Observation of bound states in the continuum embedded in symmetry bandgaps

TL;DR

This paper demonstrates that embedding symmetry-protected bound states in the continuum within a three-dimensional photonic crystal environment enables their protection across a broader frequency range, expanding design possibilities for high-Q photonic devices.

Contribution

The study introduces a novel method of protecting BICs by embedding them in a 3D photonic crystal environment, overcoming previous limitations to specific Brillouin zone regions.

Findings

BICs can be protected in a symmetry bandgap of a 3D photonic crystal.

A single layer of the photonic crystal environment suffices for BIC protection.

The approach broadens the frequency range and applications of high-Q photonic states.

Abstract

In the last decade, symmetry-protected bound states in the continuum (BICs) have proven to be an important design principle for creating and enhancing devices reliant upon states with high quality (Q) factors, such as sensors, lasers, and those for harmonic generation. However, as we show, current implementations of symmetry-protected BICs in photonic crystal slabs can only be found at the center of the Brillouin zone and below the Bragg-diffraction limit, which fundamentally restricts their use to single-frequency applications. By 3D-micro printing a photonic crystal structure using two-photon polymerization, we demonstrate that this limitation can be overcome by altering the radiative environment surrounding the slab to be a three-dimensional photonic crystal. This allows for the protection of a line of BICs by embedding it in a symmetry bandgap of the crystal. Moreover, we experimentally verify that just a single layer of this photonic crystal environment is sufficient. This concept significantly expands the design freedom available for developing next-generation devices with high-Q states.