Observation of polaritonic flat-band bound states in the continuum in a 2D magnet.
basic_science · Level V
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- Record sourced from PubMed, PMID 42749706.
- Also identified by DOI 10.1038/s41467-026-76785-w.
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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 high-order polaritonic BICs on a wide-angle flat band utilizing a subwavelength (~ <math xmlns="http://www.w3.org/1998/Math/MathML"> <msub><mrow><mi>λ</mi></mrow> <mrow><mn>0</mn></mrow> </msub> </math> /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.