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Journal of Experimental and Theoretical Physics

Raman echo under medium excitation by extremely short pulses

С. В. Сазонов, A. F. Sobolevskii

Published May 1, 2003• 12 citations
Publisher

Abstract

A generalization of the Bloembergen-Shen model to Raman active molecules with an arbitrary number of normal modes was suggested. The generalized model was used to study Raman echo signal characteristics when a system of molecules was excited by pulses of widths up to one period of optical oscillations. It was shown that a large number of echo responses on Stokes and anti-Stokes components could arise even under a two-pulse action in a continuous monochromatic pumping field. The number of echo responses depended on the number of molecular normal modes and the geometry of measurements. At small exciting pulse “areas,” the echo responses whose Stokes and anti-Stokes components corresponded to normal vibrational modes of a molecule had the highest intensity, whereas the components formed by normal mode combinations were strongly suppressed.

DOI
10.1134/1.1581935

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Raman echo under conditions of medium excitation by extremely short pulses

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A generalization of the Bloembergen-Shen model was supposed on cases those Raman active molecules with an arbitrary number of normal modes. On the basic of this model were investigated Raman echo signal characteristics when medium was excited by pulses of widths up to one period of optical oscillations. It was shown that a large number of echo responses on Stokes and anti-Stokes components could arise even under a two-pulse action in a continuous monochromatic pumping field. The number of echo responses depended on the number of molecular normal modes and the geometry of experiments. At small exciting pulse “areas,” the echo responses whose Stokes and anti-Stokes components corresponded to normal vibrational modes of a molecule.

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Soviet Journal of Quantum Electronics•Aug 31, 1976•2 citations
A generalization of the Bloembergen-Shen model to Raman active molecules with an arbitrary number of normal modes was suggested. The generalized model was used to study Raman echo signal characteristics when a system of molecules was excited by pulses of widths up to one period of optical oscillations. It was shown that a large number of echo responses on Stokes and anti-Stokes components could arise even under a two-pulse action in a continuous monochromatic pumping field. The number of echo responses depended on the number of molecular normal modes and the geometry of measurements. At small exciting pulse “areas,” the echo responses whose Stokes and anti-Stokes components corresponded to normal vibrational modes of a molecule had the highest intensity, whereas the components formed by normal mode combinations were strongly suppressed.
A generalization of the Bloembergen-Shen model was supposed on cases those Raman active molecules with an arbitrary number of normal modes. On the basic of this model were investigated Raman echo signal characteristics when medium was excited by pulses of widths up to one period of optical oscillations. It was shown that a large number of echo responses on Stokes and anti-Stokes components could arise even under a two-pulse action in a continuous monochromatic pumping field. The number of echo responses depended on the number of molecular normal modes and the geometry of experiments. At small exciting pulse “areas,” the echo responses whose Stokes and anti-Stokes components corresponded to normal vibrational modes of a molecule.
On the basis of quantum-mechanical approach, a theoretical study of the primary Raman photon echo was carried out under conditions of excitation of a medium by pulses with a duration on the order of one period of electromagnetic oscillations (ultimately short pulses) and by strong quasi-monochromatic nonresonant pumping. It is shown that the intensities of the pumps determine to a considerable extent the time of formation of the echo signal, its duration, the direction of emission, and the power and that the use of ultimately short pulses promotes the substantial simplification of the schemes of prospective experiments. The possibilities of echo generation at the Stokes and anti-Stokes frequencies in the regimes of two-, one-, and zero-pulse excitations are revealed. In the last case, the Raman-active transitions are excited by two pumps with different intensities.
A photon theory of the stimulated Raman effect is developed. A system of coupled equations for the amplitudes of scattered photons and molecular vibrations are derived. The molecular vibration coordinates are then eliminated using the random phase approximation. The equations which result assume the form of coupled equations for the first Stokes and anti-Stokes lights. The secular equation which characterizes the solution of these coupled equations is shown to be equivalent to that of Shen and Bloembergen. Using this secular equation, the condition for temporal growth of the scattered lights is obtained and is compared with that for the spatial growth, which was obtained previously by Shen and Bloembergen. It is shown that both types of growth (temporal and spatial) proceed with comparable rates if the damping of the molecular vibration is large. If its damping is small, however, the temporal growth prevails over the spatial growth. Typical line-shape of the anti-Stokes line is obtained in the case of small damping. The result shows a strong directionality, as is the case of large damping.
Echo responses of a three-level medium formed by the λ-scheme of quantum transitions under exposure to optical pulses consisting of an arbitrary number of oscillations have been studied theoretically. The cases of pulses consisting of few optical oscillations (for which the concept of an envelope cannot be used) and combinations of such pulses and quasi-monochromatic resonance signals have been considered. The approach used can be reduced to the renormalization of dipole moments of allowed quantum transitions through their multiplication by coefficients depending on the shape and duration of pump pulses and having absolute values in the range from zero (for nonresonance pulses) to unity (for resonance quasi-monochromatic pulses and broadband pulses consisting of few oscillations, whose spectrum covers the quantum transitions). A general equation has been proposed for the pulse area. In the limit of a large number of oscillations, it transforms into the well-known definition of the area of a quasi-monochromatic signal. The characteristics of primary and longlived photon echoes have been analyzed in detail. It has been shown that, when a medium is exposed to only pulses consisting of a few oscillations, three echo responses of both types can be principally generated at each frequency of the λ-scheme. Introduction of quasi-monochromatic pulses in pump pulse series decreases the number of echoes, and their qualitative character has a non-commutative property with respect to pulse permutation in a series. The extension of the proposed approach to more complex schemes of quantum transitions with the large number of quantum levels faces no principal difficulties.
An analysis is made of parametric excitation of anti-Stokes components of stimulated Raman scattering in an optical resonator filled with a medium whose refractive index is dispersion-free. The case of steady generation of the first Stokes component is considered. Expressions are obtained for the amplitudes of steady-state fluctuations of the anti-Stokes components in the case of a weak reaction of these components on the pump radiation and on the first Stokes component. Various cases of frequency dependences of the intrinsic resonator losses are considered. Conditions are found under which a large number of anti-Stokes components with similar intensities is excited so that the anti-Stokes radiation leaving the resonator is a periodic sequence of ultrashort (femtosecond) pulses with one pulse per period.