Quasi-bound flat bands in the continuum

TL;DR

This paper introduces quasi-bound flat bands in the continuum (quasi-BFICs), a novel class of optical states that exhibit high-Q resonances across a broad k-space range, leveraging disorder to enhance device performance.

Contribution

It presents the concept of quasi-BFICs, demonstrating their origin from disorder-induced effects and showing how disorder can be used to optimize optical properties.

Findings

Quasi-BFICs exhibit high Q-factors across a wide k-space range.

Disorder-induced band folding leads to mode localization and topological charges.

Experimental measurements confirm the existence and properties of quasi-BFICs.

Abstract

Bound states in the continuum (BICs) are widely known spatially localized states experimentally implemented as quasi-BICs. Although they emerged as a promising solution for achieving high-quality resonances in photonic structures, quasi-BICs are confined to a very narrow range in k-space and are highly sensitive to disorder. Here, we introduce quasi-bound flat bands in the continuum (quasi-BFICs), a class of optical states where Bloch modes are found within a photonic flat band, leading to a quasi-BIC behaviour at every k-point above the light line. We analytically and numerically demonstrate the origin of quasi-BFICs from the disorder-induced band folding, mode localization and multiple topological charges in k-space, and identify the optimal strength of structural disorder to maximise their generation probability. Angle-resolved transmission and Q-factor measurements confirm the existence of quasi-BFICs, opening new avenues for designing devices with high quality factor and wide-angle response, presenting a counterintuitive strategy that leverages disorder to enhance optical performance.