Novel propagation effects of coherent short light pulses have been studied in connection with a resonant interaction with a two-level system such as in ruby 1 and gaseous SF 6 2 " 4 and Rb. 5 Selfinduced transparency (SIT) due to a one-photon resonant transition was first investigated by Recently, coherent two-photon propagation in which twice the propagating frequency is resonant with a two-level system has also been analyzed, 6 * 7 though the experimental study has not been reported yet. We wish to present here the first observation and theoretical analysis of a new type of coherent propagation of two different-frequency optical pulses causing a two-photon transition in a gaseous three-level system. The present model excludes two simultaneous transitions in double resonance.The basic effect of coherent two-photon propagation can by analyzed in the limit of two different-frequency plane waves given as E.Low-loss, coherent two-photon propagation and pulse breakup with peak amplification are observed resulting from two-photon resonant self-induced transparency of different-, frequency optical pulses interacting with a three-level system in potassium vapor. Theoretical analysis yielding new area equations for two different-frequency pulses with computer solutions agrees well with the observed results.

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In this paper we develop a theoretical analysis which describes a coherent transient effect of two entering pulsesone laser and one Stokesin a Raman-active medium. The field (Maxwell) and atomic (Schrodinger, density-matrix representation) equations are coupled in a self-consistent manner in time and space by virtue of the resultant second-order nonlinear macroscopic polarizations at the two frequencies. This treatment is accomplished under the circumstances where atomic coherence plays a predominant role. The equations derived here may be referred to as "Raman Bloch-Maxwell equations. "The resultant equations yield a new aspect of transient stimulated Raman scattering and thus coherent Raman propagation. The transient-pulse behavior may be understood as self-induced modulation of coherent amplification and absorption. The self-induced modulation for the step-function input pulse is described by a combination of Jacobian elliptic functions depending on the input intensity. Computer calculations of transient-pulse evolution reveal interesting pulse breakup phenomena with peak amplification, energy transmission, pulse advance or delay, and pulse-narrowing effects.

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