Spectral–Angular Channel Selection of Photon‐Pair Emission via Surface Lattice Resonances in a Lithium Niobate Metasurface

ABSTRACT Spontaneous parametric down‐conversion (SPDC) provides entangled photon pairs for quantum technologies, yet ultrathin nonlinear platforms face a fundamental trade‐off: relaxed longitudinal phase matching broadens the accessible spectral–angular phase space while distributing emission over many weak radiative channels. Here, a one‐dimensional lithium niobate (LN) grating metasurface is proposed to overcome this trade‐off by using surface lattice resonances (SLRs) to convert the broad SPDC phase space into sharp, selected photon‐pair channels. The SLRs arise from coupling between a localized LN‐line resonance and a Rayleigh anomaly. The local line geometry primarily determines the resonant enhancement, whereas the grating period selects the photon‐pair emission angle at the fixed degenerate wavelength. Reciprocity‐based sum‐frequency‐generation calculations show that the SPDC maxima closely follow the linear SLR response, producing pronounced spectral–angular confinement at the designed conjugate angles. Finite‐waist‐pump analysis further reveals pump‐controlled angular anticorrelation. These results identify SLR engineering as a practical route toward compact, directionally selective quantum‐light sources.