Nonlocal metasurfaces for next-generation flat optics

Metasurfaces enable advanced light control via subwavelength structures, but traditional local models fail at subwavelength scales dominated by nonlocality. Engineered nonlocality exploits spatial correlations and wavevector-dependent resonances to overcome these limits. Three archetypes emerge: (1) Near-field coupling enables continuous phase transfer, stabilizing high-harmonic generation; (2) Momentum matching leverages periodicity for ultrahigh-Q quasi-BIC resonances, enhancing photon-pair generation; (3) Synergistic designs merge both mechanisms for complex wavefront shaping. Despite fabrication challenges, nonlocal metasurfaces promise ultracompact optical systems, high-efficient spatiotemporal light manipulation breakthroughs, and integrated quantum photonics platforms. They represent a key shift bridging nanophotonics, quantum physics, and next-generation optical engineering.