Broadband bright biphotons from periodically poled triple-resonance metasurface

Biphotons from spontaneous parametric down conversion with broad bandwidth are highly wanted in many quantum technologies. However, achieving broad bandwidth in both frequency and momentum while keeping a high rate remains a challenge for both conventional nonlinear crystals and recently emerging nonlinear metasurfaces. Here, we address this challenge by introducing a periodically poled triple-resonance metasurface (PPTM) incorporating a nano-grating atop a periodically poled LiNbO$_3$ thin film. PPTM supports high-Q guided mode resonances at pump, signal, and idler wavelengths meanwhile enabling quasi-phase matching between three guided modes in a broad frequency/momentum range. The predicted biphoton rate is over 100 MHz/mW with a frequency bandwidth of 165 nm around 1550 nm and a momentum bandwidth of $13^\circ \times 6^\circ$, improving the state-of-the-art by over three orders of magnitude in rate and one order of magnitude in bandwidths. This ultrathin broadband bright biphoton source could stimulate system-level miniaturization of various free-space quantum photonic technologies.