The forward-scattered intensity in the spontaneous parametric process is found to decrease significantly as the idler frequency goes sufficiently far into the 5.7-μ and 1.4-μ infrared absorption bands of LiNb${\mathrm{O}}_3$ and N${\mathrm{H}}_4$${\mathrm{H}}_2$P${\mathrm{O}}_4$, respectively. These results indicate that, unlike the previously studied cases in GaP and LiNb${\mathrm{O}}_3$, the corresponding optical nonlinear susceptibility has essentially no resonance increase and its ionic contribution remains small near these absorption bands. A formula for the forward-scattered intensity is obtained which agrees very well with the observed effects of idler attenuation on the spontaneous parametric scattering in both LiNb${\mathrm{O}}_3$ and N${\mathrm{H}}_4$${\mathrm{H}}_2$P${\mathrm{O}}_4$.

• Received 18 February 1969

DOI:https://doi.org/10.1103/PhysRev.184.353