PHASE-DEPENDENT ABSORPTION AND STIMULATED-EMISSION IN FEMTOSECOND PUMP-PROBE EXPERIMENTS

The pump pulse induces nonstationarity and coherence in the medium. It is shown that there are mechanisms of absorption (emission) which depend on the phase difference between the field and the matter oscillator. They are principally different from conventional absorption and stimulated emission. The absorption (emission) of the probe pulse in the nonstationary coherent medium belongs to the phase-dependent Raman absorption (stimulated emission). The sign of this absorption depends on frequency and time delay between the probe and pump pulse. The time delay dependence corresponds to the phase dependence. The possibility of amplification without the inversion of population is the characteristic feature of the phase-dependent mechanisms of absorption (emission) considered here.