Thermal emission, an omnipresent fundamental physical phenomenon in nature, is triggered by the thermal-induced motion of particles and quasi-particles within an object. Any object with a temperature above absolute zero (-273 ℃) emits thermal emission energy in the form of electromagnetic waves to the surrounding environment. Due to the random nature of the thermal motion of charged particles, the thermal radiation produced by objects in nature typically features continuous wavelength, non-polarization, and omnidirectional incoherent light. Moreover, the relationship between radiation intensity and wavelength follows Planck’s blackbody radiation law. However, in practical applications, these thermal emitters often radiate a significant amount of thermal radiation in unnecessary directions, resulting in substantial energy waste. Therefore, it is crucial to concentrate the energy generated by thermal emission from an object into a specific range of direction, so as to achieve efficient control of thermal emission direction. The ability to confine thermal emission within specific wavelength ranges and directions is of significance for improving the energy utilization efficiency of devices. With the rapid development of nanotechnology, the control of thermal emission is evolving towards micro-scale and even nano-scale devices. Micro-nano thermal emitters utilize nanophotonic structures, where at least one structure features wavelength or sub-wavelength scales, to manipulate the polarization, wavelength, phase, and amplitude of light at sub-wavelength scales. This can break the limitations of conventional thermal emitters that exhibit continuous wavelength, non-polarization, and omnidirectional in thermal emission. As a result, control over thermal emission can be achieved in terms of spectral, directional, and polarization control (Fig. 1). Compared to traditional thermal emission devices, adopting nanophotonic structures in the design of thermal emission devices enables higher degrees of freedom in controlling thermal emission. Additionally, nanophotonic thermal emitters feature compact size, light weight, and easy integration, showing great application potential in thermal imaging, infrared sensing, and communication, among other fields. In recent years, nanophotonic thermal emitters have been widely applied in research on the control of thermal emission direction.

Progress The polar materials or metallic materials can be employed to design grating structures, combined with infrared absorbing materials as infrared thermal emission sources. Under this situation, the design of grating structure parameters can induce surface phonon polariton resonances in polar materials or surface plasmon resonances in metallic materials. This makes it possible to achieve directional control of thermal emission. In 2002, Greffet’s research group investigated a coherent thermal emission based on SiC grating structure [Fig. 2(a)]. In 2005, Laroche’s research group achieved a coherent thermal emission source operating in the near-infrared band [Fig. 2(b)]. In 2016, Chalabi’s research group utilized SiC grating structures on a planar SiC substrate to achieve focused thermal emission [Fig. 2(c)]. In the same year, Park’s research group fabricated a directional thermal radiation source based on a bull’s eye-shaped grating structure using tungsten [Fig. 2(d)]. Directional control of thermal emission can be achieved by utilizing dielectric materials to design metasurfaces and combining them with infrared absorbing materials as infrared thermal emission sources. The parametric design of metasurfaces induces critical coupling, plasmon-phonon coupling, Fano resonances, and guided-mode resonances. In 2015, Costantini’s research group proposed narrowband directional thermal emission based on Au/SiN/Au metasurface [Fig. 3(a)]. In 2019, Zhang’s research group introduced angle-selective thermal emission based on Al/SiN/Al metasurface [Fig. 3(b)]. In 2020, Zhao’s research group utilized Weyl semimetal metasurface to achieve nonreciprocal thermal emission [Fig. 3(c)]. In 2023, Yu’s research group proposed a method of unidirectional thermal emission in reciprocal optical systems using metagratings [Fig. 3(d)]. By adopting photonic crystals to modulate the photon density of photonic states, it is possible to achieve manipulation and control of light with wavelength selectivity and angle selectivity. This opens up more possibilities for designing novel thermal radiation devices. In 2005, Celanovic’s research group proposed a narrowband directional thermal emitter based on vertical-cavity enhanced resonances [Fig. 4(a)]. In 2005, Lee’s research group introduced a narrowband directional thermal emitter based on SiC and one-dimensional photonic crystals [Fig. 4(b)]. In 2014, Granier’s research group utilized optimized Si/SiO₂ aperiodic structures to achieve narrowband directional thermal emission [Fig. 4(c)]. In 2022, Li’s research group proposed an approach to narrowband directional thermal emission based on monolayer tungsten disulfide and one-dimensional photonic crystal slab [Fig. 4(d)]. Epsilon-near-zero (ENZ) materials refer to special metamaterials in which the real part of the dielectric constant approaches zero within a specific wavelength range. These materials include doped semiconductors, metals, and polar materials. It is well known that these ENZ materials support a leaky p-polarized electromagnetic mode near their ENZ wavelength, termed a Berreman mode. Within a certain range of wave vectors, this mode can couple with propagating free-space modes, enabling directional control of thermal emission. In 2016, Campione’s research group proposed a narrowband directional thermal emission based on semiconductor hyperbolic metamaterials [Fig. 5(a)]. In 2017, Nordin’s research group proposed narrowband directional thermal emission based on ultra-thin phononic films [Fig. 5(b)]. In 2022, Xu’s research group utilized gradient ENZ metamaterials to realize broadband directional thermal emission [Fig. 5(c)]. In 2022, Ying’s research group proposed whole long-wave infrared directional thermal emission based on ENZ thin films [Fig. 5(d)]. The control of directional thermal emission using various nanophotonic structures offers broad prospects for applications. The ability to confine thermal emission within specific bandwidths and angle ranges is particularly relevant to sensing, communication, and energy applications. These include radiative cooling, space thermal imaging, infrared polarization conversion, information encryption, and thermal radiation sources. In 2024, Bae’s research group proposed an energy-saving window based on broadband directional thermal emission [Fig. 6(a)]. In 2014, Kyoung’s research group achieved directional control of sample thermal emission using ENZ materials, enabling high-resolution wide-field infrared imaging [Fig. 6(b)]. In 2017, Yang’s research group employed ENZ materials to prove the capacity of an infrared reflective polarizer [Fig. 6(c)]. In 2023, Ying’s research group utilized directional thermal emission covering two atmospheric windows to realize visible-infrared dual-band information encryption [Fig. 6(d)].

Research on directional control of thermal emission aims to confine heat transfer within specific wavelength ranges and directions, which has significant practical implications for applying nanophotonic thermal emitters in various aspects of daily life. We present the most representative theoretical and experimental achievements in recent years regarding directional control of thermal emission using nanophotonic structures, including grating structures, metasurfaces, photonic crystals, and ENZ materials. The main applications of such nanophotonic thermal emitters with directional control capabilities are summarized, including radiative cooling, infrared imaging, polarization conversion, and information encryption. In the future, research on directional control of thermal emission can focus on such fields as polarization-insensitive broadband directional thermal emission, dynamically tunable directional thermal emission, and directional thermal emission with low angular dispersion.