Ultrathin quantum light source enabled by a nonlinear van der Waals crystal with vanishing interlayer-electronic-coupling

TL;DR

This paper introduces a novel ultrathin 2D van der Waals crystal with negligible interlayer coupling that exhibits strong nonlinear optical properties, enabling the first demonstration of spontaneous parametric down-conversion in 2D materials.

Contribution

The study reports a new van der Waals crystal with vanishing interlayer electronic coupling and high second harmonic generation, enabling ultrathin quantum light sources in 2D materials.

Findings

Scalable second harmonic generation up to three orders higher than monolayer WS2.

First demonstration of SPDC in 2D layered materials.

SPDC source as thin as ~46 nm achieved.

Abstract

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.