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

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 high-order polaritonic BICs on a wide-angle flat band utilizing a subwavelength (~ λ 0 /35) metasurface based on a vdW magnet CrSBr. The large oscillator strength of direct excitons in CrSBr enables near-ultrastrong coupling with the photonic BIC-band, leading to a polaritonic band with strongly suppressed angular dispersions. Remarkably, the second-order polaritonic BIC-band becomes flattened, with a vanishing energy variation over a wide angular range. In addition, we find that these polaritonic BIC-bands vanish in the transverse magnetic configuration, while leading to hyperbolic exciton-polaritons within the Reststrahlen band. Our findings underscore CrSBr as an exceptional platform for exploring flat-band photonics and polaritonics, paving a new avenue for advances in next-generation optical and quantum technologies.