Nonlocal Reflectarray Metasurfaces with High-Efficiency Spin-Dependent Deflection

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We report a nonlocal reflectarray metasurface that resolves a trade-off between efficiency and spectral selectivity in nonlocal flat optics. Local reflectarrays achieve high reflection efficiency but lack spectral control, while nonlocal metasurfaces support high-Q quasi–bound states in the continuum (q-BICs) with tailorable narrowband responses but suffer from an intrinsic 25% efficiency limit. By combining nonlocal resonances with a mirror-backed cavity, our design achieves narrowband, spin-dependent beam steering with efficiencies above 93% and tailorable resonant frequency, Q-factor, and deflection direction. We demonstrate with coupled mode theory that, by leveraging a self-coupling mechanism of the resonant metasurface through the reflection of the mirror, we achieve control over the amplitude and phase of reflected light at a fixed frequency. We extend these results to show nonlocal metalenses with resonant-exclusive and spin-dependent focusing. These results position hybrid nonlocal reflectarray metasurfaces as a multifunctional platform for efficient, spin-multiplexed imaging, secure optical communication, and quantum photonics.