Reciprocal-space engineering of quasi-bound states in the continuum in photonic crystal slabs for high-Q microcavities

The bound states in the continuum (BICs) in photonic crystal (PhC) slabs presume infinite periodicity in the in-plane direction. Thus, a large number of unit cells are typically required to implement the BICs with a high quality (Q) factor. Here, we report on a method to engineer the reciprocal-space properties of BICs, which enables to keep the effect of the BIC phenomenon strong even for a microcavity of a few unit cells. For example, based on this method, a 3D microcavity of 4 unit cells can attain a Q factor of 18k. This allows for various BIC studies in a very compact platform, as well as novel functionalities for many important applications.