film placed alternately on the Au reflector, which enables directional broadband nonreciprocal thermal emission in the absence of an external magnetic field ^[68]