Hybrid Bound States in the Continuum beyond Diffraction Limit

TL;DR

This paper introduces hybrid bound states in the continuum (h-BICs) in photonic crystal slabs that operate beyond the diffraction limit, enabling advanced control over light emission and potential new nanophotonic devices.

Contribution

It presents a new class of BICs called hybrid BICs in photonic crystal slabs supporting higher diffraction orders, with tunable properties via symmetry and structural parameters.

Findings

Demonstrated control over light radiation in diffraction channels.

Characterized topological dynamics of h-BICs including generation and annihilation.

Showed symmetry breaking controls diffraction emission.

Abstract

Bound states in the continuum (BICs) have greatly impacted our ability to manipulate light-matter interaction at the nanoscale. However, in periodic structures, BICs are typically realized below the diffraction limit, thus leaving a broad spectral domains largely unexplored. Here, we introduce a new type of at-$\Gamma$ BICs of photonic crystal (PhC) slabs supporting higher diffraction orders, which we call hybrid BICs (h-BICs), whereby symmetry protection and parameter tuning are utilized to suppress light emission in the zeroth- and higher-diffraction orders, respectively. By tuning certain structural parameters of the PhC slab, we fully characterize the dynamics of the topological structure of these h-BICs, including the generation, merging, splitting, and annihilation of circularly polarized states. We further show that the relative amount of light radiated in the first-order diffraction channels can be effectively controlled by simply breaking the $C_{4v}$ symmetry of the PhC slab. Our findings reveal a versatile approach to realize new types of BICs above the diffraction limit, and could potentially inspire new efforts towards development of novel photonic nanodevices, such as multi vortex-beam generators, frequency converters, and lasers.