Formation of Quasi-Bound States in the Continuum in a Single Deformed Microcavity

TL;DR

This paper demonstrates a method to generate quasi-bound states in the continuum within a single deformed microcavity, significantly enhancing the quality factor and enabling improved control of optical resonances for photonic applications.

Contribution

The study introduces a practical mechanism using boundary deformations to create stable unidirectional radiation channels and achieve quasi-BICs in whispering-gallery-mode microcavities.

Findings

Over 3-fold increase in Q factor for a long-lived resonance.

Experimental verification of strong mode coupling.

Suppressed leakage loss in the microcavity.

Abstract

Bound states in the continuum (BIC) holds significant promise in manipulating electromagnetic fields and reducing losses in optical structures, leading to advancements in both fundamental research and practical applications. Despite their observation in various optical systems, the behavior of BIC in whispering-gallery-modes (WGMs) optical microcavities, essential components of photonic integrated chips, has yet to be thoroughly explored. In this study, we propose and experimentally identify a robust mechanism for generating quasi-BIC in a single deformed microcavity. By introducing boundary deformations, we construct stable unidirectional radiation channels as leaking continuum shared by different resonant modes and experimentally verify their external strong mode coupling. This results in drastically suppressed leaking loss of one originally long-lived resonance, manifested as more than a 3-fold enhancement of its quality (Q) factor, while the other short-lived resonance becomes more lossy, demonstrating the formation of Friedrich-Wintgen quasi-BICs as corroborated by both the theoretical model and the experimental data. This research will provide a practical approach to enhance the Q factor of optical microcavities, opening up potential applications in the area of deformed microcavities, nonlinear optics, quantum optics, and integrated photonics.