Particles captured in a trap are considered in the linear approximation with respect to a probe field. A resonance, having the width of a forbidden transition, is predicted for the stimulated Raman forward and backward scattering line profiles.
A semiclassical model of nonstationary stimulated Raman scattering with account of forward and backward waves of different frequencies is proposed. Equations for the elements of a collective density matrix and electric-field amplitudes are obtained applying the averaging method. It is shown that forward waves are generated by a united polarization component, whereas each of the backward waves is formed by its own component. The description is based on the approximation of small scattering angles and assumes that the quantities considered do not depend on transverse coordinates. The emission dynamics of forward-backward Stokes pulses is investigated both for the linear case and under the strong depletion of pumping and level populations. The domain of applicability of the model of nonstationary forward stimulated Raman scattering is determined.
It is shown that, in strong optical fields (~109 W/cm2), Raman lines may be excited due to transitions with changes by 3 and 4 in the rotational quantum number, which are forbidden in the linear approximation. This lifting of forbiddenness for Raman scattering is due to four-photon processes in which a pump photon is absorbed and a Stokes photon is emitted in an elementary event. Under quasiresonance pumping, this type of stimulated Raman scattering may be observed due to rotational transitions. Estimate are made of the conditions for observation of the spontaneous and stimulated processes in specific media.
An experimental investigation was made of the distribution of the energies of pulses of forward (concurrent) and backward first Stokes components of stimulated Raman scattering (STRS), and of pump (exciting) radiation pulses, transmitted by a scattering medium. The study was carried out under linear, intermediate, and nonlinear scattering conditions. The dependences of the coefficients of variation and of the asymmetry of the distributions on the energy of the pump pulses were determined. The statistical properties of the energy of the backward STRS pulses differed in a number of respects from the forward STRS pulses, including a relatively high coefficient of variation in the former case. An interpretation of these differences was developed. It was found that STRS gave rise to fluctuations of the energy of the transmitted pump pulses and the statistical characteristics of these pulses were determined. The results obtained for forward STRS were in satisfactory agreement with the conclusions reached on the basis of a quantum theory.