Observation of polaritonic flat-band bound states in the continuum in a 2D magnet

TL;DR

This paper reports the experimental realization of polaritonic flat-band bound states in the continuum using a vdW magnet metasurface, revealing high Q factors and potential for advanced optical and quantum applications.

Contribution

It demonstrates the first observation of flat-band BICs in a 2D vdW magnet, showing ultrastrong coupling and flat polaritonic bands over a wide angular range.

Findings

Second-order polaritonic BICs are flat across a wide angular range.

Q factors of the polaritonic BICs exceed 1500.

BICs vanish in the transverse magnetic configuration.

Abstract

Flat-band bound states in the continuum (BICs) are topological states with suppressed group velocity and robustness against radiation loss, offering a powerful platform for the exploration of non-Hermitian, nonlinear, topological phenomena and device applications. Van der Waals (vdW) metasurfaces have recently emerged as promising candidates for sustaining BICs and hybridizing with material transitions. However, the realization of flat-band BICs remains elusive. Here, we experimentally demonstrate polaritonic high-order BICs on a wide-angle flat band utilizing a subwavelength metasurface made of a vdW magnet CrSBr. The large oscillator strength of direct excitons in CrSBr enables near ultrastrong coupling with BICs, leading to strongly suppressed polaritonic angular dispersions. Remarkably, second-order polaritonic BICs become flat-band across a wide angular range, with corresponding Q factors exceeding 1500. Additionally, we find that these polaritonic BICs vanish in the transverse magnetic configuration, while leading to fascinating surface hyperbolic exciton-polaritons within the Reststrahlen band. Our findings underscore CrSBr as an exceptional platform for exploring flat-band photonics and polaritonics, paving the new avenue for advances in next-generation optical and quantum technologies.