  Interlayer electronic coupling in two-dimensional (2D) materials enables
tunable and emergent properties by stacking engineering. However, it also
brings significant evolution of electronic structures and attenuation of
excitonic effects in 2D 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 novel van der Waals crystal, niobium oxide dichloride, featuring a
vanishing interlayer electronic coupling and scalable second harmonic
generation intensity of up to three orders higher than that of exciton-resonant
monolayer WS2. Importantly, the strong second-order nonlinearity enables
correlated parametric photon pair generation, via a spontaneous parametric
down-conversion (SPDC) process, in flakes as thin as ~46 nm. To our knowledge,
this is the first SPDC source unambiguously demonstrated in 2D 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, and high-performance photon modulators in both classical and quantum
optical technologies.
