Quantum Pair Generation in Nonlinear Metasurfaces\nwith Mixed and Pure Photon Polarizations

Metasurfaces\nare highly effective at manipulating classical light\nin the linear regime; however, effectively controlling the polarization\nof nonclassical light generated from nonlinear resonant metasurfaces\nremains a challenge. Here, we present a solution by achieving polarization\nengineering of frequency-nondegenerate biphotons emitted via spontaneous\nparametric down-conversion in GaAs metasurfaces, utilizing quasi-bound\nstates in the continuum (qBIC) resonances to enhance biphoton generation.\nThrough comprehensive polarization tomography, we demonstrate that\nthe emitted photons’ polarization directly reflects the qBIC\nmode’s far-field properties. Furthermore, we show that both\nthe type of qBIC mode and the symmetry of the meta-atoms can be tailored\nto control each single-photon polarization state, and that the subsequent\ntwo-photon polarization states are nearly separable, offering potential\napplications in the heralded generation of single photons with adjustable\npolarization. This work provides a significant step toward utilizing\nmetasurfaces to generate quantum light and engineer their polarization,\na critical aspect for future quantum technologies.