Spectrally Tunable Generation of Entangled Photon Pairs from a Tilted Nonlinear Metasurface

Entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) are the cornerstone of many quantum optical applications. Nonlinear metasurfaces have recently been introduced as a new class of compact SPDC sources [1], and the next step is to enable active tunability of their resulting quantum state. Here, we demonstrate the generation of narrow-band yet frequency tunable, spectrally entangled photon pairs from a nonlinear metasurface. For this, we use guided-mode resonances (GMR) with high angular dispersion. In previous works that use Mie- or BIC-type resonances, for a fixed pump wavelength, the demonstrated SPDC spectrum remained fixed by the respective resonance wavelength [1], [2]. Here, we theoretically and experimentally show that in a metasurface supporting GMRs, the non-degeneracy of the generated photon pairs can be tuned by tilting the metasurface while the pump wavelength can remain constant, see the conceptual rendering in Fig. 1(a).