Frequency-domain coherent control of femtosecond two-photon absorption: intermediate-field versus weak-field regime

Coherent control of femtosecond two-photon absorption in the intermediate-field regime is analysed in detail in the powerful frequency domain using an extended fourth-order perturbative description. The corresponding absorption is coherently induced by the weak-field non-resonant two-photon transitions as well as by four-photon transitions involving three absorbed photons and one emitted photon. The interferences between these two groups of transitions lead to a difference between the intermediate-field and weak-field absorption dynamics. The corresponding interference nature (constructive or destructive) strongly depends on the detuning direction of the pulse spectrum from one-half of the two-photon transition frequency. The model system of the study is atomic sodium, for which both experimental and theoretical results are obtained. The detailed understanding obtained here serves as a basis for coherent control with rationally-shaped femtosecond pulses in a regime of sizeable absorption yields