Fig. 4: Enhanced spontaneous parametric down-conversion (SPDC) via QPM of 3R-MoS₂ homo-structure.

a Schematic of SPDC process on 3R-MoS₂ van der Waals stacking. A single photon \({\gamma }_{p}\) with higher energy (1.60 eV) undergoes a nonlinear interaction in 3R-MoS₂ homo-structure with the stacked angle 60° (respect to the preceding 3R-MoS₂ sample in Fig. 3a) and each layer at thickness of \({t}_{c}\), generating the entangled photon pair with lower energy (0.8 eV), known as the signal photon \({\gamma }_{s}\) and idler photon \({\gamma }_{i}\). b Schematic of custom-built setup for photon-pair correlation measurements (orange pump photons and purple SPDC signals): 775 nm continuous wave (CW) pump generated from a single-frequency fiber laser, polarized beamsplitter (PBS), half-wave plate (HWP) for pump power control, spherical lenses, bandpass filter at 1550 nm with a 12 nm full-width at half-maximum, fiber coupler, and 50:50 fiber beamsplitter, SPAD; single-photon avalanche diode, time-correlated single photon counting (TCSPC) system. c Normalized second-order correlation functions \({g}^{\left(2\right)}(\tau )\) from 3R-MoS₂ stack (i), 3R-MoS₂ area B (ii), area A (iii), and sapphire substrate (gray square dots) measured at 15 mW for 10 min with 1 ns binning time. d Comparison of power-dependent coincidence rate of the A (blue), B (purple) and AB areas (orange) of the 3R-MoS₂ homo-structure (with respect to the preceding sample in Fig. 3a) measured for 10 min. The extracted slopes are 0.00771, 0.00168, 0.00188 for AB, A and B respectively. The error bars represent the standard deviations of SPAD counts per each measurement.