Unidirectional guided resonance continuum of Dirac bands in WS bilayer metasurfaces.

Unidirectional guided resonances are crucial for enhancing the efficiency and performance of various photonic devices, such as couplers and antennas. However, unidirectional guided resonances have been reported only under discrete frequency-wavevector points on a dispersion band, which require accidental interference configurations. Here we show that unidirectional guided resonances can continuously exist across nearly the entire band structure in glide-symmetric bilayer metasurfaces. This continuous excitation of unidirectional guided resonances originates from a synergistic effect between anomalous orthogonality and vertically asymmetric geometry, which is achieved by a Dirac crossing band that preserves glide symmetry. We realize the glide-symmetric bilayer metasurfaces by stacking two WS metasurface layers. Angle-resolved emission spectra directly reveal this unidirectional guided resonance continuum. Our work suggests a fundamental solution to existing narrow-band constraints on unidirectional emission and absorption.