Perfect Absorption and Refractive-Index Sensing by Metasurfaces Composed of Cross-Shaped Hole Arrays in Metal Substrate

Achieving perfect electromagnetic wave absorption with a sub-nanometer bandwidth is challenging, which, however, is desired for high-performance refractive-index sensing. In this work, we theoretically study metasurfaces for sensing applications based on an ultra-narrow band perfect absorption in the infrared region, whose full width at half maximum (FWHM) is only 1.74 nm. The studied metasurfaces are composed of a periodic array of cross-shaped holes in a silver substrate. The ultra-narrow band perfect absorption is related to a hybrid mode, whose physical mechanism is revealed by using a coupling model of two oscillators. The hybrid mode results from the strong coupling between the magnetic resonances in individual cross-shaped holes and the surface plasmon polaritons on the top surface of the silver substrate. Two conventional parameters, sensitivity (S) and figure of merit (FOM), are used to estimate the sensing performance, which are 1317 nm/RIU and 756, respectively. Such high-performance parameters suggest great potential for the application of label-free biosensing.