Photo-Thermally Tunable Photon-Pair Generation in Dielectric Metasurfaces

Photon-pair sources based on spontaneous four-wave mixing (SFWM) in integrated photonics are often spectrally static. We demonstrate and model a fundamental thermo-optical mechanism that modulates photon-pair generation in amorphous silicon (a-Si) thin films and metasurfaces via SFWM. Femtosecond-pulsed excitation yields g⁽²⁾(0) > 400 in unpatterned a-Si, confirming high-purity nonclassical emission. Resonant a-Si metasurfaces produce photon pairs at rates exceeding 3.8 kHz under 0.6 mW pump power through Mie-type modes. Pump absorption induces localized heating that redshifts resonances, altering modal overlap and SFWM efficiency, leading to deviations from the quadratic power scaling expected in the undepleted regime. Coupled electromagnetic and heat-transfer simulations quantitatively reproduce these trends. Polarization-resolved measurements show nearly isotropic nonlinear responses, with $\left|{\chi }_{a-Si}^{\left(3\right)}\right|\approx 3\times \left|{\chi }_{poly-Si}^{\left(3\right)}\right|$. This work positions a-Si as a bright, CMOS-compatible quantum photonics platform and identifies thermo-optical detuning as a key mechanism that should be considered─and potentially harnessed─in integrated photon-pair sources.