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Journal of Physics B Atomic Molecular and Optical Physics

Hard-collision broadening of the probe-field spectrum in a -system

Yu. I. Belousov, David A. Shapiro

Published Apr 1, 2003• 2 citations
Publisher

Abstract

The probe-field spectrum of a three-level Λ-gas with hard collisions between particles is studied. The shape and positions of components of the Autler–Townes doublet are obtained at different collision frequencies. When the Doppler broadening is much greater than the collision frequency the width of the peaks is found to be proportional to the collision frequency. In contrast when collisions are elastic the peaks are shown to narrow due to the Dicke effect.

DOI
10.1088/0953-4075/36/8/304

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Journal of Experimental and Theoretical Physics•Mar 1, 2006•2 citations
The probe-field spectrum of a three-level Λ-gas with hard collisions between particles is studied. The shape and positions of components of the Autler–Townes doublet are obtained at different collision frequencies. When the Doppler broadening is much greater than the collision frequency the width of the peaks is found to be proportional to the collision frequency. In contrast when collisions are elastic the peaks are shown to narrow due to the Dicke effect.
The absorption spectrum of a gas with a large Doppler width and soft collisions between particles is studied. The particles are assumed to have a nonlinear dependence of the resonance frequency on velocity. The shape of the narrow peak in the spectrum resulting from an extremum of this dependence is calculated analytically for the first time. In the absence of collisions it has a characteristic asymmetric shape. Collisions are shown to broaden the line and change its shape. The profile of the probe-field spectrum is obtained for a three-level system with the strong field at an adjacent transition. The components of the Autler-Townes doublet are found to spread and repel each other because of collisions.
An expression is found for the profile of Bennett hole with different level lifetimes under strong field and weak collisions. It is shown that the square of the total width being equal to the sum of the squares of the diffusion and field widths. Probe-field spectrum of the three-level system is obtained in the presence of the strong field at the adjacent transition. The Autler - Townes doublet components are shown to broaden and repel each other under the collisions. The main reason of both the effects occurs to be the diffusion of phase due to quadratic frequency shift.
We investigate theoretically the spectrum of weak probe field absorption by three-level atoms with the Λ configuration of levels in the field of a strong electromagnetic wave acting on an adjacent transition and colliding with buffer gas atoms. Analysis is carried out for the general case of arbitrary collisional relaxation of low-frequency coherence at a transition between two lower levels. It is shown that, in the absence of collisional relaxation of low-frequency coherence, the probe field spectrum always exhibits clearly manifested anisotropy with respect to mutual orientation of wavevectors of the strong and probe radiation (even under small Doppler broadening). It is found that the probe field spectrum may acquire under certain conditions supernarrow resonances with a width proportional to the diffusion coefficient for atoms interacting with radiation. This fact may form the basis for a spectroscopic method for measuring transport frequencies of collisions between absorbing and buffer particles. A large-amplitude supernarrow resonance (with an amplitude much larger than the amplitude of the resonance near the line center), which is observed in the far wing of the absorption line, exhibits collisional narrowing (a nonlinear spectroscopic analog of the Dicke effect) at collision frequencies several orders of magnitude lower that the Doppler linewidth. Simple working equations proposed for describing the probe field spectrum are convenient for experimental data processing.
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.
An analytical expression is found for the profile of Bennett hole with different level lifetimes under strong field and weak collisions. The expression is valid while the diffusion and field broadening being more than the homogeneous one, but less than the thermal width. It is shown that the square of the total width being equal to the sum of the squares of the diffusion and field widths. The shape of saturation curve is calculated including weak collisions. The formula obtained describes the smooth transition from homogenous to inhomogeneous saturation throughout the new intermediate region. Probe-field spectrum of the three-level system is computed in the presence of the strong field at the adjacent transition. The Autler-Townes doublet components are shown to broaden and repel each other under the collisions. The main reason of both the effects occurs to be the diffusion of phase due to quadratic frequency shift.
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 ).