Spontaneous four-wave mixing in a thin layer with second-order nonlinearity

TL;DR

This paper demonstrates that in ultrathin second-order nonlinear layers, direct spontaneous four-wave mixing (SFWM) can dominate photon pair generation, offering new avenues for integrated quantum photonics.

Contribution

The study shows that in thin lithium niobate layers, direct SFWM surpasses cascaded processes, highlighting a novel mechanism for efficient photon pair production in integrated platforms.

Findings

Direct SFWM dominates in thin layers due to smaller wavevector mismatch.

Photon pairs generated via SFWM in lithium niobate were successfully demonstrated.

Broader quantum state engineering opportunities arise from simultaneous second- and third-order nonlinear processes.

Abstract

Pairs of entangled photons are crucial for photonic quantum technologies. The demand for integrability and multi-functionality suggests 'flat' platforms - ultrathin layers and metasurfaces - as sources of photon pairs. With the success in demonstrating spontaneous parametric down-conversion (SPDC) from such sources, an alternative process to generate photon pairs, spontaneous four-wave mixing (SFWM), also starts to attract interest. In materials with nonzero second-order nonlinear susceptibility $\chi^{(2)}$, SFWM can generate photon pairs both directly, through the third-order nonlinear susceptibility $\chi^{(3)}$, and in a cascaded way, through second harmonic generation (SHG) followed by SPDC. Usually, the cascaded process is more efficient. Here, we show that in a thin layer, direct SFWM dominates, because the wavevector mismatch for SFWM is much smaller than for SHG or SPDC. To demonstrate it, we implement the photon pair generation via SFWM in a second-order nonlinear material - a thin layer of lithium niobate (LN). The existence of both second- and third-order nonlinear processes offers broader opportunities for quantum state engineering.