Fig. 4: Characterization of photon pair generation from NbOCl2/metal heterostructures.
From: Boosting classical and quantum nonlinear processes in ultrathin van der Waals materials
a Schematic drawing of the crystal structure of NbOCl2, denoting the polar axis b, nonpolar axis c, and out-of-plane axis a. Red, oxygen atoms; yellow, chlorine atoms; blue, niobium atoms., Inset: An illustration of the quantum spontaneous parametric down conversion (SPDC) process, in which a photon (\({\omega }_{p}\)) incident upon a nonlinear crystal is spontaneously converted into signal and idler photons of lower frequency (\({\omega }_{i}\) and \({\omega }_{s}\)). b Coincidence counts rate from 275 nm thick NbOCl2 on gold (orange), NbOCl2 on SiO2/Si (blue) and the bare SiO2/Si substrate (dark blue). c Coincidence rate in H-V basis from NbOCl2 on gold substrate. H-axis is defined as the polar axis (b-axis), and the fundamental wave is also polarized along the H-axis. d Coincidence count rate from NbOCl2 on gold film with different wavelength integration ranges. lp long pass. e Power dependent coincidence counts rate from NbOCl2 on gold film, showing a linear response. The solid line represents the fitted curve. f Power dependent coincidence-to-accidental ratio (CAR) from NbOCl2 on gold film, indicating that a clear correlation peak above the classical limit (\({{{\rm{CAR}}}} > 2\)) is obtained from the sample. The solid line represents the fitted curve, which exhibits an inverse relationship. The results in (d–f) are measured under co-polarized configuration. g Coincidence count rate as a function of the fundamental wave polarization in the co-polarized detection configuration. Points are experimental results, and the broad line is a theoretical fitting.
