Dual Flat-Bands of Bound State in the Continuum and Radiative Mode via TE-TM Coupling

TL;DR

This paper introduces a symmetry-controlled method to create dual flat-bands of bound states in the continuum and their radiative counterparts in photonic crystal slabs, enhancing photonic device design flexibility.

Contribution

It presents a novel symmetry-breaking approach to achieve dual flat-bands via TE-TM coupling, applicable across various materials and polarization states, with a physics-based coupling model.

Findings

Dual flat-bands can be realized through symmetry breaking in photonic crystals.

The mechanism is governed by geometric tuning, not accidental degeneracies.

Enhanced angular bandwidths are achieved in high-index systems.

Abstract

A general symmetry-controlled mechanism is proposed for realizing dual flat-bands of bound state in the continuum (BIC) and its radiative counterpart in photonic crystal slabs. By breaking the vertical mirror symmetry of slab, inter-polarization coupling between TE-like and TM-like modes is activated, while intra-polarization coupling among modes within the same polarization class is simultaneously preserved. The cooperative action of these two coupling channels gives rise to the concurrent flattening of both the BIC-hosting band and the radiative band, resulting in a dual flat-band system with strongly contrasting quality (Q) factors. An effective two-step coupling model is constructed to capture the essential physics and show that the emergence of the flat bands is governed by geometric tuning rather than accidental degeneracies. The mechanism is shown to be generic with respect to polarization and material platform, enabling dual flat-band states in both low- and high-index systems, with substantially enhanced angular bandwidths in the latter. These finding establish a unified route for flat-band photonic engineering and provide a robust platform for angle-tolerant resonant photonic functionalities.