Optical parametric generation covering the sodium D1, D2 lines from a 532-nm pumped periodically poled lithium niobate (PPLN) crystal with ionic-nonlinearity enhanced parametric gain

We report optical parametric generation from a 532-nm-pumped periodically poled lithium niobate with a wavelength spectrum covering the sodium D₁, D₂ wavelengths 589.6 and 589.0 nm. Despite the 8 cm^-1 attenuation at the 5.45 μm idler wavelength, the PPLN generates a 9 μJ/pulse energy near the 589-nm sodium wavelength when pumped by a 130 μJ/pulse frequency-doubled Nd:YAG Q-switched laser. The observed effective nonlinear coefficient is about 40% higher than its value at the visible and near infrared wavelengths. We believe that this is the first observation of ionic susceptibility enhanced parametric gain in the mid-infrared absorption region of lithium niobate.