Design of hollow metasurfaces for absorption sensors and surface enhanced infrared absorption

Abstract

Absorption sensors detect variations in the imaginary part of refractive index with selectivity in sensing since analytes can be distinguished according to their characteristic absorption bands. In this study we combine the theoretical analysis based on temporal coupled mode theory and numerical calculations in order to derive design principles of optimal metasurfaces aimed for absorption sensing and surface enhanced infrared absorption (SEIRA). We consider hollow metal–insulator-metal metasurfaces with an empty space between two metallic layers. This space acts as a channel for the infiltration of fluid analytes in the region with the maximal electric field enhancement, which provides high sensitivity. We demonstrate that optimal metasurfaces operate in overcoupled regime where radiative decay rate of the resonant mode is larger than non-radiative decay rate. The operation in this regime is adjusted by choosing appropriate channel thickness (the vertical distance between two metallic layers), which should be around three times larger than the channel thickness at the critical coupling point, associated with equal decay rates and zero reflectance. Metasurface period should be as large as possible, whereas the operating frequency should be equal to the resonant frequency of metasurfaces. The same conclusions hold for hollow metasurfaces aimed for SEIRA, while in addition, their resonant frequency should match the vibrational frequency of an analyte under investigation. The absorption sensitivity (reflectance change divided by the change in the imaginary part of refractive index) of an optimal metasurface is larger than , which provides detection of the imaginary part of refractive index below .