Topologically Protected Polaritonic Bound State in the Continuum

TL;DR

This paper demonstrates topologically protected phonon-polaritonic bound states in the continuum in hBN nanoresonator arrays, achieving high-Q modes with minimal radiation loss, and explores their tunability and robustness for nanophotonic applications.

Contribution

It introduces the first experimental validation of topologically protected BICs in phonon-polaritonic systems using hBN nanoresonators, advancing nanophotonics technology.

Findings

Topologically protected BICs exist at the mma-point in hBN nanoresonator arrays.

High-Q polaritonic modes are achieved with minimal radiation leakage.

Breaking symmetry transitions BICs into tunable quasi-BICs with strong field confinement.

Abstract

Bound states in the continuum (BICs) have emerged as powerful tools for realizing ultra-high-Q resonances in nanophotonics. While previous implementations have primarily relied on dielectric metasurfaces, they remain limited by the diffraction limit. In this work, we theoretically and numerically demonstrate and experimentally validate the existence of topologically protected phonon-polaritonic BICs in periodic arrays of cylindrical nanoresonators composed of isotopically enriched hexagonal boron nitride (h11BN), which have the availability of two restrahlen bands (lower (type-I) and upper (type II)), operating in the lower Reststrahlen band (RB-1). Owing to the uniaxial anisotropy of hBN and the rotational symmetry of the structure, these systems support topologically symmetry-protected BICs at the {\Gamma}-point, where radiative losses are fully suppressed. The total quality factor is ultimately bounded by the intrinsic phonon damping of h11BN, enabling high-Q polaritonic modes with minimal radiation leakage. When cylindrical symmetry is broken via angular tilting of incident light away from the normal incidence, these BICs transition into quasi-BICs (q-BICs) with strong field confinement and tunable radiation leakage. This topological protection enables robust control over mode lifetimes and confinement, paving the way toward scalable polaritonic platforms for mid-infrared optoelectronics, sensing, and quantum nanophotonics.