Efficient generation of entangled photons typically relies on spontaneous parametric down-conversion (SPDC) in phase-matched macroscopic nonlinear media. However, generating entanglement under phase-matching constraints requires additional bulk optics or interferometers. In contrast, ultrathin van der Waals semiconductors - such as transition metal dichalcogenides (TMDs) - exhibit strong enough optical nonlinearities for SPDC to be observed from subwavelength-thick media, thereby bypassing conventional phase-matching constraints. In this microscopic domain, the intrinsic crystal symmetry governs the nonlinear optical response, enabling the native generation of polarization-entangled photon pairs. However, generating these states efficiently has been fundamentally restricted by the material's coherence length (
Lc), which limits the attainable conversion efficiency. Here, we investigate periodically-poled TMDs (PPTMDs) designed to scale up this interaction via quasi-phase matching. We demonstrate that mechanically flipping the sign of the nonlinearity at precise intervals of
Lc introduces quasi-phase matching, that scales the pair-production rate while preserving the pristine, symmetry-generated polarization entanglement, with fidelities exceeding 99%. Backed by a rigorous theoretical model, our work clarifies the interplay between crystal symmetry and propagation effects in thin nonlinear media, providing a new avenue for engineering quantum light in nanophotonic systems.
Miniaturization of nonlinear optical elements is one of the major trends of modern photonics. In particular, in the ‘flatoptics’ geometry, the sample is ultrathin along the light propagation direction. For nonlinear optical effects in such samples, phase matching is satisfied automatically, which allows for the use of materials with giant nonlinear susceptibilities. The tiny thickness reduces the efficiency but it can be compensated for by using geometric and material resonances. Recently also spontaneous parametric down-conversion (SPDC), leading to the generation of entangled photons, has been implemented in ultrasmall sources: subwavelength layers, metasurfaces, even nanoantennas. The lifted constraint of phase matching gives to SPDC even more freedom than to classical frequency conversion effects. The reason is that SPDC, as a spontaneous effect, is stimulated by quantum vacuum fluctuations, which populate all modes uniformly. SPDC in ultrathin samples demonstrates very broad spectral and angular width, extremely high degrees of continuous-variable entanglement, tunable polarization entanglement, and orders of magnitude enhancement of photon pair production rate due to the geometric resonances of dielectric metasurfaces.