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.     

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.     

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.     

Observation of stimulated raman scattering in CVD-diamond

We report the first experimental observation of a nonlinear laser effect, stimulated Raman scattering (SRS), in manmade diamond grown from the gaseous phase by the chemical vapor deposition (CVD) technique. Multiple Stokes and anti-Stokes generation in the visible and near-IR ranges was excited under nanosecond and picosecond pumping in a 350-μ-thick plate. All the registered Raman-induced lasing wavelengths were identified. We classify the CVD-diamond as a promising χ⁽³⁾-active material for Raman laser converters in a record wide spectral range.     

Generation of single dispersion precompensated 1-fs pulses by shaped-pulse optimized high-order stimulated Raman scattering

We propose and theoretically analyze a new approach for generating and shaping 1-fs pulses. It combines the ideas of strong-field molecular optics and optimal control to manipulate light generation in a pump-probe Raman regime. Flexible phase control over the generated spectrum of about 3 eV width is achieved by controlling the input pulses and maximizing the coherence of medium excitation by adiabatically aligning molecules in the medium with a specially shaped pump pulse. The generated pulse is optimized for an output window, precompensating for its dispersion to all orders.     

Surface enhanced raman scattering (SERS) from silver,copper, and gold films in UHV: excitation spectra.

SER excitation spectra of different vibrational lines of various molecules (C₅H₅N, C₂H₄, O₂, CO) adsorbed to Ag-, Cu-, and Au-films in UHV have been studied. The experimental results are explained by assuming an electromagnetic origin of the observed excitation profile resonances. Within this frame we estimate a size of ～ 1 – 2 nm of the ‘SERS relevant’ surface roughness (bumps) and consequently a short range ‘classical’ enhancement restricted to mainly the first layer of adsorbed molecules.     

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.     

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.     

Determination of temperature by stimulated raman scattering of molecular nitrogen,oxygen, and carbon dioxide

We have determined the temperature from SRS spectra of N₂-N₂, N₂-CO₂, O₂-O₂, and CO₂-CO₂ recorded in wide pressure and temperature ranges. The fitting procedure takes simultaneously into account the Dicke effect and motional narrowing. We have quantified the accuracy of the MEG and ECS-P models for rotational relaxation. The temperature extracted from each model is compared with thermocouple measurements. The influence of vibrational broadening and shifting is discussed in detail.     

Temperature dependence of the raman spectra of carbon nanotubes with 1064 nm excitation

In this report, the near infrared 1064 nm line of an Nd:YAG laser, which has strong thermal effect, was used as the excitation. A temperature dependence of the Raman spectra of carbon nanotubes was observed at different temperatures by varying the incident laser power. The results show that the relative Raman intensities to the tangential stretching mode (G mode) of the higher-order Raman modes within 2500–3500 cm⁻¹ increase with increasing excitation laser power at the sample and the changes in the relative Raman intensities are linear in the excitation laser power. This has not been reported elsewhere. Thorough analysis shows that this is a temperature dependence of double-phonon Raman scattering and maybe provide important information for the studying of CNTs and double-phonon Raman scattering.     

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.     

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.