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Soviet Journal of Quantum Electronics

Spectral and kinetic manifestations of a phase memory in collisions during interaction of radiation with a Lorentz gas

Faris Gel’mukhanov, А. И. Пархоменко

Published Apr 30, 1990• 4 citations
Publisher

Abstract

A study is reported of the influence of a phase memory in the presence of collisions on the work done by a field and on a light-induced drift of two-level gas particles in the Lorentz model of a gas (heavy buffer particles). It is shown that a phase memory may result in amplification of the incident radiation by particles at certain velocities in the absence of a population inversion (when practically all the particles are in the ground state). A phase memory is shown to have a strong influence on the light-induced drift if the collision frequency is much higher than the Doppler width of an absorption line: the drift calculated ignoring the influence of a phase memory may be strongly underestimated.

DOI
10.1070/qe1990v020n04abeh005683

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Physical Review A•Jan 1, 1990•8 citations
A study is reported of the influence of a phase memory in the presence of collisions on the work done by a field and on a light-induced drift of two-level gas particles in the Lorentz model of a gas (heavy buffer particles). It is shown that a phase memory may result in amplification of the incident radiation by particles at certain velocities in the absence of a population inversion (when practically all the particles are in the ground state). A phase memory is shown to have a strong influence on the light-induced drift if the collision frequency is much higher than the Doppler width of an absorption line: the drift calculated ignoring the influence of a phase memory may be strongly underestimated.
The influence of phase memory under collisions on the field work and the light induced drift (LID) effect of two-level gas particles is studied. It is shown that the phase memory effects may lead to the case where about half of the particles interacting with radiation amplify the incident radiation. The integral absorption coefficient for this remains positive due to growing radiation absorption by another half of the particles. It has been found that the phase memory strongly affects the LID under v ≫ k ( v is the collision frequency, k is the Doppler line width): calculation of LID without regard for the phase memory can lead to underestimation of the LID effect value by a factor of ( v / k ) 2 ≫ 1. Interaction of the particles in the hard sphere model is used to illustrate that the phase memory begins to affect the LID when the relative difference of the transport cross-sections of the particles is ground and excited states is small: Δσ/σ ≲ / R , where is the de Broglie wavelength at thermal velocities, R is the sphere radius.
The absorption (amplification) spectrum of a weak probe field by two-level atoms located in a strong resonant laser field and colliding with buffer-gas atoms is analyzed theoretically. The analysis is performed for low collision frequencies compared to the Doppler absorption linewidth (low gas pressure) and with allowance made for an arbitrary change in the phase of the radiation-induced dipole moment at elastic collisions between gas particles. The phase memory effects have been found to lead to a strong qualitative and quantitative transformation of the probe-field spectrum even at rare collisions, when the well-known Dicke manifestation mechanism of the phase memory effects (the removal of Doppler broadening due to the restriction of the spatial particle motion by collisions) is inoperative. The strong influence of the phase memory effects on the spectral resonances at low gas pressures stems from the fact that the phase-conserving collisions change the velocity dependence of the partial refractive index n ( v ) (the refractive index for particles moving with velocity v ).
The spectrum of weak probe field absorption (amplification) by two-level atoms experiencing collisions with buffer gas atoms in a strong resonance laser field is studied theoretically. Analysis is carried out for systems with a weak Doppler broadening under relatively mild constraints on the strong field intensity for the general case of an arbitrary change in the phase of the radiation-induced dipole moment in elastic collisions of gas particles. It is shown that, in spite of uniform broadening of the absorption line, the probe field spectrum exhibits a clearly manifested anisotropy to mutual orientation of the wavevectors of strong and probe radiation. It is found that the width of resonances in the probe field spectrum under definite conditions (that can easily be created in experiments) is proportional to the diffusion coefficient for atoms interacting with radiation. This fact can form the basis of the spectroscopic method for measuring the transport frequencies of collisions between particles absorbing radiation and buffer particles. It is shown that phase memory effects in collisions strongly modify the probe field spectrum both qualitatively and quantitatively. Simple operative formulas proposed for the probe field spectrum are convenient for experimental data processing.
The influence of phase memory effects on the spectrum of resonance Raman scattering by three-level atoms with the Λ configuration of levels experiencing collisions with buffer gas atoms in a strong monochromatic radiation field is studied theoretically. Systems with a small Doppler broadening compared to the collision frequency (large buffer gas pressures) are analysed in the general case of an arbitrary change (from complete change to complete preservation) in the phase memory at any of three transitions in the Λ system. It is shown that in the absence of the collision relaxation of the low-frequency coherence at the transitions between two lower levels of the Λ system, the radiation scattering spectrum has a spectrally narrow component at the Raman frequency, which, despite the homogeneous broadening of the absorption line, exhibits a strongly pronounced anisotropy. In the direction, close to the propagation direction of exciting radiation, this line maximally narrows down. It is significant that upon optical pumping to the level unaffected by a strong field the resonance Raman spectrum noticeably differs from the spectrum in the case of the probe field. A simple expression is proposed for calculating the degree of the phase memory preservation in collisions from the relative amplitude of the Raman resonance.
The effect of collisions on the resonance fluorescence spectrum of three-level atoms with a V-configuration of levels when excited by two monochromatic resonance fields has been theoretically investigated. The analysis has been carried out for the systems with a small Doppler broadening compared to the collision frequency (high buffer gas pressures) and for the general case of an arbitrary change (from complete failure to complete preservation) of the phase memory at any of the three transitions in the V-system. It has been shown that in the three-level V-type atoms excited by two monochromatic fields, the resonant fluorescence spectrum (the number of spectral lines and their widths) at one of the transitions strongly depends on the presence of resonant radiation at the adjacent transition. Under certain conditions, the line widths will be completely determined by the relaxation characteristics of the adjacent transition and, in particular, can be repeatedly narrowed with an increase in the radiation intensity at the adjacent transition. It has been shown that despite the homogeneous nature of the broadening of the absorption lines, the resonance fluorescence spectrum has a pronounced anisotropy with respect to the mutual orientation of the wave vectors of spontaneous and exciting radiation. It has been established that the line widths in the resonance fluorescence spectrum are proportional to the diffusion coefficient of the atoms interacting with radiation under certain conditions. This fact can serve as the basis for a spectroscopic method for measuring the transport frequencies of the collisions of the radiation-absorbing particles with the buffer ones. It has been shown that the effects of phase memory in the collisions lead to a strong qualitative and quantitative modification of the resonance fluorescence spectrum.
A new light-induced drift effect is discussed in which a white-light source separates a gas mixture containing a single optically active (absorbing) species and a buffer gas which does not interact with the light. A three-level model with one upper level and two closely spaced lower levels is used for the active atoms, and strong collisions with the more abundant surrounding buffer gas atoms are assumed. The intensity spectrum in the vapor (including the effects of hyperfine pumping), and the resulting drift velocity are calculated as a function of optical depth into the vapor.