Control Of Thermal Emission Using Polar Materials With Layered Structures

Thermal emission source is known as incoherent source,which has a broad spectrum and is quasi-isotropic.However,spectrally and spatially selective emission sources are crucial for improving the efficiency of many energy conversion systems,e.g.solar cells and thermophotovoltaic devices.According to Kirchhoffs law,for an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium,the emissivity is equal to the absorptivity.Therefore,we can regulate the wavelength,direction and polarization of thermal emission by tailoring absorptivity of the devices.Devices based on surface plasmon resonance have been widely used to regulate thermal emissions.However,metals are highly lossy and have low quality factor due to the presence of free charge carriers.We utilize polar materials with lower losses to achieve high Q-factor(quality factor)thermal emission.In this work,we use polar crystals to control spectral and spatial property of thermal emission.In the aspect of spatial regulation of thermal radiation,by using aluminium oxide and silicon carbide,we combine Berreman mode and Fabry-Perot resonance to achieve the enhancement of thermal emission at small emission angles without the change of emission wavelength.In the aspect of spectral regulation of thermal radiation,we use anistropic hexagonal boron nitride to design high Q-factor thermal emitters with ultra-thin layered structures.When incident angle is 15° degree,the absorption spectrum of the structure with different thicknesses of hBN and under different polarization incident light is simulated using COMSOL Multiphysics.It is shown that the absorption peak around 7 ?m is polarization insensitive,but the absorption peak around 12 ?m can only be stimulated by p-polarized light.When the thickness of hBN is 150 nm,this device can achieve the absorptivity of 0.33 and the Q-factor of 640 around the wavelength of 12 ?m.In experiment,when the thickness of hBN is 109 nm,this device can achieve the absorptivity of 0.41 and the Q-factor of 99 around the wavelength of 12 ?m.The structures we design have advantages of simple fabrication and low cost,which would facilitate them to be widely used in the future.