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Fortschritte der Physik

Stimulierte Resonanz‐Raman‐Streuung an angeregten und nicht angeregten Molekülen

Joachım Herrmann

Published Jan 1, 1977• 7 citations

Abstract

Abstract A review of the theory of the stimulated resonance Raman scattering (SRRS) is given. The calculation of the gains for the SRRS with the ground level (SRRS I) or an excited one (SRRS II) as initial states shows that there are some restrictions for their appearance. Due to the strong resonance absorption of the pumping wave and the disturbing influence of stimulated fluorescence the SRRS I is not observable in a Franck‐Condon excitation and in an absorption of Lorentzian line shape. The SRRS II has a sufficiently large gain on the condition τel ≪vib (τel, τvib – transversal relaxation time of the electronic or vibronic transition). In case of a strong deviation of the wings of the absorption profile from the Lorentzian shape, however, the SRRS I is favoured rather than the two other processes. Another possibility to avoid the resonance absorption of the pumping wave exists for the transversal excitation of the SRRS. Thereby the limit of application of the SRRS may be extended to other substances which are no saturable absorbers. Apart from the saturable absorption and the transversal excitation the self‐induced transparency is the third possibility to generate SRRS of ultrashort light pulses.

DOI: 10.1002/prop.19770250106

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Abstract A review of the theory of the stimulated resonance Raman scattering (SRRS) is given. The calculation of the gains for the SRRS with the ground level (SRRS I) or an excited one (SRRS II) as initial states shows that there are some restrictions for their appearance. Due to the strong resonance absorption of the pumping wave and the disturbing influence of stimulated fluorescence the SRRS I is not observable in a Franck‐Condon excitation and in an absorption of Lorentzian line shape. The SRRS II has a sufficiently large gain on the condition τel ≪vib (τel, τvib – transversal relaxation time of the electronic or vibronic transition). In case of a strong deviation of the wings of the absorption profile from the Lorentzian shape, however, the SRRS I is favoured rather than the two other processes. Another possibility to avoid the resonance absorption of the pumping wave exists for the transversal excitation of the SRRS. Thereby the limit of application of the SRRS may be extended to other substances which are no saturable absorbers. Apart from the saturable absorption and the transversal excitation the self‐induced transparency is the third possibility to generate SRRS of ultrashort light pulses.

Abstract The gain of the stimulated resonance RAMAN scattering (SRRS) and stimulated fluorescence is obtained by means of an analytical solution of the density matrix equation and MAXWELL's equations for laser‐ und STOKES‐radiation. The transient effects, which are caused by the population change of the molecule levels, and the influence of the resonance absorption on the pumping and the STOKES radiation are studied. The strong laser absorption yields a maximal gain, which shows, why the stimulated resonance RAMAN scattering could not be observed in many substances. As a result of our paper we give a possible interpretation of the experimentally observed relation between the stimulated fluorescence and the resonant RAMAN scattering.

An analysis is made of some features of spontaneous resonance Raman scattering representing a noise signal which is amplified by a strong field in a nonlinear medium. A detailed review is made of the experimental results obtained in investigations of the spectra of stimulated preresonance and resonance Raman scattering in nonlasing and lasing systems. It is shown that, depending on the experimental conditions, these spectra may be due to transitions between vibrational levels of molecules which are in the ground or first excited electronic states. The example of several compounds is used to demonstrate the information which can be derived from such spectra. Possible practical applications of studies of such spectra are indicated. The physical basis of the theory of stimulated resonance Raman scattering is discussed and conclusions are drawn on the nature and mechanism of this Raman effect.

The nonlinear regime of stimulated Raman scattering (SRS) may be reached in two ways. The best known is that of depletion of the pump laser as a large number of photons are scattered into the Stokes field. There is, however, another way in which gain saturation can occur, that is, by depletion of the atomic population while the laser remains undepleted. The equations which describe SRS, including damping and population inversion,1 are remarkably similar to those of well-known theories of two-level superfluorescence, and it is possible by analogy to define a delay time τ D for cooperative Raman scattering (CRS). This delay time is inversely proportional to the spontaneous scattering rate, which depends on the pump laser intensity. A sufficient condition for CRS is that the spontaneous Stokes linewidth Γ is very much greater than τ D − 1 . The only other requirement is that there be more laser photons than scatterers, so that τ D is independent of the number of scatterers. The S(1) transition in molecular hydrogen, when excited by a short (≅ 100-ps) laser pulse, provides a possible system for observing pure CRS. Similar studies have been carried out previously2 for Γ ≅ τ D − 1 .

The passage of strong monochromatic laser radiation through a resonance medium consisting of three-level atoms is investigated. It is shown that if collisions are taken into account in the scattered waves the resonances at combination frequencies occur to first order in the laser field intensity. A numerical analysis of the dependence of the stimulated electronic Raman scattering (SERS) components' gain factors on the pump frequency is carried out. The dispersion curves of the SERS components' gain (absorption) factors for fixed values of pump frequency and intensity are plotted.

The theory of resonance Raman effect, resonantly enhanced two-photon absorption, and resonance fluorescence---valid also for strong fields---is treated in a unified and simplified way. Dressed states, Bloch equations, and perturbation theory are used to calculate line positions, and integrated intensities both for steady state and transient excitation. The case of adiabatic following is solved explicitly; it occurs when an off-resonance incident pulse is turned on and off slowly. It leads to the identification of Raman scattering, two-photon absorption, and Rayleigh scattering with an adiabatic process while fluorescence and consecutive two-photon absorption with nonadiabaticity. Time-dependent spectra are defined in a rigorous way. Our formulas agree in the various limits with those that appear in the literature.