Ultrathin quantum light source with van der Waals NbOCl2 crystal

Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies(1-4).

Automated summary

A van der Waals crystal, niobium oxide dichloride (NbOCl2), has been found to exhibit weakened interlayer electronic coupling and monolayer-like excitonic behavior in the bulk form. It also has a scalable second-harmonic generation intensity that is three orders higher than that in monolayer WS2. Additionally, the strong second-order nonlinearity in thin flakes as thin as 46 nm enables correlated parametric photon pair generation through a spontaneous parametric down-conversion (SPDC) process. This is the first SPDC source demonstrated in two-dimensional layered materials and the thinnest SPDC source ever reported. This research opens avenues for developing van der Waals material-based ultracompact on-chip SPDC sources and high-performance photon modulators in both classical and quantum optical technologies.