Enhanced $Q$ factor and robustness of photonic bound states in the continuum merging at locally bent trajectories

TL;DR

This paper demonstrates how locally bent trajectories of bound states in the continuum (BICs) in photonic structures can significantly enhance their quality factor and robustness, leading to ultrahigh-Q resonances with broad tunability.

Contribution

It introduces a novel 'V'-shaped trajectory of BICs in momentum space and shows how merging with symmetry-protected BICs creates a 'K'-shaped ultrahigh-Q region with improved stability.

Findings

Enhanced Q factor near the trajectory's turning point.

Formation of a 'K'-shaped ultrahigh-Q region through merging BICs.

Wide-range stability of high-Q BICs in finite gratings.

Abstract

Bound states in the continuum (BICs) in planar photonic structures have attracted broad scientific interest owing to their exceptional capability to confine light. Topological robustness of certain BICs allows them to be moved in the momentum space by tuning the geometric parameters of the structure. In this work, we study such a BIC in a one-dimensional periodic grating, and find that its momentum-space position can be made a non-monotonic function of a geometric parameter, forming a locally bent ''V''-shaped trajectory. We show that, near the turning point of this trajectory, the robustness of the BIC and its $Q$ factor can be greatly enhanced. We tune such ''V-BICs'' to almost merge with a symmetry-protected BIC at the $\Gamma$-point. This creates a ''K''-shaped ultrahigh-$Q$ region containing a BIC with a much higher and more stable $Q$ factor compared to the ordinary merging BICs. The ''K-BICs'' are also found to provide a strong enhancement of the $Q$ factor in finite gratings over an extremely wide range of geometric parameters. Our findings enable further advancements in the development of ultrahigh-$Q$ BICs and their applications.