Symmetric Bound States in the Continuum in an All Graphene Metasurface—Design and Sensor Applications

Abstract

In this article, we have proposed an all-graphene-based 2-D symmetric bound state in the continuum (BIC) metasurface for the THz region. The structure consists of rectangular dimers etched on the graphene layer on a dielectric substrate. We have demonstrated our design in the transmission mode of operation. Within the structure’s unit cell, two dimers are arranged at a regular spacing. At equal lengths and thicknesses, the quality factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> -factor) diverges, marking the signature of BIC. The in-plane asymmetry is introduced by varying the size of one of the dimers. Due to this asymmetry, plasmonic quasi-BIC (Q-BIC) dips appear in the transmission spectrum with finite <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> -factor and resonance linewidth. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> -factor follows an inverse square dependency with the asymmetry parameter. The chemical potential of the graphene can actively tune the Q-BIC transmission dips. By varying the chemical potential from 0.95 to 0.45 eV, a spectral shift of Q-BIC from 11.43 to 7.7 THz is obtained, maintaining the same structure. We then exploited the Q-BIC plasmonic transmission dip for various sensing applications. We have shown that we can design a biosensor for detecting malaria stages in human blood by tuning the Q-BIC point. A maximum sensitivity of 10.56 THz/RIU has been obtained for the schizont stage of malaria. We have also demonstrated that the same sensor design can be used to detect hazardous solvents such as toluene and methanol, reporting a maximum sensitivity of 3.505 THz/RIU.