Experimental Observation of Vector Bound States in the Continuum

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.