Non-linear absorption of laser radiation by atomic systems [by] H. K. Shepard and H. T. Yura.

A three-level atomic system whose energy level structure allows the absorptionof two laser photons via a real intermediate state is considered, and the rates of single and double photon absorption is calculated 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 4th power, respectively, while for strong fields both the single and double absorption rates tend to constants independent of E. Situations are found where the rates of the double photon absorption rate to the single photon rate is of the order 6. Also, the case of two different laser beams (of different intensities and frequencies) is considered, interacting with a three-level atomic system. The rate of absorption of two photons, one from each beam is calculated. Under certain circumstances, the two-photon absorption rate is a few times larger than the single phonton rate. Finally, numerical estimates are given of a double phonton absorption process in neutral chromium and potassium vapor.