As is known, traditional thermal emission is broadband, unpolarized, and incoherent, typically altered by changing temperature to modify spectral line shapes and intensities. Conventional materials face challenges in accurately controlling the radiation characteristics with multiple degrees of freedom, limiting their applications in infrared spectra. In recent years, two-dimensional metasurface structures with sub-wavelength size and ultra-thin thickness have overcome the limitations of traditional research on thermal emission manipulation due to their flexible and controllable optical response. Metasurface structures obtained via various designs have successfully manipulated thermal emission in multiple degrees of freedom, such as wavelength, polarization, direction, time, and coherence. This has promoted the miniaturization and integration of infrared devices.

Both realizing rational wavelength-selective emission and manipulating the emission at other wavelengths as much as possible are essential for practical infrared applications. In 2011, Liu’s research group experimentally developed a narrow dual-band mid-infrared thermal emitter [Fig. 1(a)]. In 2013, Argyropoulos’ research group discussed the possibility of realizing ultra-broadband omnidirectional absorbers and angularly selective coherent thermal emitters based on properly patterned plasmonic metastructures [Fig. 1(b)]. In 2015, Hossain’s group designed and experimentally demonstrated a metamaterial thermal emitter for highly efficient radiative cooling [Fig. 1(c)]. In contrast to unpolarized blackbody thermal emission, metasurface-based thermal emitters with fabricated subwavelength meta-atoms typically emit polarized thermal emission. For linearly polarized thermal emission, Liu’s research group experimentally demonstrated a new type of macroscopic perfect and tunable thermal emitters in 2017 [Fig. 2(a)]. Additionally, circularly polarized thermal emission is another hot spot for polarization manipulation of thermal emission. In 2010, Dahan’s research group experimentally demonstrated spin-dependent dispersion splitting of the emitted light and analyzed it in terms of a geometric Doppler shift [Fig. 2(b)]. In 2023, Nguyen’s research group reported the emission of polarized mid-wave infrared (MWIR) radiation from a 700 nm thick incandescent chiral metasurface [Fig. 2(c)]. In 2023, Wang’s research group designed nonvanishing optical helicity by engineering a dispersionless band that emits omnidirectional spinning thermal radiation [Fig. 2(d)]. Further wavelength-selective thermal emission within the demanded emission directions should be restricted to improve the emission efficiency. In 2010, Han’s research group theoretically examined thermal emission from metallic films with surfaces that are patterned with a series of circular concentric grooves (a bull’s eye pattern) [Fig. 3(a)]. In 2015, Costantini’s research group introduced a plasmonic metasurface to control the spectrum and directivity of blackbody radiation [Fig. 3(b)]. In 2021, Overvig’s research group introduced a platform for thermal metasurfaces and completed the compactification program of optical systems [Fig. 3(c)]. In 2017, Zhang’s research group found that thermal emission of phonon can be controlled by the magnetic resonance mode in a metasurface [Fig. 4(a)]. In 2019, Zhang’s research group employed Al/SiN/Al metasurface to manipulate the thermal emission in the infrared range [Fig. 4(b)]. In 2020, Zhong’s research group established angle-resolved thermal emission spectroscopy as an alternative platform to characterize the intrinsic eigenmode properties of non-Hermitian systems [Fig. 4(c)]. In 2021, Zhong’s research group proposed a scheme to construct and probe the mid-infrared surface wave radiation of the interface state in the waveguide by thermal emission [Fig. 4(d)]. In recent years, dynamic tunable thermal emitters possessing switchable thermal emission properties under high-speed modulation have caught extensive attention to develop adaptive thermal management devices. In 2019, Kang’s research group demonstrated the electrical modulation of a narrowband MWIR thermal emission at high temperatures of up to 500 ℃ by adopting GaN/AlGaN multiple quantum well photonic crystals [Fig. 5(a)]. In 2017, Liu’s research group proposed and demonstrated the idea of a metamaterial microelectromechanical system capable of tailoring the energy emitted from a surface [Fig. 5(b)]. In 2017, Coppens’ research group achieved simultaneous spatio-temporal emission manipulation [Fig. 5(c)]. In 2021, Xu’s research group experimentally demonstrated a nonvolatile optically reconfigurable mid-infrared coding radiative metasurface [Fig. 5(d)]. The nonreciprocal system is fundamentally vital for solar energy harvesting systems to reach their efficiency limit and is appealing to thermal management devices, which are usually designed by following Kirchhoff’s law. In 2014, Zhu’s research group validated general principles by direct numerical calculations based on fluctuational electrodynamics and thermal emitters constructed from magneto-optical photonic crystals [Fig. 6(a)]. In 2020, Zhao’s research group indicated that the axion electrodynamics in magnetic Weyl semimetals can be adopted to construct strongly nonreciprocal thermal emitters that nearly completely violate Kirchhoff’s law overbroad angular and frequency ranges without requiring any external magnetic field [Fig. 6(b)]. In 2022, Ghanekar’s research group exploited spatio-temporal refractive index modulation of a grating to drive photonic transitions between guided resonance modes [Fig. 6(c)]. The above-mentioned studies have been conducted to study far-field thermal emission, which is bounded by the Planck thermal-emission limit. However, subwavelength thermal emitters appear to exceed the limit. In 2015, Liu’s research group investigated the near-field radiative heat transfer of 1D and 2D metasurfaces [Fig. 7(a)]. In 2017, Fernández-Hurtado’s research group proposed a novel mechanism to further enhance near-field radiative heat transfer (NFRHT) with the utilization of Si metasurfaces [Fig. 7(b)]. In 2017, Shi’s research group proposed multilayer graphene-hBN heterostructures to further enhance the near-field thermal radiation [Fig. 7(c)]. In 2018, Ilic’s research group theoretically demonstrated a near-field radiative thermal switch based on thermally excited surface plasmons in graphene resonators [Fig. 7(d)]. Numerous research on manipulating thermal emission has brought new perspectives for various infrared applications, including radiative cooling, thermophotovoltaic devices, thermal camouflage, thermal imaging, and biochemical sensing. In 2013, Rephaeli’s research group presented a metal-dielectric photonic structure capable of radiative cooling in daytime outdoor conditions [Fig. 8(a)]. In 2017, Zhai’s research group demonstrated efficient day and nighttime radiative cooling with a randomized and glass-polymer hybrid metamaterial [Fig. 8(b)]. In 2021, Zeng’s research group demonstrated a hierarchically designed polymer nanofiber-based film, which enables selective mid-infrared emission, effective sunlight reflection, and excellent all-day radiative cooling performance [Fig. 8(c)]. In 2018, Chang’s research group demonstrated tungsten-based refractory metasurfaces with desired spectral selectivity for solar thermophotovoltaics (STPVs) applications [Fig. 8(d)]. In 2018, Salihoglu’s research group reported a new class of active thermal surfaces enabling efficient real-time electrical control of thermal emission over the full infrared spectrum without changing the surface temperature [Fig. 8(e)]. Thermal emission manipulation in multiple degrees of freedom is mostly performed on a single metasurface, therefore resulting in limited capabilities of manipulating thermal emission. Further extending the manipulation degrees of freedom is essential for practical infrared applications. A pixelated metasurface array is a promising solution to this problem. In 2022, Chu’s research group proposed a micro-meta-cavity array by combining nanohole metasurfaces and Fabry‒Pérot cavity [Fig. 9(a)]. Polarization, wavelength, and spatial multiplexing thermal emission with high spatial resolution have also been experimentally demonstrated by utilizing nanohole patterns. In 2023, Chu’s research group experimentally demonstrated an integrated technology that allows for indirect absorption spectrum measurement via thermal emission of a meta-cavity array. This indirect measurement method opens a new avenue for compact infrared spectroscopy analysis.

Wavelength-selective thermal emission is the key to improving the efficiency of various thermal management applications. Metasurface-based thermal emitters have successfully achieved the emission spectrum required by plenty of infrared devices. Meanwhile, we discuss the flexible manipulation of the radiation angle, polarization, and coherence properties of thermal emission, with a focus on nonreciprocal thermal emission and near-field thermal emission research. An integrated metasurface array on a single chip can be utilized promisingly to obtain more tunable degrees of freedom. Given that integration and miniaturization are the development goals for future flat and compact infrared applications, there are still several challenges for on-chip thermal emission manipulation. In recent years, various interesting physics mechanisms have been explored and applied to optical research.