High-Q refractive index sensor with an ultrawide detection range based on topological bound states in the continuum

Previous studies on refractive index sensors have shown that their sensing characteristics are limited by variations in the background environment refractive index, resulting in a significant decrease in the figure of merit (FOM) and sensitivity of the sensor. Here, we design a high-Q refractive index sensor, which is composed of a Dirac semimetal. The proposed sensor is based on topological bound states in the continuum (BICs), which have a diverging quality factor, and exhibits extremely high FOM and detection sensitivity over a wide variation range of the background environment refractive index. Its operation is based on the reciprocating motion of two pairs of BICs in the k_x and k_y high-symmetry lines of the momentum space. Specifically, two pairs of BICs, which are characterized by topological charges, can be merged and generated by varying the Fermi energy of the Dirac semimetal. Furthermore, we extract the relation between the Fermi energy and the background environment refractive index for the merging-BIC. This ensures that the FOM is extremely high over a very wide variation range of the background environment refractive index. Our findings provide a perspective for investigating ultrahigh performance refractive index sensors based on merging-BICs.