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 nc, the topological charges continuously depart from the merging state, and result in the robust and considerable high-Q factors (above 7 × 10
⁴
) in a large detection range of 0.456. In such a range, sensing sensitivity of ~36 nm/RIU and figure-of-merit from ~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.