NON-LINEAR ABSORPTION OF LASER RADIATION BY ATOMIC SYSTEMS,

Abstract : A three-level atomic system is considered whose energy level structure allows the absorption of two laser photons via a real intermediate state, and the rates are calculated of single and double photon absorption as a function of the electric field strength E. For weak fields it is found that the single and double photon absorption rates are proportional to E-squared and E to the fourth power, respectively, while for strong fields both the single and double absorption rates tend to constants independent of E (i.e., they saturate). Situations are found where the ratio of the double photon absorption rate to the single photon rate is of the order 6. Also considered is the case of two different laser beams (of different intensities and frequencies) interacting with a three-level atomic system. The rate of absorption of two photons, one from each beam, is calculated. Under certain circumstances it is found that the two photon absorption rate is a few times larger than the single photon rate. Numerical estimates are given of a double photon absorption process in neutral chromium and postassium vapor. (Author)