Design of a high-sensitivity graphene-silicon hybrid micro-disk in a square cavity whispering gallery mode biosensor

Whispering gallery mode (WGM)–based sensors offer high sensitivity, good linearity, and wide measurement range and hence are promising for the field of biosensors. In this paper, a novel Si-graphene-integrated micro-disk biosensor is presented with increased Q-factor and sensitivity. The structure is composed of a Si micro-disk on top of six graphene layers. This composite structure is placed inside a square cavity which is etched in Su-8 substrate. It is shown that graphene reduces the bandwidth and FWHM of the biosensor which can be attributed to the high absorption due to optical transitions in graphene. On the other hand, the square cavity increases the surface area of the resonator thus reducing the electric field losses of the structure. The structure is numerically simulated by FDTD method and optimized for highest sensitivity and Q-factor. It is shown that in the proposed structure, two WGM modes are present. The sensitivity for the first and second modes are as high as 355.5 nm/RIU and 326 nm/RIU, respectively and the figure of merit (FOM) is equal to 640.5 RIU⁻¹ for the first mode and 480.8 RIU⁻¹ for the second mode. This biosensor can be used to detect pathogens such as cancer cells, glucose, influenza, and corona.