Ultra-narrowband perfect absorption in hybrid hetero-metasurface for sensing and optical switching

Abstract Recently, the concept of the bound states in the continuum (BICs) has emerged, demonstrating a highly enhanced light-matter interaction. To achieve perfect absorption, symmetry-protected BICs have been explored as it is convenient to realize and can easily transition into excitable quasi-BICs by inducing asymmetry. However, in most studies, small symmetry breaking parameters are required to achieve perfect absorption with ultra-narrowband, which poses a significant challenge in nanofabrication. In this paper, we propose a hybrid hetero-metasurface, composed of a hetero-nanostructure and a monolayer graphene. The hetero-nanostructure incorporates a hetero-bilayer grating, consisting of a SiO2 upper grating and a Si lower grating, which is positioned on a SiO2 spacer above a perfect electric conductor (PEC) mirror. This design significantly increases symmetry breaking parameters for achieving ultra-narrowband perfect absorption, leading to perfect absorption with a full width at half maximum (FWHM) of 0.045 nm at a symmetry breaking parameter of 15 nm. This represents an order of magnitude larger symmetry breaking parameter compared to the hybrid non-hetero-metasurface. The hybrid hetero-metasurface boasts superior performance, with a sensitivity of 190 nm/RIU and a figure of merit (FOM) reaching up to 5428 RIU-1. As well as narrowband optical switching can be accomplished by altering the polarization direction of the incident light. This work offers a novel approach for achieving ultra-narrowband absorption based on symmetry-protected quasi-BICs, applicable to sensing and narrowband optical switching.