Saturation in the principal (excited) channel of four-level medium as an effective two-level configuration

Saturation of complex molecules affects the nonlinear processes occurring in the medium. The medium can be described by two-, three-, and four-level configurations. The principal singlet-singlet and excited triplet-triplet channels of a four-level, N-type, configuration were used to study a nonlinear medium, which was excited by radiation with two frequencies. Under weak population of energy levels in the triplet-triplet channel (a small ratio of the total probability of spontaneous and irradiate transitions for both channels p23/p31), the principal (singlet-singlet) channel is similar to a two-level configuration. At the same time, the effective population of the energy levels in the triplet-triplet channel (the ratio p23/p31 is large and the radiation intensity in the principal channel is high) makes the excited (triplet-triplet) channel coincide with the two-level configuration. The saturation intensity of radiation in principal (excited) channel, as effective two-level configuration, and nonlinear processes in this channel can be controlled by external radiation acting on excited (principal) channel. Finally, the average excitation of molecules into the energy levels in the triplet-triplet channel (ratio p23/p31 ≈ 1) makes the four-level configuration similar to the three-level configuration with effective level as a result of combination between second singlet level S2 and first triplet level T1.