Four-photon transitions in ZnS

The non-linear photoconductivity in ZnS has been studied by means of three-photon and four-photon excitation with a ruby and neodymium laser, respectively. By using the experimental results obtained at the two different wavelengths and the previous value of three photon non linear cross section γ₃, a quantitative determination of γ₄ is obtained. The results are compared with three different theoretical calculations: semiclassical perturbation theory, quantum electrodynamic theory and Keldysh's theory.