Molecular coherence effects in stimulated raman scattering

A semi-classical calculation of the three-level system consisting of the ground state, the vibrationally excited state and the electronic excited state under the laser and the Stokes perturbation is given. The induced molecular polarization produces gain modulation of the Stokes and loss modulation of the laser at a frequency that is dependent on the optical intensity. With the optical intensity in self-trapped filaments in nonlinear liquids such as CS₂, the period of modulation becomes of the order 10^?11 s and a large amplitude modulation of the laser and the Stokes waves will result. The amplitude modulation is not much reduced, if the molecular relaxation time of the order 10^?11 s is taken into account. Effects of non-uniform field distribution and the width and shape of the incident laser pulse are discussed. The frequency broadening caused by the three-level effect is shown to be larger than, or at least as large as, the broadening caused by the optical Kerr effect.     

Multiwave stimulated raman scattering under quasi-phase-matching conditions

The effect of the generation of higher Stokes and anti-Stokes components of steady-state and transient stimulated Raman scattering (SRS) on the realization of quasi-phase-matching conditions in the course of generation of the first anti-Stokes SRS component is numerically analyzed by using a system of coupled equations derived with regard to diffraction effects. The calculations show that the application of a periodic layered structure in order to realize quasi-phase-matching conditions virtually does not reduce the anti-Stokes SRS efficiency and allows efficient conversion of pulses with a duration of 3 ns or longer. It is found that focusing of waves inside the medium allows partial compensation for diffraction spreading and provides for an increase in the efficiency of anti-Stokes SRS conversion, and, at a Fresnel number above three, the effect of diffraction on the realization of quasi-phase-matching conditions during SRS can be disregarded.     

Nonstationary intracavity stimulated raman scattering by polaritons

Stimulated Raman scattering by polaritons when employing a nonlinear crystal inside the cavity of the laser with Q-switching and locked to fundamental frequency was theoretically studied. The equations were formulated describing the dynamics of the fundamental, Stokes and polariton waves under the assumption of instant Q-switching and uniformity of radiation except in the longitudinal direction. Numerical solutions of the equations were found and analysed. We demonstrate the influence, on generation characteristics, of the most important parameters, such as the effective nonlinearity, the initial level of population inversion, the feedback of the Stokes wave and the group mismatch between a Stokes and a pump wave. Optimum generation conditions are determined. Possibilities of the method are discussed for the generation of a) giant pulses which are controllable with regard to carrier frequency and duration but are unachievable with other methods, b) Stokes pulses of very small duration and high energy having sharp trailing edges, and c) analogous polariton pulses in the middle and far infrared range. The generation using a number of polariton branches in LiNbO₃ was considered as an example. Calculations are in qualitative agreement with experimental data.     

Image formation using stimulated raman scattering gain

Theoretical analysis of the spatial, noise, and energy characteristics of an amplifier has been performed in the mode of spectral and time selection using subnanosecond stimulated Raman Scattering gain of weak echo signals in crystalline active media that are known for high (up to 10^–1 cm/MW) gain coefficients. The possibility to reach high gain values has been demonstrated for weak signals from objects at acceptable angular sizes of the field of vision of an amplifier. To provide a signal-to-noise ratio that exceeds unity over the entire field of vision, the number of photons at the input to an amplifier that is required has to exceed the number of its resolution elements. Accurate determination of the possibilities of recording of weak echo signals and quality of images of targets that are obtained using amplifiers under stimulated Raman Scattering requires additional special experiments.     

Model of transient amplification by stimulated raman scattering

A semiclassical model for light amplification by stimulated Raman scattering is presented. The phases of the Stokes, anti-Stokes, and laser waves are assumed to be perfectly matched. Depletion of the laser pump is neglected. The corresponding system of equations for input pulses of arbitrary shape is solved analytically by the Riemann-Volterra method. Gain is suppressed for close Raman polarizations.     

On the interaction of stimulated spin-flip raman scattering and stimulated recombination radiation in InSb

We have studied the interaction of stimulated spin-flip Raman scattering and stimulated recombination radiation in n-InSb using a CW CO pump laser. A change in linewidth and output power of second Stokes spin-flip radiation is observed when the tuning curve of the first Stokes stimulated spin-flip radiation and the stimulated recombination radiation cross. The observations are explained using a simple rate equation model.     

Observation of stimulated electron-acoustic-wave scattering

A diffraction-limited laser interacts with a plasma whose conditions are uniform on the scale of the focused laser spot. Two distinct, narrow waves are observed in the backscattered spectrum with phase velocities of v(phi)/v(e) = 1.4+/-0.08 and 4.2+/-0.1, where v(e) is the electron thermal speed. The high-velocity wave is ordinary stimulated Raman scattering (SRS) from a Langmuir wave. The low-velocity wave corresponds to stimulated scattering from an electron-acoustic wave (SEAS), and implies strong electron trapping. Previous SRS data from low-density plasmas are reinterpreted in terms of SEAS.     

Quantum beats in stimulated electronic raman scattering of ultrashort laser pulses

A theory of quantum beats in a Stokes signal observed in pump-probe experiments upon excitation of four-level atoms with two close upper lying levels by femtosecond laser pulses is developed. Quantum beats arise in the intensity of the Stokes field due to the interference of two atomic wave packets generated by ultrashort pump and probe pulses. An analytical solution of the non-steady-state Maxwell-Bloch equations is obtained, and the dependence of the Stokes radiation intensity on the delay time between two laser pulses is investigated. The theoretical results describe qualitatively well the observed features of quantum beats.     

Capture of the stokes wave under coherent stimulated raman scattering

Interaction of coherent light pulses with a three-level medium of the “lambda” configuration is studied analytically and numerically. It is shown that in the course of Raman scattering the Stokes wave can be captured in the region of inversion. Conditions are found for complete energy transfer from the pump 2π-pulse to the Stokes wave.     

Photodynamic damage (PDD) study using stimulated raman scattering

In the current study, photodynamic damage in different cell cultures was examined using a pulsed laser light. Two different experiments were performed to analyse the photodynamic damage. For the first one, a stimulated Raman scattering laser has been obtained by exciting DMSO liquid with Nd-YAG laser, second harmonic generation, 532 nm. The resulted SRS wavelength is pulsed 630 nm. 1 ml ALA (200 μg/ml) was added to cell suspension and keep it for incubated for 4 h then irradiate the suspension with SRS wavelength 630 nm at different light dose 8, 12, 17, 25, 30, 40, 60, 100, 150, 200 μJ for 10 pluses and obtain the cell degradation. We repeat the step above but for 30 pluses. Finally for the second experiment, 1 ml ALA (200 μg/ml) was added to cell suspension and was incubated for 4 h and then irradiated with Nd-YAG Laser at wavelength 532 nm. Different doses range between 8 up to 200 μJ for 10 pluses only and the cell degradation rate was measured.     

A Vlasov code for the numerical simulation of stimulated raman scattering

Numerical simulations of the stimulated Raman scattering are presented using an Eulerian relativistic Vlasov code. Such a code allows a finer resolution in phase space than a particle code and provides a better understanding of the acceleration process for the particles at relativistically high energy. Forward Raman scattering as well as backward Raman scattering are considered to illustrate the possibilities of the Eulerian Vlasov code.     

Polarization resolved stimulated raman scattering: probing depolarization ratios of liquids

Polarization resolved stimulated Raman scattering (SRS) signal is described in the case of isotropic media and linearly polarized incident fields. The model gives simple expressions for the two perpendicularly polarized SRS signals I_X and I_Y, detected along the X and Y directions, respectively, as a function of the incident pump and Stokes polarization angles. We find that Raman depolarization ratio can be simply obtained from the ratio of the SRS intensities detected along the X and Y axis. These theoretical findings are supported by polarization resolved SRS measurements performed on polarized and depolarized bands of cyclohexane. Copyright © 2011 John Wiley & Sons, Ltd.     

Transient stimulated raman scattering in crystals during motion of populations of vibrational states

Transient stimulated Raman scattering (SRS) in crystals is analyzed taking into account the motion of populations of vibrational states under the action of subpicosecond (shorter than the dephasing time) pump pulses. Analytic expressions describing the dynamics of excitation of vibrations in SRS are derived. It is found that for a small wavelength of SRS interaction and high intensities of pump radiation observed for femtosecond SRS in crystals, avalanche excitation of vibrations can be responsible for SRS suppression. It is shown that when phase matching of Stokes-anti-Stokes parametric coupling in transient SRS is ensured, it is possible to elevate the efficiency of frequency conversion under conditions of motion of populations of vibrational states; this explains recent successful results in the experimental implementation of femtosecond SRS in crystals pumped by a Bessel beam.     

Quantum theory of stimulated raman scattering in an inhomogeneously broadened three-level gaseous system

A quantum-statistical treatment of stimulated Raman scattering in a gaseous system is presented using a density-matrix formalism. The molecular (atomic) system is described by three energy levels. Both atomic system and the radiation fields are quantized. The effects of atomic motion and detuning are incorporated in the analysis. Higher order nonlinearities and loss terms are included to render the problem more realistic. The equations of motion describing the photonstatistics of pump and Stokes fields are obtained. The equation, without detailed balance, is solved in the steady-state by a slowly varying function technique in the case of two variables. The steady state characteristics of the Stokes field are studied. The coherence properties, occurrence of antibunching phenomena are studied for different initial distributions.     

Compression of ultraintense laser pulses in inhomogeneous plasma upon backward stimulated raman scattering

The potential for time compression of ultraintense laser pulses in plasma based on nonlinear interactions between waves during backward stimulated Raman scattering is analyzed analytically and numerically. The optimal conditions for obtaining a maximal energy conversion coefficient of 10-ps laser pulses to the energy of 100-fs Stokes pulses are found.     

Theory of stimulated resonance raman scattering in dyes with ground or excited initial state

The theory of stimulated resonance Raman scattering (SRRS) in dyes with ground or excited initial state (up-down or down-up mechanism) is investigated. The gains for these processes are obtained by means of an analytical solution of the rate equation. It is shown that under pumping conditions the down-up SRRS is more probable than the up-down SRRS.