# Quasi Bound States in the Continuum with Few Unit Cells of Photonic Crystal Slab

## TL;DR

This paper demonstrates that bound states in the continuum (BICs) can be realized as high-Q quasi-BICs in photonic crystal slabs with only a few unit cells, enabling compact high-performance microcavities.

## Contribution

It introduces a new approach to achieve high-Q quasi-BICs in minimal unit cell structures using a tight-binding perspective and reciprocal-space matching.

## Key findings

- Quasi-BICs with high Q-factors are achievable in structures with only two or three unit cells.
- The Q-factor of these microcavities can reach levels comparable to defect-based photonic crystal microcavities.
- The approach facilitates experimental realization of BICs in compact, mirrorless microcavity platforms.

## Abstract

Bound states in the continuum (BICs) in photonic crystal slabs represent the resonances with an infinite quality(Q)-factor, occurring above the light line for an infinitely periodic structure. We show that a set of BICs can turn into quasi-BICs with a very high Q-factor even for two or three unit cell structures. They are explained by a viewpoint of BICs originating from the tight binding of individual resonances of each unit cell as in semiconductors. Combined with a reciprocal-space matching technique, the microcavities based on quasi-BICs can achieve a Q-factor as high as defect-based PhC microcavities. These results may enable experimental studies of BICs in a compact platform as well as realizing a new concept of high-Q mirrorless microcavities.