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Journal of Physics D Applied Physics

Laser focusing and defocusing via anomalous dispersion and density inhomogeneity in Tl vapour

Jingyuan Zhang, Terence A. King

Published Jul 14, 1991• 0 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3727/24/7/003

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•Jan 1, 1982•0 citations
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.
The results are reported of investigations into the spectral and energy characteristics of a thallium photodissociation laser, emitting due to the 7S/sub 1/2/--6P/sub 3/2/ (lambda = 535 nm) and 7S/sub 1/2/--6P/sub 1/2/ (lambda = 337.6 nm) transitions. The experiments were performed using two types of resonator, and this made it possible to increase the radiation power density above the superradiance level and to operate in two-wavelength lasing regimes. On reaching a certain power density the structure of the laser emission spectrum changed, the nature of this change being different for the different lines. A discussion of the results obtained is given.
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.
This paper reports the first time achievement of large population inversion between thallium 7 2S1/2 and 6 2P3/2 states through two-photon dissociation of pure Tl2 molecules. The second harmonic of a Nd–YAG laser at 532 nm is used to cause two-photon absorption in Tl2 from ground state Ou− to the repulsive state associated with the atomic 7 2S and 6 2P1/2 states. The relative population of the product atomic species is determined by using a tunable dye laser pumped by the third harmonic of a Nd–YAG laser. The dye laser beam focused for complete spatial and temporal overlap with the photodissociaiton pulse excites the product states to known atomic Rydberg states with subsequent ion detection. Prompt population inversion amounting to 100% is achieved. It is shown that photodissociation transition is mediated through predissociation of the upper bound state to a repulsive state correlated with Tl(7 S1/2) in the region of large internuclear separation.
The behaviour of an optically thick column of barium vapour resonantly 10 -2 illuminated at 553.5m by a 50 ns, 10 Wm pulse from a flashlamp pumped dye 19 21 -3 laser system was investigated for barium number densities in the range 10 -10 m in the presence of He buffer gas at 100 Torr.A variety of optical saturation effects associated with the principal resonance transition were studied.In marked contrast to earlier work, saturation of the intensity of the scattered light near 553.5 nm with increasing laser power was found for a suitable geometry of the collection optics but at a laser intensity typically two orders of magnitude higher than that predicted by simple theory, despite the fact that this theory was found to describe correctly transmission measurements along the length of the vapour column.The apparent contradiction between the fluorescence and transmission results, which indicated that the scattered radiation intensity continued to rise with incident laser power, despite the population of the upper resonance level having reached a limiting saturation value, could be understood in terms of a simple model which took into consideration the spectral distribution of the scattered light and its transfer through the unexcited vapour.However, in common with other similar experiments, saturation of the scattered radiation intensity with laser power could not be totally achieved when a more usual collection optics geometry was employed.This was interpreted as a consequence of laser intensity variation across the face of the laser beam.A number of subsidiary experiments were performed.These included spatially resolved intensity measurements of the scattered radiation and time resolved metastable level population measurements.In addition, evidence for laser induced ionization was sought using two different methods and this enabled an upper limit of 10% ionization to be placed on the experiments in the present work.