Bound States-to-Bands in the Continuum in Cylindrical Granular Crystals

TL;DR

This paper explores the creation and control of bound states in the continuum within cylindrical granular crystals, demonstrating their transition to quasi-BICs and formation of bound bands with high-Q, dispersionless resonances.

Contribution

It introduces a tunable mechanical system that supports BICs and shows how these states can form bound bands in periodic structures, with experimental validation.

Findings

Demonstrated transition from BICs to quasi-BICs via tunable contact boundaries

Observed high-Q, dispersionless resonances in a chain of resonators

Established formation of bound bands in a finite periodic structure

Abstract

We theoretically investigate and experimentally demonstrate that genuine bound states in the continuum (BICs) -- polarization-protected BICs -- can be completely localized within finite-size solid resonators. This bound mode is realized in a highly tunable mechanical system made of cylindrical granular crystals, where tunning the contact boundaries enables the in situ transition from the BICs to quasi-BICs in a controllable manner. Since a single-particle resonator can support BICs itself, these bound states can extend to form bound bands within periodic structures composed of such resonators. We experimentally demonstrate the emergence of a quasi-bound (flat) band in a finite chain with broken resonator symmetry, using a laser Doppler vibrometer. Remarkably, we show that all cylindrical resonators within the entire chain exhibit high-Q and dispersionless resonance.