Investigation of two-stage parametric and four-photon processes

An investigation was made of the intensity, frequency spectrum, and angular distribution of the spontaneous anti-Stokes scattering of ruby laser radiation in a lithium iodate crystal. When the pump field was a superposition of ordinary and extraordinary waves, the strongest spontaneous radiation was due to a two-stage process involving up-conversion of the frequency of three-photon parametric luminescence. When only an ordinary or extraordinary pump wave was scattered, the output radiation had angular and frequency distributions which–within the limits of the experimental error–were identical with the distributions predicted theoretically for four-photon parametric luminescence. It was found that, in the investigated wavelenght range 0.45–0.54 μ, the component χ₃₅ of the cubic nonlinearity tensor of the LiIO₃ crystal was of the same order as the component χ₁₁.