ADIABATIC INTERACTION OF A LASER-PULSE WITH A TRANSPARENT MEDIUM AND THE RELATION OF SELF-PHASE MODULATION WITH MICROSCOPIC PROCESSES

This paper considers the interaction of a nonmonochromatic plane elect romagnetic wave with separate atoms and with an entire medium. The pur pose of this work is to ascertain the relation between macroscopic phe nomena that accompany the propagation of a pulse in the medium and ele mentary processes of the interaction of photons from the pulse field w ith an atom. We analyze this problem by considering self-phase modulat ion. Because the carrier frequency of the pulse is close to the freque ncy of one of the atomic transitions, perturbation theory is inapplica ble and we solve the problem within the framework of the adiabatic app roximation. The bandwidth of the pulse is much less than the detuning from the resonance. Therefore, we can assume that the medium is transp arent. The probabilities of various multiphoton coherent processes of scattering from separate atoms are calculated. It is demonstrated that self-phase modulation is due to multiphoton coherent forward scatteri ng of photons involved in the frequency expansion of a pulse that prop agates in the medium. The dependence of the characteristics of propaga ting radiation on the parameters of the medium and on the characterist ics of the pulse provides the opportunity to experimentally detect the effect predicted in this paper.