High-sensitive refractive index sensing enabled by topological charge evolution

A high-sensitive refractive index sensor is proposed and demonstrated by utilizing merging bound states in the continuum, namely a set of integer topological charges in the momentum space. Through varying cladding refractive index $n_c$ , the topological charges continuously depart from the merging state, and result in the robust and considerable high- $Q$ factors (above $7\times 10^4$ ) in a large detection range of 0.456. In such a range, sensing sensitivity of $\sim$ 36 nm/RIU and figure-of-merit from $\sim$ 5990 (air) to 1607 ( $n_c$ = 1.456) are achieved. Meanwhile, the detection limit on the order of $10^{-5}$ RIU is clearly distinguishable from the measured spectrum under tiny variation of cladding refractive index. Our work paves the way for promising integrated high-sensitive sensors in many biological and chemical applications.