Optical Brewster Metasurfaces Exhibiting Ultrabroadband Reflectionless Absorption and Extreme Angular Asymmetry

Abstract

Impedance mismatch between free space and absorptive materials is a fundamental issue plaguing the pursuit of high-efficiency light absorption. In this work, we design and numerically demonstrate a type of nonresonant impedance-matched optical metasurface exhibiting ultrabroadband reflectionless absorption based on the anomalous Brewster effect, which is denoted as an optical Brewster metasurface here. Interestingly, the Brewster metasurface exhibits a type of extreme angular asymmetry: a transition between perfect transparency and perfect absorption appears when the sign of the incident angle is changed. Such a remarkable phenomenon originates from the coexistence of traditional and anomalous Brewster effects. Guidelines for material selection based on an effective-medium description and strategies such as the integration of a metal back-reflector or a folded metasurface are proposed to improve the absorption performance. Finally, a gradient optical Brewster metasurface exhibiting ultrabroadband and near-omnidirectional reflectionless absorption is demonstrated. Such high-efficiency asymmetric optical metasurfaces may find applications in optoelectrical and thermal devices like photodetectors, thermal emitters, and photovoltaics.