Abstract
ABSTRACTBroadband absorption is needed for a wide spectrum of applications such as solar–thermal conversion, radiative cooling, and photodetection. In this work, we theoretically show that trapezoidal gratings made of a natural hyperbolic material on a metal substrate can be used to achieve omnidirectional perfect absorption in a relatively broad spectral region. Hexagonal boron nitride (hBN) is taken as a mid-infrared polar material with hyperbolic characteristic in the wavelength region from 6.2 to 7.3 μm. The anisotropic rigorous-coupled wave analysis is used to calculate the absorptance as well as the local power dissipation and field distributions. The main mechanism for the broadband perfect absorption is elucidated by considering the slow-light effect in a hyperbolic waveguide, which can trap the incident light of different wavelengths in different regions of the trapezoid. The spectral range and bandwidth of near perfect absorption can be adjusted within the hyperbolic range of the material by changing the shape of the trapezoid. Moreover, the substrate can play a role on the reflectance and transmittance of the structure for the wavelengths that do not support the slow-light effect. Similar designs can be applied to other hyperbolic materials to achieve perfect absorption in various wavelength regions.
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