Omnidirectional photonic chiral flatband in nonlocal membrane metasurfaces

Omnidirectional photonic flatbands combine enhanced density of states with angular robustness, yet integrating these properties with optical chirality remains challenging. Here, we experimentally realize an omnidirectional chiral flat-band resonance in a nonlocal silicon membrane metasurface patterned with periodic square-lattice air holes. Increasing the lattice period suppresses evanescent coupling between neighboring guided resonances, driving the system into the weak-coupling regime of a tight-binding model and producing flat dispersions along orthogonal in-plane directions. Breaking the in-plane mirror symmetry further imparts chirality without compromising the flat-band character. The resulting resonance remains robust over incidence angles of ±5° and across azimuthal directions, while maintaining a nearly invariant resonance wavelength, a Q-factor of 886 and a circular dichroism of 0.86. The chiral flatband further enhances third-harmonic generation, which inherits its spin selectivity and angular robustness, enabling chirality-controlled frequency-upconversion imaging. Our results establish a planar platform for omnidirectional chiral flat-band physics, nonlinear nanophotonics and imaging. This work presents a general strategy for realizing omnidirectional photonic chiral flat bands in nonlocal silicon membrane metasurfaces, enabling efficient third-harmonic generation and photonic-spin-controlled nonlinear imaging.