Photonic devices rarely provide both elaborate spatial control and sharp spectral control over an incoming wavefront. In optical metasurfaces, for example, the localized modes of individual meta-units govern the wavefront shape over a broad bandwidth, while nonlocal lattice modes extended over many meta-units support high quality-factor resonances. We experimentally demonstrate dielectric metasurfaces that offer both spatial and spectral control of light, realizing a metalens focusing light only over a narrowband resonance while leaving off-resonant frequencies unaffected. Our devices realize such functionality by supporting a quasi-bound state in the continuum encoded with a spatially varying geometric phase. We also show that our resonant metasurfaces can be cascaded to realize hyperspectral wavefront shaping, which may prove useful for augmented reality glasses, transparent displays and high-capacity optical communications.
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