Experimental Observation of Vector Bound States in the Continuum

Abstract Bound states in the continuum (BICs) provide an important way for creating photonic devices reliant upon optical modes with high Q‐factors. However, current experimental implementations of BICs in photonic crystal (PhC) slabs only possess a certain polarization, which fundamentally restricts their use to surface‐enhanced applications assisted by scalar‐polarized near fields. In this work, vector BICs in PhC slabs are theoretically designed and experimentally realized. In theory, the symmetry‐protected vector BICs can be constructed by tuning geometric parameters of the PhC slab, where the coalescence of eigenfrequencies for a pair of TE‐ and TM‐like modes appears. It is found that the homogeneous superchiral fields with three orders of magnitude larger than circular polarized lights can be achieved, assisted by vector quasi‐BICs. Furthermore, Si₃N₄ PhC slabs are experimentally fabricated and their band structures are measured. It is shown that a pair of symmetry‐protected BICs with TE‐ and TM‐like polarizations can coalesce with each other at a suitable value for the diameter of array holes, indicating the realization of vector BICs. This work can possess important applications in the field of ultrasensitive detections of molecular chirality.