Effect of low-Raman window position on correlated photon-pair generation in a chalcogenide Ge_11.5As₂₄Se_64.5 nanowire

We investigated correlated photon-pair generation via spontaneous four-wave mixing in an integrated chalcogenide Ge_11.5As₂₄Se _64.5 photonic nanowire. The coincidence to accidental ratio, a key measurement for the quality of correlated photon-pair sources, was measured to be only 0.4 when the photon pairs were generated at 1.9 THz detuning from the pump frequency due to high spontaneous Raman noise in this regime. However, the existence of a characteristic low-Raman window at around 5.1 THz in this material's Raman spectrum and dispersion engineering of the nanowire allowed us to generate photon pairs with a coincidence to accidental ratio of 4.5, more than 10 times higher than the 1.9 THz case. Through comparing the results with those achieved in chalcogenide As₂S₃ waveguides which also exhibit a low Raman-window but at a larger detuning of 7.4 THz, we find that the position of the characteristic low-Raman window plays an important role on reducing spontaneous Raman noise because the phonon population is higher at smaller detuning. Therefore the ultimate solution for Raman noise reduction in Ge_11.5As₂₄Se_64.5 is to generate photon pairs outside the Raman gain band at more than 10 THz detuning.