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Soviet Journal of Quantum Electronics

Possibility of observing spontaneous and stimulated Raman scattering by forbidden rotational and vibrational–rotational transitions in a strong optical field

Victor R. Blok, Gennady M. Krochik, Yu. G. Khronopulo

Published Nov 30, 1979• 1 citation
Publisher

Abstract

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.

DOI
10.1070/qe1979v009n11abeh009748

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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 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 non-classical effect of squeezing of light in spontaneous Raman and stimulated Raman processes are investigated under a short-time approximation based on a fully quantum mechanical approach. A Raman process is looked upon as a two-photon and one-phonon interaction process. Squeezing in amplitude, amplitude squared and amplitude cubed have been investigated. The occurrence of squeezing is observed in the fundamental mode in spontaneous Raman and stimulated Raman scattering. The squeezing in a Stokes field is found to be dependent on squeezing of the initial pump field.
Abstract The theory of multifrequency stimulated Raman scattering (SRS) of light on rotational transitions of molecules, which are simulated by a three-level quantum system, is developed. It is shown that the application of bichromatic pumping increases the scattered radiation spectrum width. The exact algebraic solution for the wave amplitudes is obtained in the simplest version of synchronous SRS on transitions with a negligibly low eigenfrequency. It is found that an increase in the ratio of the Raman transition frequency to the pump radiation frequency leads to predominance of the anti-Stokes scattering branch. The effect of wave asynchronism emerging due to linear and nonlinear dispersion of the medium as well as the phase difference of the pump waves on SRS is considered. All types of asynchronism narrow generated frequency spectrum and suppress the anti-Stokes scattering branch. Nonlinear dispersion ensures positive values of wave amplitudes due to phase jumps by π during the propagation of waves in a medium.
The traditional Raman process alluded to in Chap. 13 is a three-state sequence of transitions in which radiative excitation (induced by a pump field) is followed by spontaneous emission that produces a final state differing from the initial state [Her50b] [Sho90, § 17.5]. When the final state of the sequence is more energetic than the initial state the resulting emission line (to the red of the pump wavelength) is known as a Stokes spectral line. The difference between the pump frequency and the Stokes frequency, the Raman frequency , defines the excitation energy of the final state relative to the initial state. When, instead, the final state has lower energy than the initial state, as can occur when the initial quantum state is already excited, then the emission is an anti-Stokes line, at a bluer wavelength than the pump field. The overall Raman scattering is a two-photon process. Typically Raman spectroscopy deals with molecules; the two-photon transitions are then between vibrational-rotational states, through electronically excited intermediate states, that are characterized in part by vibrational quantum number v and rotational angular momentum quantum numbers J, M . From any given excited electronic state there are many fluorescing transitions, corresponding to various vibrational and rotational quantum numbers of the final state. The wavelengths of the various Stokes and anti-Stokes lines (i.e. the Raman frequencies) characterize the particular molecular species, and so they have provided a valuable diagnostic tool for spectroscopists.
The Raman effect is the inelastic scattering of photons by matter. When a monochromatic light beam (pump beam) propagates in an optical fibre, spon- taneous Raman scattering transfers some of the photons to new frequencies. The probability of a photon scattering to a particular frequency shift, depends on that frequency shift, forming a characteristic spectrum. The scattered photons may lose energy (Stokes) or gain energy (anti-Stokes). If the pump beam is linearly polarized, then the polarization of scattered photons may be the same (parallel scattering) or orthogonal (perpendicular scattering). If photons are already present at other frequencies then the probability of scattering to those frequencies is enhanced (stimulated scattering). In this thesis, the equations governing the Raman effect, both in bulk glass and single mode fibre, are restated. The probability of spontaneous scattering (parallel and perpendicular) in an optical fibre has been measured for frequency shifts between 4THz and 58THz. Particular attention has been given to those frequency shifts where the scattering is intrinsically weak. This has identified those frequency shifts where it might be possible to eliminate Raman noise from experiments designed to generate quantum states of light in optical fibres using four wave mixing. From the measured scattering probabilities, the Raman equations can be used to predict the scattering cross-section in bulk glass. The prediction is found to be in reasonable agreement with measurements of the bulk glass cross-section published by other authors. By increasing the pump power, the transition to stimulated scattering has been observed. In the case of Stokes scattering there is reasonable agreement between the Raman equations and experimental observations. In the case of anti-Stokes scattering, and in the limit of very high pump power, behaviour is observed which is not predicted by the Raman equations, indicating that other processes must be included to successfully describe such experiments.