Using fine-structured gratings to implement mid-infrared dual-band absorbers

Abstract

A dual narrowband absorber operating at mid-infrared (mid-IR) frequencies was numerically investigated. The structure consists of a fine-structured silicon grating on a gold film. Each unit cell of the fine-structured silicon grating is composed of two different silicon bars. When illuminated by a transverse-magnetic (TM) polarized plane wave, the absorber will create two absorption bands. At normal incidence, the two absorption bands have respective peak wavelengths of ∼3.864 µm and ∼3.994 µm, and respective bandwidths of ∼28 nm and ∼36 nm. The level of absorption can be higher than 0.998. It is shown that the two absorption bands are related to different silicon bars in each unit cell. Moreover, the physical origin of the two absorption bands is attributed to the different surface-plasmon-polariton (SPP) modes excited in the absorber.