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Journal of Modern Optics

Focusing and defocusing dynamics of weak-light beam in a resonant three-level atomic system

Yihong Qi, Shiqi Jin, Hui Sun, Hongju Guo, Yueping Niu, Shangqing Gong

Published Jan 10, 2010• 0 citations
Publisher

Abstract

Focusing and defocusing dynamics of weak-light ring-shaped beams in a Λ-type atomic system are investigated. It is found that focusing of weak light is possible due to the quantum interference effect. The focusing or defocusing regime with vanishing absorption can be manipulated by adjusting detunings from the resonant transition. When the fields are tuned appropriately larger than resonant frequencies, the beam propagates in the defocusing regime and diffraction of the beam takes place. Conversely, the beam focuses in the medium when the laser frequencies are tuned smaller than the resonant frequencies.

DOI
10.1080/09500340903521868

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Focusing and defocusing dynamics of weak-light ring-shaped beams in a Λ-type atomic system are investigated. It is found that focusing of weak light is possible due to the quantum interference effect. The focusing or defocusing regime with vanishing absorption can be manipulated by adjusting detunings from the resonant transition. When the fields are tuned appropriately larger than resonant frequencies, the beam propagates in the defocusing regime and diffraction of the beam takes place. Conversely, the beam focuses in the medium when the laser frequencies are tuned smaller than the resonant frequencies.
An ultrashort weak pulse that propagates resonantly on the $\ensuremath{\mid}a⟩\ensuremath{\rightarrow}\ensuremath{\mid}b⟩$ transition of a three-level system $(\ensuremath{\mid}a⟩,\ensuremath{\mid}b⟩,\ensuremath{\mid}c⟩)$ experiences reshaping effects on a time scale longer than its duration. When a strong field is applied (nonresonantly) on the $\ensuremath{\mid}b⟩\ensuremath{\rightarrow}\ensuremath{\mid}c⟩$ transition, important light shifts are induced. We have theoretically studied the novel behavior of both the medium and weak propagating laser pulses in such conditions. For the transmitted field profile, drastic changes occur: at long times, efficient reduction of the distortion is obtained, while strong oscillations appear for short times, mapping out the dynamical light shift. These shaping effects, which are accompanied by new features in the spectrum for the propagating pulse, are interpreted as the result of the interference between incident field and induced dipole radiated field, whose frequency sweeps in time because of the induced light shift.
The propagation of two-component laser radiation in a medium consisting of atoms with a Λ level scheme is investigated. The simultaneous self-focusing of two beams is considered. The main features of this phenomenon are: 1) lowering of the self-focusing threshold by several orders of magnitude in comparison with the known case of saturation of a transition in a two-level atom; 2) a strong dependence of the character of the propagation of the radiation on the difference between the detunings of the two frequency components of the field from resonance, which is associated with fulfillment of the two-photon resonance condition.
We consider propagation of a weakly nonlinear probe light beam in a resonant three-level atomic system where an optical lattice is induced by a standing-wave control field and conditions for electromagnetically induced transparency are created. We employ a unified theory to solve the respective nonlinear system of equations in various regimes and investigate formation of spatial optical solitons and instabilities, both analytically and numerically. We show that the effect of the lattice is twofold. On the one hand it allows one to reduce the probe beam intensity necessary for observation of nonlinear effects until the level of one-photon energy, and on the other hand the lattice allows for implementation of different dynamical regimes either by simple manipulation of its parameters or by varying one- and two-photon detunings or by changing the geometry of the incident probe beam.
A numerical and analytic investigation is made of the self-interaction of Gaussian beams under two-photon resonance conditions when the pulse duration is shorter than the relaxation times and the frequency detuning from resonance exceeds appreciably the spectral width of the radiation (adiabatic model). An analysis is made of defocusing (up-detuning) and self-focusing processes (down-detuning) in two-photon resonance. Three characteristic adiabatic self-focusing regimes are identified: a steady-state waveguide and weak and strong quasiwaveguide propagation. Expressions are derived for the critical powers and minimal self-focusing length.
We report a study on light force on a beam of neutral two-level atoms superimposed upon a few-cycle-pulse Gaussian laser field under both resonant and off-resonant conditions. The phenomena of focusing, defocusing, and steering of the neutral atoms in the laser field are analyzed by solving the optical Bloch equation beyond the rotating-wave approximation and the force equation self-consistently. We find that two-level atoms in an atomic beam can be focused and defocused for large, positively and negatively detuned interactions even in the regime of extreme nonlinear optics. The so-called optical potential may be used for stable trapping of the neutral two-level atoms for large positively detuned interaction. This work successfully reproduces some of the features reported in recent experimental and theoretical works.
Summary form only given. The interaction between an intense pump beam and a weak probe beam in an atomic vapor has been the subject of numerous investigations. Besides the well-known features in the frequency spectrum of the light, interesting spatial phenomena are observed. They are due to the inhomogeneous intensity distribution of a laser beam, coupled to propagation, and range from self-focusing or defocusing effects to the generation of localized distributions of light that propagate self-similarly (bright or dark solitons). We investigate the reshaping of a probe beam due to the interaction with a pump in sodium vapor, a problem that is important in many different contexts.