Broadband Bright Biphotons From Periodically Poled Brillouin Zone Folding 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 Brillouin zone folding (PP-BZF) metasurface incorporating a periodically perturbed silica grating atop a periodically poled lithium niobate film. PP-BZF metasurfaces support high-Q guided mode resonances at pump, signal and idler wavelengths while 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 20.5 THz (1472.5-1637.8 nm) and a momentum bandwidth of (Formula presented.), improving the state-of-the-art for metasurface biphoton sources 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.