APL Photon. 3, 021301 (2018)

Spontaneous parametric down-conversion (SPDC) spectroscopy is attracting attention as it offers a means for performing mid-infrared (mid-IR) spectroscopy without the need for dedicated sources and detectors in the mid-IR, which can be expensive and are limited in performance. Instead, the scheme harnesses the principles of nonlinear optics to split a short-wavelength pump photon into a pair of photons (a long-wavelength (mid-IR) idler photon and a short-wavelength signal (visible or IR) photon) using a nonlinear crystal. Correlations between the signal and idler photons mean that collecting information about one also provides information about the other. As a result, the short-wavelength signal photon can be read out using affordable, high-performance detection equipment while the mid-IR idler photons probe the sample. To date, demonstrations of the technique have relied on the use of bulk nonlinear crystals, but scientists in Australia and Germany have now shown that it should be possible to create an integrated on-chip version using lithium niobate waveguides to perform the SPDC. Their experiments with pump wavelengths in the 740–780 nm range and Ti-diffused LiNbO3 waveguides suggest that idler wavelengths as long as 2.5 μm can be generated. An on-chip system employing such waveguides would not only be compact but should also offer a high SPDC efficiency of up to 9 × 106 photon pairs per second for just 0.5 mW of pump power .