Nanogap enhancement of the refractometric sensitivity at quasi-bound states in the continuum in all-dielectric metasurfaces

All-dielectric metasurfaces have great potential as highly sensitive refractometric sensors relying on their spectral shifts because of an extensive range of design flexibilities and their smaller absorption losses than plasmonic platforms. However, simultaneously realizing both high quality (Q) factors and the large interplay of light with external medium in such photonic sensors remains one of the key challenges for their better performance. This study proposes silicon block metasurfaces with nanogaps to overcome this challenge based on quasi-bound states in the continuum (BICs). We show that the metasurface has two quasi-BIC modes—magnetic dipole (MD) and electric quadrupole (EQ)—and their electric fields experience large enhancement at the ∼30 nm nanogap regions. Consequently, introducing nanogaps into the metasurfaces increases the environmental refractive index sensitivity by up to 2.7 times in the MD mode while keeping the high Q factors and achieves the figure-of-merit (FOM) of 239. In addition, we show that the appropriate selection of the amount of asymmetry is needed under the trade-off between the FOM and spectral signal-to-noise ratio, which provides design guidelines for highly sensitive biosensors based on quasi-BICs.