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Journal of the Optical Society of America B

Pressure-induced stimulated Rayleigh scattering in a saturated two-level system

C. J. Hsu

Published Nov 1, 1990• 2 citations
Publisher

Abstract

In a two-level system that is subjected to a resonant coherent pump, an extraresonance that resembles stimulated Rayleigh scattering takes place when the pump reaches a moderate intensity. In addition to the major features obtained in pressure-induced extraresonance, this scattering is also verified as a field-induced extra-resonance. A nonlinear power-induced decay rate was found and is predicted to give new features of the extraresonance in the gain and in the line shape. Because of the existence of this decay, Lorentzian-shaped contribution to the gain, in addition to the well-known dispersive one, was found. These effects are especially pronounced in a dephasing mechanism and may produce an extraresonance with a frequency shift that is considerably larger than that of the conventional pressure-induced one.

DOI
10.1364/josab.7.002155

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Journal of the Optical Society of America B•Aug 1, 1988•10 citations
In a two-level system that is subjected to a resonant coherent pump, an extraresonance that resembles stimulated Rayleigh scattering takes place when the pump reaches a moderate intensity. In addition to the major features obtained in pressure-induced extraresonance, this scattering is also verified as a field-induced extra-resonance. A nonlinear power-induced decay rate was found and is predicted to give new features of the extraresonance in the gain and in the line shape. Because of the existence of this decay, Lorentzian-shaped contribution to the gain, in addition to the well-known dispersive one, was found. These effects are especially pronounced in a dephasing mechanism and may produce an extraresonance with a frequency shift that is considerably larger than that of the conventional pressure-induced one.
Pressure‐ and radiation‐induced stimulated Rayleigh scattering in the Rayleigh wing are discussed quantitatively for the case where both the pump laser at fixed frequency ω1 and the probe laser with variable frequency ω2 are arbitrarily strong. The absorption spectra of the pump and probe fields, both expressed as a function of ω2, exhibit a narrow dispersive lineshape centered at the fixed frequency ω1 but with opposite tendencies: the field whose frequency is closest to the resonance frequency, ωba, of the two‐level system always undergoes stimulated emission, the other field being absorbed. The intensity of these processes increases, up to a limit, with the intensity of both the pump and probe fields. A qualitative explanation for the origin of the dispersive lineshape is suggested.
The phenomenon of stimulated Rayleigh-wing scattering in anisotropic molecular liquids is analyzed. Representative combinations of polarization for the incident and scattered light are considered. It is shown that in regions where the focused laser beam is intense, saturation occurs in the molecular response to external optical fields, in that perfect alignment of the individual liquid molecules in the optical fields is approached. Accordingly, the nonlinear amplification associated with the stimulated radiation is reduced. The results of the analysis qualitatively explain several important recent observations which are in conflict with previous theroretical predictions. For example, it is shown that the nonlinear gain is considerably greater when the incident light is circularly polarized (the stimulated radiation being circularly polarized in the reverse sense) than it is when the incident light is linearly polarized. Despite saturation, sufficient amplification exists in the former case to allow the efficient conversion of incident to stimulated Rayleigh light at moderate power levels.
By using the Schenzle-Brewer model, in which the ground state of a two-level system also has a finite lifetime, it is shown that two pressure-induced extraresonant degenerate four-wave mixing signals are obtained. The first extra resonance is similar to that predicted and observed by Bogdan et al. [Opt. Lett. 6, 82, 348 (1981)]. It is characterized by a linewidth equal to the inverse lifetime of the upper state and appears only in the presence of collisional redistribution. The second extra resonance is characterized by a linewidth equal to the inverse lifetime of the ground state and does not depend on collisional redistribution for its appearance.
The effect of pump fluctuations on various coherent processes that arise in three-level systems interacting with two external fields is examined. Such coherent processes include the forward Hanle effect and various four-wave mixing effects such as the generation of phase-conjugate signals. A general formulation that enables one to calculate the influence of laser linewidth on the coherent signals produced in various directions is presented. Ensemble averages, over laser temporal fluctuations, of various physical quantities, such as atomic polarization, are calculated. The spectrum of polarization fluctuations is shown to consist of several new features which lead to coherent radiation in different directions depending on the resonant frequencies in the polarization fluctuations. The influence of pump linewidth on pressure-induced extra resonance (PIER) is treated in detail. The possibility of producing a laser-fluctuation-induced coherent signal at one of the atomic frequencies is examined. This new signal, which is produced in a direction different from that of the PIER signal, but has the same type of resonant character as PIER, is found to have significantly different pressure dependence than the PIER signal. The results of our numerical computations are qualitatively explained in terms of the convolutions of products of third-order susceptibilities and pump field-correlation functions.
A frequency-unshifted and backward stimulated scattering can be efficiently generated in one-photon-absorption free but two-photon absorbing materials. Using a number of novel two-photon absorbing dye solutions as the scattering media and nanosecond pulsed laser as the pump beams, a highly directional backward stimulated scattering at the exact pump wavelength can be readily observed once the pump intensity is higher than a certain threshold level. The spectral and spatial structures as well as the temporal behavior and optical phase-conjugation property of this new type of backward stimulated scattering have been experimentally studied. This stimulated scattering phenomenon can be explained by using a model of two-photon-excitation enhanced standing-wave Bragg grating initially formed by the strong forward pump beam and much weaker backward Rayleigh scattering beam; the partial reflection of the pump beam from this grating provides an positive feedback to the initial backward Rayleigh scattering beam without suffering linear attenuation influence. Comparing to other known stimulated (Raman, Brillouin, Rayleigh-wing, and Kerr) scattering effects, the stimulated Rayleigh-Bragg scattering exhibits the advantages of no frequency-shift, low pump threshold, and low spectral linewidth requirement.
We study the effect of resonant light pressure on the optical response of a Doppler-broadened atomic medium to a single monochromatic laser field. As a main consequence of the spontaneous scattering force, a novel contribution to the optical dispersion of the atomic gas is predicted, which shows a pronounced nonlinear intensity dependence; moreover, for a given laser intensity, its strength is closely related to the laser-beam diameter. Most importantly, this light-pressure-induced dispersion displays an even symmetry with respect to the optical Doppler detuning; as a result, the total dispersion curve of the medium can be changed substantially. In addition to a detailed theoretical description of the phenomenon, we propose a simple experiment based on frequency-modulation spectroscopy for the measurement of the predicted nonlinear dispersion feature.