Thermal emission is the most critical optical property in thermal radiation. Precise regulation of the thermal emissivity of materials is essential for practical applications of thermally related devices such as energy harvesting, chemical sensing, and dynamic camouflage. Designing micro- or nano-structured metasurfaces is an effective way to modulate spectral emissivity. All-dielectric metasurfaces supporting surface phonon polaritons (SPhPs) can achieve strong light-matter interaction with low optical loss and play an essential role in coherent thermal emission. Among them, silicon carbide (SiC) is a promising candidate due to its rich molecular vibrations and thermal phenomena. Numerous studies have successfully efficiently tuned the coherence of thermal emission by constructing SiC metasurface. However, the physical origin of SPhP emission modes in SiC and their coupling mechanism have not been clearly elucidated yet. In addition, temperature affects the emission properties of the material by changing its optical properties. Still, the current lack of data on the high-temperature dielectric function of SiC is not conducive to further expansion in this direction. Therefore, the physical mechanisms regulating the coherent thermal emission of hexagonal SiC (4H-SiC) all-dielectric metasurfaces are systematically investigated by adjusting the geometric period and temperature.

In this paper, the ellipsometric parameters ψ and Δ are obtained with the help of IR-VASE Mark Ⅱspectroscopic ellipsometry (SE), and the temperature-dependent dielectric functions (ε=ε'+iε″) of 4H-SiC are derived by fitting the B-spline optical model. The obtained dielectric functions are chosen as input for the finite element modeling (FEM) simulations in the form of interpolation functions. The FEM simulations investigate the emitting mode's quality and the metasurface's thermal emission potential in terms of temperature, incidence angle, and structural period. Absorption energy (W) calculations are used to investigate the absorption mechanism of the metasurface at different geometric periods.

Firstly, the dielectric functions of 4H-SiC are obtained experimentally, including anisotropic and temperature-dependent dielectric functions (Fig. 2). The results show that the high temperature reduces the polarization intensity of 4H-SiC. The excitation conditions of the metasurface's emitting modes are obtained with the help of FEM, where the SPhPs mode of the grating structure with a period of P=6.6 μm is excited at an incidence angle of about 7.5° [Fig. 3(d)], and the local surface phonon polarizations (LSPhPs) mode of the micron pillar array with a period of P=3 μm is excited at an incidence angle of about 3° [Fig. 3(e)]. When the period of the micron pillar is expanded (P>5 μm), both SPhPs and LSPhPs modes can be generated together. Figure 5 shows that W of the LSPhPs mode is mainly distributed on the top and bottom surfaces of the column. W tends to be more localized at the bottom as the period increases, and the substrate surface gradually shows the absorption distribution. It is caused by the reduction of inter-column coupling that allows the diffusion of W to be released. The large period increases the spatial coherence of the emissivity (Fig. 6) and the Q-factor of the LSPhPs mode [Fig. 8(a)], while the high temperature decreases the spatial coherence (Fig. 7) and Q-factor of both modes [Fig. 8(b)].

In summary, the effects of geometric period and temperature on the coherent thermal emission characteristics of the 4H-SiC all-dielectric metasurface resonator are systematically investigated by using SE and FEM. The experimental results show that the dielectric function ε_zof the parallel optical axis in anisotropic 4H-SiC has more robust polarization properties than that of the vertical optical axis ε_x, and high temperature significantly tunes the dielectric function of the material and reduces its polarization intensity. FEM results indicate that SPhPs induce coupling between optical modes and dominate the modulation of coherent thermal emission, while the zone-folded longitudinal optical phonon (ZFLO) modes contribute a higher Q-factor. In addition, the geometric period positively affects the excitation and coupling of each emission mode, which contributes to the spatial coherence of the emissivity but not the temporal coherence of the ZFLO and SPhPs modes. The high temperatures reduce the coherent thermal emission by weakening the excitation of SPhPs. It is concluded that the low effective propagation length of SPhPs at small periods and high temperatures is the direct reason for the low coherent ability of thermal emission. This work comprehensively reveals the emission characteristics of the 4H-SiC resonators from both geometric design and material properties and guides exploring potential applications of near-field thermal radiation and thermal imaging with high spatial resolution.