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Journal of the Optical Society of America B

Collisionally induced phenomena in nonlinear optics

E. Giacobino, P. R. Berman, M. Pinard, G. Grynberg

Published Jan 1, 1986• 0 citations
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Abstract

Collisional perturbation of atoms in a vapor is well known to induce additional resonances in degenerate or nearly degenerate four-wave mixing.1,2 Actually the effect of collisions in nonlinear optics includes a much broader class of phenomenon, in which the dephasing of the wave functions during the collisions modifies the response of the medium to the input waves. Two examples of such effects are described: coherent emission of radiation induced by collisions and collisionally aided amplification.

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Collision enhanced resonances represent an interesting class of nonlinear-optical processes. They occur because collisional dephasing can rephase quantum mechanical amplitudes that ordinarily cancel out exactly, thereby allowing otherwise unobservable wave-mixing resonances to be seen. This is an especially interesting phenomenon because these resonances are coherent effects that are induced by an incoherent process (collisional dephasing). First predicted1 in the late 1970s and eventually observed2 in 1981, these novel effects have now been seen in a wide variety of four-wave-mixing experiments, ranging from self-focusing to coherent anti-Stokes Raman spectroscopy.

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Pressure-induced effects in nonlinear optics and spectroscopy are studied using a nonstandard-dressed-atom formalism for a set of two-level atoms. We consider a two-level atom interacting with two radiation fields whose frequencies differ by \ensuremath{\delta} and undergoing dephasing collisions with a perturber bath. The atom-field detunings are large compared with all relaxation rates in the problem. The system is described by a Hamiltonian of an atom dressed by a single-mode field with a coupling constant slowly varying in time at frequency \ensuremath{\delta}. We show that the collisionally aided excitation of the initially unpopulated dressed states is modulated in time. All the characteristics of the pressure-induced effects are deduced from the properties of this time-modulated excitation. Apart from already known phenomena (pressure-induced extra resonances in four-wave mixing, pressure-induced phase shifts, etc.) that are reinterpreted using this approach, we present a new effect that can be described as fluorescence beats. Finally, we show that this approach gives a firm basis for use of a model involving collision-induced gratings and we interpret the pressure-induced effects in the context of this model.

Physical Review A•Apr 1, 1989•22 citations
Collisional perturbation of atoms in a vapor is well known to induce additional resonances in degenerate or nearly degenerate four-wave mixing.1,2 Actually the effect of collisions in nonlinear optics includes a much broader class of phenomenon, in which the dephasing of the wave functions during the collisions modifies the response of the medium to the input waves. Two examples of such effects are described: coherent emission of radiation induced by collisions and collisionally aided amplification.
Collision enhanced resonances represent an interesting class of nonlinear-optical processes. They occur because collisional dephasing can rephase quantum mechanical amplitudes that ordinarily cancel out exactly, thereby allowing otherwise unobservable wave-mixing resonances to be seen. This is an especially interesting phenomenon because these resonances are coherent effects that are induced by an incoherent process (collisional dephasing). First predicted1 in the late 1970s and eventually observed2 in 1981, these novel effects have now been seen in a wide variety of four-wave-mixing experiments, ranging from self-focusing to coherent anti-Stokes Raman spectroscopy.
A very large number of physical phenomena in the resonant interaction of atomic-molecular systems with the radiation fields can be studied by treating the atomic system as a two level system and by examining its response to the applied fields. Various perturbative results are well known. A number of recent experimental works require the knowledge of the nonperturbative response functions, when both the applied fields are strong. These include a variety of experiments on instabilities(1) in lasers involving the switching from a single mode to for example two mode operation. Similarly recent nonlinear mixing experiments, where not only four wave mixing but also higher mixing in saturating fields has been studied. Pump probe experiments have also been repeated with probes whose intensity could be large. The purpose of this paper is to formulate the general theory and use it to understand the experiments. The formulation we present is also well suited for studying the effect of collisions. Details of various collisional induced coherences in fourwave and six wave mixing etc. can be studied.
Abstract : Work is summarized in areas of: Laser Assisted Collisions; Velocity Changing collisions in Atomic Vapors; Coherent Transients; Collision Induced Extra Resonances; Breakdown of the Optical Bloch Equations; Saturation Spectroscopy; Dynamics of Two-and Three-level Atoms Interacting with Radiation Fields; Pulse Propagation. Keywords: Laser spectroscopy, Optical collisions, Radiative collisions, Two-level atom, Atomic coherence, Degenerate four-wave mixing, Saturation spectroscopy, Laser propagation,, Dressed atom, Photon echo, Bloch equations, Collision kernal, Collisions, Optical noise.
Pressure-induced effects in nonlinear optics and spectroscopy are studied using a nonstandard-dressed-atom formalism for a set of two-level atoms. We consider a two-level atom interacting with two radiation fields whose frequencies differ by \ensuremath{\delta} and undergoing dephasing collisions with a perturber bath. The atom-field detunings are large compared with all relaxation rates in the problem. The system is described by a Hamiltonian of an atom dressed by a single-mode field with a coupling constant slowly varying in time at frequency \ensuremath{\delta}. We show that the collisionally aided excitation of the initially unpopulated dressed states is modulated in time. All the characteristics of the pressure-induced effects are deduced from the properties of this time-modulated excitation. Apart from already known phenomena (pressure-induced extra resonances in four-wave mixing, pressure-induced phase shifts, etc.) that are reinterpreted using this approach, we present a new effect that can be described as fluorescence beats. Finally, we show that this approach gives a firm basis for use of a model involving collision-induced gratings and we interpret the pressure-induced effects in the context of this model.