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Soviet Journal of Quantum Electronics

Transit effects in the theory of a Lamb dip in a strong field at very low gas pressures

Alexandre N. Titov

Published May 31, 1985• 1 citation
Publisher

Abstract

A theoretical analysis is made of the interaction of a spatially bounded plane standing optical wave with a molecular gas at a very low pressure. This theory applies to weak and strong (saturating) fields. Calculations are made of the profile, amplitude, and width of a Lamp dip as a function of the intensity of the saturating field and of the dimensions of the light beam. The role of slow molecules in the formation of the dip is identified. It is shown that in weak saturating fields the width of the resonance is governed by the average thermal velocity of molecules in a gas and by the transverse size of the light beam.

DOI
10.1070/qe1985v015n05abeh007082

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A theoretical analysis is made of the interaction of a spatially bounded plane standing optical wave with a molecular gas at a very low pressure. This theory applies to weak and strong (saturating) fields. Calculations are made of the profile, amplitude, and width of a Lamp dip as a function of the intensity of the saturating field and of the dimensions of the light beam. The role of slow molecules in the formation of the dip is identified. It is shown that in weak saturating fields the width of the resonance is governed by the average thermal velocity of molecules in a gas and by the transverse size of the light beam.
An important problem in saturated absorption spectroscopy is discussed: the width of a nonlinear absorption resonance in a low-pressure gas in a standing wave field. It is shown that the results reported by A.N. Titov et al. are incorrect. These authors stated that at weak saturations the resonance width is of the order of the reciprocal transit time of an atom traveling at the thermal velocity across a light beam. The specific source of the error is indicated, which is an incorrect mathematical treatment of the correct initial equations. It is also shown that the saturated absorption line profile in the transit limit is almost independent of the laser field configuration.
The field dependence of the susceptibility of a gas interacting with a standing wave is calculated near the center of a line for a field of arbitrary profile. An allowance is made for the effects of spatial modulation of the gas. Numerical calculations are reported for a field with a Gaussian profile allowing for degeneracy. These calculations can be checked directly against experimental results for a transition corresponding to a line at λ = 3.39 μ in CH4 , which should make it possible to determine accurately the dipole moment of the transition. The amplitude and half-width of a Lamb dip are calculated for a field of arbitrary profile on the basis of perturbation theory. The problem of approximation of a real Gaussian field with a model field characterized by an amplitude constant over a circular cross section is considered. The precision of model calculations is determined for weak and strong fields. Resonances in atomic beams are discussed under transit conditions. It is shown that in this case there is a slow atom effect, which is responsible for lines with a characteristic width that is a linear function of the homogeneous width of a transition when excited particles are detected on the basis of their fluorescence.
A theory of saturated absorption resonances is developed for the transit range of pressures of an absorbing gas with an arbitrary degree of saturation. The rate equation approximation is used to obtain expressions for the absorption coefficient in the cases of one traveling wave, weak and strong opposite waves, and a standing wave with a rectangular radial distribution of the intensity. An analytic study is made of the behavior of resonance widths when the saturating field intensities are varied and it is shown that, in particular, at relatively low densities the widths are proportional to the fourth root of the intensity.
It is shown that the interaction of three standing-wave light beams with a low-pressure gas gives rise to a narrow absorption power resonance. The width of this resonance is related to the transit time of gas atoms between the beams. The weak-field case is analyzed. A formula for a Held of arbitrary intensity is obtained for the situation when the dimensions of the beams are much smaller than the distances between them. Numerical calculations are reported of the resonance amplitude and profile. Optimal (in respect of the field intensity) conditions for the observation of the effect are determined. If the distances between the beams are sufficiently large, it should be possible to observe resonances of width equal to the homogeneous width of atomic radiation lines in spite of the considerable field and transit broadening. The possibility of using this power resonance in stabilization of the emission frequency of gas lasers is considered.
This paper presents a calculation of the depth of the saturation dip observed when laser radiation of a Gaussian TEM00{\mathrm{TEM}}_{00}TEM00​ mode is absorbed in a thermal molecular beam or in a very-low-pressure gas. Transit-time broadening is assumed to be the only broadening mechanism, with neglect of collisions and other relaxations. There is no restriction to small saturation parameters. The absorption experienced by one molecule has been calculated via the optical Bloch equations. Averaging over all relevant parameters of the molecular trajectory and the laser field followed. To get a reference for the saturation, from which to measure the dip depth, simply twice the absorption from a running-wave field has been taken, which therefore also had to be calculated in addition to the calculation of the absorption in a standing wave. Preliminary experiments have been carried out for comparison, and the data agree very well with the calculated shape of the curve and the absolute values of the calculated dip depth. From the laser-power scaling factor, necessary to fit the experimental data to the universal theoretical curve, the dipole moment can be derived without having to know such hard-to-determine quantities as absorber density, partition function, collision relaxation rate, or even the diameter of the laser beam.
A three-dimensional theory for the resonant interaction of electromagnetic waves with a gas of two-level atoms is formulated in terms of macroscopic variables. The theory is utilized to find the steady-state attenuation of a plane wave in the presence of another plane wave running in the opposite direction with different amplitude. Contributions are included from the reflection of the oppositely running wave by an induced standing-wave inhomogeneity in the population inversion of the medium. The resulting attenuation and reflection coefficients are expressed as velocity integrals of continued fractions. Correspondence is made with existing gas-laser theories, yielding the formulation of a high-intensity ring-laser theory. Analytic approximations for the coefficients are presented for the Doppler-limit cases of both waves weak, one wave weak, and negligible reflection (rate-equation approximation). More-general cases have been calculated numerically. The attenuation coefficients exhibit a Lamb-dip feature. The relative depth of the dip increases rapidly with power at low saturation levels, slowly at high saturation, and is greater in the attenuation of the weaker wave. The width of the dip is nonlinearly power broadened. The shape of the dip is very nearly Lorentzian, except for one special case at high power in which the line splits. The propagation equations for the two waves are integrated over long absorption paths. A large resulting attenuation increases the relative size of the dip while decreasing the power broadening.