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Journal of Physics B Atomic Molecular and Optical Physics

Focusing and defocusing of a near-resonant dye laser beam by photodissociation-produced Tl atoms

Jie Zhang, Terence A. King

Published May 14, 1990• 0 citations
Publisher

Abstract

A new optical effect showing focusing and defocusing of a near-resonant dye laser beam in Tl atomic vapour produced by photodissociation is reported. The effect is attributed to wavefront distortion of the dye laser beam, caused by Tl vapour number density gradient produced via an inhomogeneous intensity distribution of the dissociation KrF excimer laser beam, and the anomalous dispersion of Tl atoms near the resonance of the 6 2 P 3/2 -7 2 S 1/2 transition.

DOI
10.1088/0953-4075/23/9/007

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A new optical effect showing focusing and defocusing of a near-resonant dye laser beam in Tl atomic vapour produced by photodissociation is reported. The effect is attributed to wavefront distortion of the dye laser beam, caused by Tl vapour number density gradient produced via an inhomogeneous intensity distribution of the dissociation KrF excimer laser beam, and the anomalous dispersion of Tl atoms near the resonance of the 6 2 P 3/2 -7 2 S 1/2 transition.
An optical effect showing focusing and defocusing of a dye laser beam near the atomic resonance of the Tl*6p 2 P 3/2 -7s 2 S 1/2 transition in Tl vapour produced by photodissociation is reported. The experimental investigation of the effect as a function of detuning, vapour number density, dissociation laser energy and dye laser energy is presented. The effect is attributed to distortion of the dye laser wavefront by anomalous dispersion near the resonance and the inhomogeneous number density of Tl atomic vapour produced by an inhomogeneous intensity distribution of the dissociation laser.
A collection of ultracold atoms has an index of refraction that varies depending on the spatial density distribution of the gas. For many confined gases commonly found in ultracold atom experiments, there can be a substantial gradient in the spatial density distribution and hence the index of refraction. In addition, these gases can have a smaller spatial extent than that of the cross section of a laser beam that illuminates them. With a sufficient gradient in density under these conditions, the resulting index variation leads to frequency-dependent focusing or defocusing of incident near-resonant light as demonstrated by the calculations in this work. Nonintuitive intensity pattern formations result within the gas from the combination of refraction and diffraction of the incident light, including focusing effects that substantially increase the intensity of the light as compared to its incident value despite the nominally absorptive nature of the gas.
In this paper, we address the propagation of a near resonant laser inside an atomic vapor in the case when, due to the Kerr effect, the laser beam would either self-focus or self-defocus. We show, both theoretically and experimentally, how to get rid of such an alteration in the transverse beam profile without changing any of the characteristics of the laser light under consideration (wavelength, intensity, etc.), nor of the atomic vapor. Moreover, our proposed method offers a lot of control on the beam profile, whose transverse size after propagation may be chosen at will by making use of a second, copropagating laser, whose required wavelength and intensity may be derived analytically.
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.
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.