Lines of Bound States in the Continuum in a Phononic Crystal Slab

TL;DR

This paper reveals the existence of symmetry-protected bound states in the continuum forming continuous lines in a phononic crystal slab, which are topologically protected and experimentally demonstrated.

Contribution

It introduces the concept of BIC lines in phononic crystals, showing their topological protection and off-center occurrence, supported by simulations and experiments.

Findings

BICs form continuous lines along high-symmetry directions.

BICs are symmetry-protected both at the gamma point and off-center.

BICs are topologically protected and robust to parameter changes.

Abstract

We demonstrate that bound states in the continuum (BICs) form continuous lines along high-symmetry directions of momentum space in a simple phononic crystal slab. Contrary to common sense, these BICs are symmetry-protected (SP) BICs not only at the center of the Brillouin zone (gamma point) but also off the gamma point. We utilize numerical simulations, a group theory method, and a mode expansion method to comprehensively understand the formation of the BICs. It is revealed that these BICs correspond to phase singularity lines of far-field radiation, and the phase winding number can be defined as a topological index. This makes the BICs topologically protected and robust to any parameter variation that maintains periodicity and rotational symmetry. Finally, the generation of the BICs lines is experimentally demonstrated.