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Broadband and polarization-independent asymmetric transmission of visible light through a three-dimensional trapezoidal metallic metasurface

In modern optical applications, it has become an important need to flow light unidirectionally. An optical diode realizes this task as an electrical counterpart manipulates the flow of electrons in semiconductor materials. In this study, we show a broadband and polarization-independent optical diode-like mechanism in a metasurface configuration in the visible spectrum. The approach is passive such that the operating principle does not depend on any type of external biasing scheme. The constituted metasurface composed of a periodic type three-dimensional nanoarray of trapezoidal-shaped aluminum metal on a sapphire substrate is designed to produce the desired optical response for opposite directions of illumination. The optical transmission properties were systematically investigated using finite-difference time-domain computations. The asymmetric transmission frequency interval of the designed metasurface is associated with the Wood–Rayleigh anomaly, and the physical principle lies in the generation of the different number of higher order modes upon oppositely incident light. Our design has forward transmission of greater than 50%, backward transmission of less than 28%, and contrast ratio of greater than 3 dB in the entire visible spectrum. Specifically, a maximum forward transmission of 88% at 550 nm wavelength and a very high contrast ratio (∼23  dB) at a wavelength of 461 nm were obtained. It is numerically shown that the asymmetric transmission has been directly related to the appearance of high-order diffractions for only one direction excitation case. This study provides a path toward the realization of optical diodes for applications, such as optical communications and laser systems.