基于全介质的超表面谐振器热发射相干性研究

Thermal Emission Coherence Based on All-Dielectric Metasurface Resonators

以六方碳化硅(4H-SiC)全介质谐振器为例,借助椭圆偏振实验与有限元电磁模拟系统地研究了温度与结构几何周期对其相干热发射的影响机制。研究结果表明:高温削弱了4H-SiC晶体的介电极化与超表面的相干热发射,而几何周期正向影响表面波传输模的激发与耦合强度,从而影响发射率的空间相干性;二者主要通过改变表面波有效传输长度来调节热发射的相干性。此外,全介质超表面谐振器的高Q值对温度与结构周期有很强的依赖性。所得结果可为相关极化超表面谐振器件在热发射调控领域的应用提供参考。

Objective 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 nanostructured metasurfaces is an effective way to modulate spectral emissivity.Alldielectric metasurfaces supporting surface phonon polaritons(SPhPs)can achieve strong lightmatter 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 hightemperature 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(4HSiC)alldielectric metasurfaces are systematically investigated by adjusting the geometric period and temperature.Methods In this paper,the ellipsometric parametersψandΔare obtained with the help of IRVASE MarkⅡspectroscopic ellipsometry(SE),and the temperaturedependent dielectric functions(ε=ε′+iε″)of 4HSiC are derived by fitting the Bspline 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.Results and Discussions Firstly,the dielectric functions of 4HSiC are obtained experimentally,including anisotropic and temperaturedependent dielectric functions(Fig.2).The results show that the high temperature reduces the polarization intensity of 4HSiC.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=6.6μm 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=3μm is excited at an incidence angle of about 3°[Fig.3(e)].When the period of the micron pillar is expanded(P>5μm),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 intercolumn coupling that allows the diffusion of W to be released.The large period increases the spatial coherence of the emissivity(Fig.6)and the Qfactor of the LSPhPs mode[Fig.8(a)],while the high temperature decreases the spatial coherence(Fig.7)and Qfactor of both modes[Fig.8(b)].Conclusions In summary,the effects of geometric period and temperature on the coherent thermal emission characteristics of the 4HSiC alldielectric metasurface resonator are systematically investigated by using SE and FEM.The experimental results show that the dielectric functionεz of the parallel optical axis in anisotropic 4HSiC has more robust polarization properties than that of the vertical optical axisεx,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 zonefolded longitudinal optical phonon(ZFLO)modes contribute a higher Qfactor.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 4HSiC resonators from both geometric design and material properties and guides exploring potential applications of nearfield thermal radiation and thermal imaging with high spatial resolution.