Dispersion-free photon-pair generation enabled by tailored multipolar flatband resonances from a nonlinear metasurface.

Nonlinear metasurfaces have emerged as a versatile platform for photon-pair generation through spontaneous parametric down-conversion (SPDC). However, the angular dispersion of optical resonances restricts the emission range and momentum-space uniformity of the generated photon pairs, thereby hindering practical applications in quantum sensing and imaging. Here, we propose and numerically investigate an all-dielectric InGaP metasurface that supports angle-robust photon-pair generation enabled by tailored multipolar flatband resonances. By introducing elliptical nanopores into nonlinear nanodisks, we engineer the far-field coupling and polarization response of weakly dispersive flatband modes while preserving strong field confinement. As a result, the tailored structure exhibits a more than one-order-of-magnitude enhancement in second-harmonic generation efficiency compared with the unperforated design. Based on the quantum-classical correspondence between SPDC and sum-frequency generation (SFG), we further reveal a pronounced enhancement of the nonlinear response and demonstrate stable photon-pair generation with rates up to approximately 2×105 Hz over an angular range of ±12°. In addition, the generated photon pairs remain robust over a broad momentum range and show tunable polarization properties under different pump wavelengths. These results provide a practical route toward compact and efficient quantum light sources in nonlinear metasurfaces, and highlight the potential of flatband engineering for applications in quantum imaging and integrated quantum photonics.