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Journal of Experimental and Theoretical Physics

Scattering of atoms in a bichromatic field of oppositely propagating light pulses

V. I. Romanenko, Леонід Петрович Яценко

Published Mar 1, 2000• 11 citations
Publisher

Abstract

The interaction of a two-level atom with two light pulses which have different carrier frequencies and propagate in opposite directions is studied in the adiabatic approximation. It is shown that when there is a delay between the light pulses, the momentum of the atom changes, as a result of the interaction with the laser field, by a definite amount that depends on the amplitude of the light pulses and on the difference of their carrier frequencies. It is predicted that the momentum transfer to the atom is a step function of the amplitude of the light pulses.

DOI
10.1134/1.559120

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arXiv:2011.11312•Physical Review A•Apr 9, 2021•3 citations
The interaction of a two-level atom with two light pulses which have different carrier frequencies and propagate in opposite directions is studied in the adiabatic approximation. It is shown that when there is a delay between the light pulses, the momentum of the atom changes, as a result of the interaction with the laser field, by a definite amount that depends on the amplitude of the light pulses and on the difference of their carrier frequencies. It is predicted that the momentum transfer to the atom is a step function of the amplitude of the light pulses.
A theoretical analysis is presented of the change in the momentum of a three-level atom due to its interaction with counterpropagating laser pulses that overlap in time. The two lower energy states of the atom are metastable; i.e., a lambda-type configuration of atomic levels is considered. The cases of two and four counterpropagating pulses having different carrier frequencies are considered. In the case of adiabatic atom-field interaction, it is shown that the atom’s momentum can change by an integer multiple of the photon momentum and the corresponding standard deviation is small as compared to the photon momentum squared.
In this paper we investigate how Rydberg atoms respond to perturbation by two contrapropagating ultrashort laser pulses. We consider the case where the durations of both pulses ${\ensuremath{\tau}}_{1}$ and ${\ensuremath{\tau}}_{2}$ are shorter than the inverse of the initial-state energy ${\ensuremath{\epsilon}}_{i}^{\ensuremath{-}1}$. When acting alone such a pulse passes through the atom without noticeable alteration in the atomic state. The situation is different if two such pulses interfere in the region of atom localization. In this case the atomic response is significantly enhanced. This is due to the nonzero momentum transferred to the electron by the interplay of the electric field of one pulse and the magnetic field of the other. The sudden perturbation approximation is used to evaluate the transition probabilities. They are shown to depend on the atom position with respect to the pulse interference region. This dependence is determined by the relationship between the atomic diameter di{d}_{i}di​ and the interference-region size $l=c({\ensuremath{\tau}}_{1}+{\ensuremath{\tau}}_{2})$ (ccc is the speed of light). If ${d}_{i}\ensuremath{\ll}l$ this dependence is sensitive to the function form of the pulses. For sufficiently strong fields the atoms can be ionized completely. In the opposite case of ${d}_{i}\ensuremath{\gg}l$ the transition probabilities are sensitive to the electron density distribution along the propagation direction. The probabilities of the initial-state destruction and atom ionization drop as l∕dil∕{d}_{i}l∕di​ irrespective of the characteristics of the pulses.
A modified perturbation theory is developed for the calculation of the time-dependent wave function describing the propagation of an atom in a two-mode laser field. With the mode frequencies nearly equal, a first approximation is obtained that reproduces and extends known results, for both resonant and nonresonant transitions. This lowest-order solution sums terms that describe the absorption of a photon from one mode and emission into the other, leaving the total number of photons unchanged and shifting the momentum of the atom accordingly. Corrections are generated by expansion of a resolvent from which the nearly degenerate states have been projected out. These approximate solutions are used to construct asymptotic states in a formulation of atom-atom scattering in the two-mode field. In close analogy with the soft-photon approximation, a ``soft-pair'' approximation is developed based on the dominance of those asymptotic interactions in which the transfer of energy from the field to the atom is small, allowing one to relate the scattering amplitude in the presence of the field to the field-free amplitude. When the atomic ground state and an excited state are closely coupled by the field, the asymptotic wave function can have mixed parity allowing for long-range contributions to the effective potential. In addition to an enhancement of forward-scattering probabilities, this leads to a threshold anomaly that is analyzed here, in the resonant case, with the aid of a modified effective-range theory.
We study the dynamics of the phase pulses of laser radiation with two resonant frequency components propagating in an atomic three-level Λ type medium under coherent population trapping. Using the density matrix adiabatic approach we show that the effect of a very large slowing down occurs not only for amplitude pulses, but also for phase pulses. The group velocity of phase pulses can be controlled by the amplitudes of resonant fields. The analytical solution describing the mutual influence of the phase modulation of one field component on another is obtained.
We study numerically the evolution of the velocity distribution of atoms under the action of the bichromatic force. The comparison of the time dependencies of the distribution width and the average acceleration of atoms reveals the correlation of these quantities. We show that the estimation of the momentum diffusion coefficient on the basis of the analogy between the interaction of atoms with the counterpropagating bichromatic waves and the interaction of atoms with the counterpropagating sequences of the $\ensuremath{\pi}$ pulses roughly corresponds to the results of numerical calculations. To separate the influence of the momentum diffusion on the evolution of atomic momentum distribution from the influence of the time-dependent Doppler shift, we study the motion of a ``heavy'' atom, for which the velocity change during the interaction of an atom with the field can be neglected. Provided that the parameters of the atom-field interaction are optimal, we show that the momentum diffusion coefficient is proportional to the intensity of the laser radiation. We used the Monte Carlo wave-function method for the numerical simulation of the atomic motion.