High-Q quasi-BIC in photonic crystal nanobeam for ultrahigh sensitivity refractive index sensing

We propose and numerically demonstrate the existence of bound state in the continuum (BIC) in silicon-based one-dimensional photonic crystal nanobeam (1D-PCN). The 1D-PCN is formed by introducing rectangular air nanoholes in a simple 1D nanobeam waveguide. By engineering the structure parameters, an ideal BIC with infinite quality (Q) factor is demonstrated in 1D-PCN. On this basis, we further investigate the refractive index sensing performance with quasi-BIC by introducing defects into the aforementioned 1D-PCN structure, and an ultrahigh Q factor of ∼ 4.1 × 107 is obtained. Compared to the conventional sensing mechanism based on the shift of forbidden bands modes, the 1D-PCN sensor based on quasi-BIC has a comparable figure of merit (FOM) higher than 106. It is worth mentioning that the quasi-BIC can be directly excited by external light sources without the special coupling mechanism, which provides a new method for ultrasensitive refractive index sensing.