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Optics Letters

Optical collisions in an intense laser field

A. Szöke

Published Feb 1, 1978• 5 citations
Publisher

Abstract

The collisionally broadened optical absorption cross section that is constant at low intensities is predicted to decrease at high intensities. The prediction is based on a nonlinear change of the effective interaction of the atom with the strong incident electromagnetic field and a distortion of the atomic interaction potential by it. An experiment is analyzed here that observes this falloff on the “wing” of the 460.73-nm strontium resonance line, collision broadened by an argon buffer gas. Qualitative agreement is reached between theory and experiment when the degeneracy of the levels is taken into account.

DOI
10.1364/ol.2.000036

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The collisionally broadened optical absorption cross section that is constant at low intensities is predicted to decrease at high intensities. The prediction is based on a nonlinear change of the effective interaction of the atom with the strong incident electromagnetic field and a distortion of the atomic interaction potential by it. An experiment is analyzed here that observes this falloff on the “wing” of the 460.73-nm strontium resonance line, collision broadened by an argon buffer gas. Qualitative agreement is reached between theory and experiment when the degeneracy of the levels is taken into account.
An analysis is made of the spectrum of absorption of a weak probe electromagnetic by two-level atoms in a strong resonant laser field, undergoing collision with buffer gas atoms. The analysis is made using an approach that allows for the direct influence of a strong electromagnetic on the dynamics of an elastic collision between an active atom and a buffer gas atom. Rate equations are analyzed for a combined atom–strong electromagnetic field system (an atom dressed by the field) allowing for spontaneous and optical collisional transitions, and also for the interaction with the probe field. In the steady-state case, an expression is derived for the electric susceptibility of the medium at the small-signal frequency. This expression contains the rates of the optical collisional transitions that depend nontrivially on the parameters of the strong electromagnetic field. The phenomenological characteristics of optical collisional transitions generally used are only valid at low intensities and for small frequency detunings of the strong electromagnetic field, i.e., in the impact limit.
An analysis of near-resonant collisions between dissimilar atoms in the presence of a radiation field reveals their effect on the far wings of the absorption profile. Extending a model due to Cavalieri, Arimondo, and Matera, we take into account the collisional shifts of atomic levels, as well as the collisional coupling between atoms and the coupling with the radiation field. A general formula for the absorption profile is derived and expressed in terms of the universal line-shape function of the unified Franck-Condon treatment of pressure broadening. We show that in some cases collision-induced coherence can give rise to a strongly depressed red wing that exhibits transparency for some values of the laser detuning. \textcopyright{} 1996 The American Physical Society.
The authors present an investigation of the modification caused by an intense laser field to the rates of m J changing collisions for Hg perturbed by argon. They have studied experimentally the competition between radiative and collisional coupling between m J states with a strong laser field tuned to the Hg J=1 to J=1 transition at lambda 435.8 nm. Population is pumped into the strongly dressed manifold from the ground state with a weak UV 'probe' laser detuned slightly from the lambda 253.7 nm intercombination line. The fluorescence observed at lambda 404.7 nm depends critically on whether the excitation proceeds via collisional redistribution or resonance through Stark shifted levels. The authors present data that clearly exhibit the effects of this competition. The data show in particular how one can produce excitation to transient Stark shifted levels in the presence of collisions. This will be critical in future experiments on collisions in strong fields.
The close-coupled theory of collisions in a radiation field is used to calculate the absorption profile for the Sr $^{1}\ensuremath{\leftarrow}^{1}$S resonance line broadened by collisions with Ar. The calculations predict the polarization ratios of Sr 1P^{1}\mathrm{P}1P fluorescence following line wing excitation by either linear or circular polarized light. Ab initio calculations were used to obtain the ground and excited SrAr molecular-potential curves, which were adjusted to give improved agreement with experiment. The radiative-scattering theory gives a unified description of the absorption coefficient and polarization redistribution from the small detuning impact limit region to the far spectral wings. The cross sections for elastic and inelastic depolarizing collisions of Sr 1P1^{1}\mathrm{P}_{1}1P1​+Ar were also calculated. The calculated absorption coefficient, impact-broadening rate, linear- and circular-polarization ratios, and depolarization rate coefficients are for the most part in good agreement with experiment.
We examine theoretically the behavior of cross sections for laser-induced collisional energy transfer (LICET) over a wide range of detunings and laser intensities for the two-atom pairs Li-Sr and Eu-Sr which have been previously studied experimentally. These two cases are complementary in that the laser and collisional interactions enter in opposite orders, and their level structures are such as to give weak-field line shapes of opposite asymmetry and intensity shift. A three-state model is used and the Bambini-Berman formula for the weak-field quasistatic wing is verified. At high intensities the peak cross section is found to go through a maximum and the LICET profile is found to become symmetric about a shifted center with notable departures from a uniform falloff in the far quasistatic wing, apparently arising from radiative collisional interference effects.