# Topologically Enabled Ultra-high-Q Guided Resonances Robust to Out-of-plane Scattering

## TL;DR

This paper introduces a topological photonic design that significantly suppresses out-of-plane scattering in optical resonators, achieving ultra-high quality factors and robustness against fabrication imperfections, advancing photonic device performance.

## Contribution

The work demonstrates a novel topological approach to enhance optical resonator quality factors by merging bound states in the continuum, with experimental validation of record-high Q-factors.

## Key findings

- Achieved Q-factors up to 4.9×10^5 in the telecommunication regime.
- Resonance enhancement is robust across fabricated samples.
- Topological design reduces out-of-plane scattering losses.

## Abstract

Due to their ability to confine light, optical resonators are of great importance to science and technology, yet their performances are often limited by out-of-plane scattering losses from inevitable fabrication imperfections. Here, we theoretically propose and experimentally demonstrate a class of guided resonances in photonic crystal slabs, where out-of-plane scattering losses are strongly suppressed due to their topological nature. Specifically, these resonances arise when multiple bound states in the continuum - each carrying a topological charge - merge in the momentum space and enhance the quality factors of all resonances nearby. We experimentally achieve quality factors as high as $4.9\times 10^5$ based on these resonances in the telecommunication regime, which is 12-times higher than ordinary designs. We further show this enhancement is robust across the samples we fabricated.Our work paves the way for future explorations of topological photonics in systems with open boundary condition and their applications in improving optoelectronic devices in photonic integrated circuits.