An angular dispersion-free resonant metasurface for quantum photon pair generation

Metasurfaces supporting high quality factor resonances have shown the ability to enhance spontaneous parametric down-conversion (SPDC) process and are therefore seen as a promising platform for entangled quantum photon pair generation. We propose a high quality (Q) factor flat-band optical metasurface for efficient quantum light generation via SPDC. The structure consisting of periodic GaAs nano-resonators on SiO2 was numerically optimised to realise a metasurface that is dispersionless along the Γ–Y axis near the telecommunications band (∼1550 nm). Our simulations show that the metasurface hosts two quasi–bound-states-in-the-continuum (q-BIC) modes with quality factors exceeding 109; the intentional merging of the BICs results in high-Q (>105) across a finite in-plane wave-vector range. This combination of angle-independent resonance frequency with persistently high Q mitigates the off-Γ degradation typical of symmetry-broken q-BICs. The design increases the optical density of states and allows for efficient collection of the generated photons using a high numerical aperture. It is anticipated that the resulting devices could be used for both quantum photon pair generation and for classical nonlinear second-harmonic generation. Our results demonstrate flat-band, high-Q metasurfaces that can be fabricated as a compact route to on-chip nonlinear and quantum photonics.