Thermal radiation spectrum characteristics of sub-wavelength graded refractive index structures

Abstract

In this study, the thermal radiation spectrum characteristics of subwavelength graded refractive index structures, which included ladder, cone, and pyramid structures, were investigated based on the electromagnetic field theory. The finite difference time domain (FDTD) method was adopted, and the material of the structure was monocrystalline silicon. Perfectly matched layer (PML) absorbing boundary conditions were used in the incident light direction ( z axis), whereas periodic boundary conditions (PBCs) were employed in the horizontal direction ( x , y axis) along the sample surface. The effect of the structure parameters and structure morphology on the spectral characteristics was analyzed. Owing to the ladder structure, the refractive index gradually increased from air to the silicon substrate, and a graded refractive index layer was formed. The reflection was substantially reduced, especially in the short-wave band. When the period and the total height were determined, the anti-reflection effect increased with the number of layers. Furthermore, the reflection of the structures, including cone, ladder, and pyramid, was investigated. The light incident on the surface of the ladder was scattered by the ladder structure, and the absorption of the ladder was greatly enhanced. Among the three subwavelength structures, the ladder exhibited the largest anti-reflection activity. Therefore, the thermal radiation spectrum characteristics of sub-wavelength structures can be tuned by optimizing the structure parameters. In addition, the effect of the incident angle on the spectral properties was studied. For incident angles larger than 0°, the influence of the polarization state on the spectral reflectivity was analyzed. When the incident angle increased from 0° to 30°, the average reflectivity was less than 5%. When the incident angle was 60°, the reflection was enhanced, and the average reflectivity was about 13.96%. The results show that the antireflection properties of the ladder structure in a wide wavelength range (300–2500 nm).