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Physical Review Letters

Transverse ‘‘bouncing’’ of polarized laser beams in sodium vapor

R. Holzner, Patrik Eschle, A. W. McCord, D. M. Warrington

Published Oct 12, 1992• 21 citations
Publisher

Abstract

Two intersecting laser beams of orthogonal circular polarization near resonant to the homogeneously broadened D1 transition of Na vapor were observed to deflect each other upon contact. The experimental results are in good agreement with calculations based on a model which includes diffraction and a local nonlinear atomic susceptibility wihch fully takes into account saturation and optical pumping effects. The contact deflection (or ``bouncing'') was also observed when both beams were tuned to the atomic resonance frequency.

DOI
10.1103/physrevlett.69.2192

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Annual Meeting Optical Society of America•Jan 1, 1988•0 citations
Two intersecting laser beams of orthogonal circular polarization near resonant to the homogeneously broadened D1 transition of Na vapor were observed to deflect each other upon contact. The experimental results are in good agreement with calculations based on a model which includes diffraction and a local nonlinear atomic susceptibility wihch fully takes into account saturation and optical pumping effects. The contact deflection (or ``bouncing'') was also observed when both beams were tuned to the atomic resonance frequency.
Circularly polarized laser beams propagating through sodium vapour and tuned to the buffer-gas-broadened atomic transition can optically pump sodium atoms into a non-absorbing ground state. This causes an intensity-dependent refractive index gradient along as well as transverse to the laser beam propagation direction, giving rise to a number of nonlinear spatio-temporal intensity and polarization pattern creating processes. In the case of a single circularly polarized laser beam we have observed self-focusing and defocusing, the transformation of the incident Gaussian beam intensity profiles into ring profiles, a large shift of about 5 GHz of the maximum of the absorption profile when suitable magnetic fields are applied and the deflection of a beam by the inhomogeneous transverse magnetic field of a current-carrying wire. When two beams of opposite circular polarization are superimposed, astonishing effects such as the mutual deflection of both beams (beam bouncing), the mutual extinction of both beams (beam switching), the separation of initially overlapping beams (beam splitting) and the mutual attraction of both beams (beam attraction) can be observed. While most of the effects can be well described for the stationary state by a to atomic transition model, the correct description of the dynamics requires the consideration of all hyperfine states.
The interaction of left- and right-hand circularly polarized narrow-band cw laser beams that copropagate through sodium vapor and are tuned to the homogeneously broadened D1 transition can lead to the mutual extinction of both beams. Such beam switching is caused by an intensity- and polarization-dependent refractive index due to optical pumping even at low intensities. The dynamics of beam switching is investigated experimentally and explained qualitatively by the conventional J = 1/2 to J = 1/2 transition model. The accurate quantitative description, however, is only achieved by an extended model comprising hyperfine ground states in spite of homogeneous line broadening by argon buffer gas, which is larger than the hyperfine splitting.
Circularly polarized laser beams propagating through sodium vapor and tuned to the buffer gas broadened atomic D1 transition can optically pump sodium atoms into a nonabsorbing groundstate. The corresponding groundstate populations depend on the intensity distribution, the polarization and the magnetic fields. They lead to refractive index gradients along as well as transverse to the laser beam propagation direction and therefore modify the light path giving rise to a number of nonlinear spatiotemporal intensity and polarization pattern creating processes.
We describe a sensitive technique for observing small transverse modifications to intensity, phase, and polarization arising in nonlinear propagation of a laser beam through an atomic vapor. The technique, which in essence is an interferometric one, is based on the use of crossed polarizers that are nearly but not exactly pure circular. Illustrative results are presented for cw dye-laser radiation propagating through collisionally broadened atomic sodium that is near resonant to the D1 line. Well-defined ring patterns produced by nonlinear interactions are observed at both positive and negative laser detunings.
A circularly polarized laser beam propagating through sodium vapor is shown to be deflected by the inhomogeneous magnetic field from a nearby electric current-carrying thin wire aligned parallel to the beam. The effect arises from the refractive index gradient induced by magnetic-field-modified optical pumping. The experimentally observed beam deflection is in good agreement with predictions from a $J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2\ensuremath{\leftrightarrow}J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ model for the full four-dimensional spatialtemporal evolution of the interaction between radiation field and atoms. The time of order 100 ns needed for the beam to switch between undeflected and deflected positions is determined by the average Larmor precession period and is 2 orders of magnitude shorter than the optical pumping time.
We report the first observations of the cw self-bending effect1 using an asymmetrical laser beam propagating in sodium vapor and discuss some concerns of practical and physical importance. The laser frequency was detuned below the D2 resonance until the maximum self-bending angle was observed. At ~200°C the nonlinear refractive index was found to be predominantly Kerrlike (Δn ≃ n 2 I) at experimental peak intensities up to ~220 W/cm2. This result is inconsistent with our numerical calculations of the saturation of the anomalous dispersion that includes power-broadened hole-burning of an inhomogeneously broadened two-level system,2 which predicts strong saturation of Δn. By analyzing either the self-deflection angle or the intensity at a single point in the far-field region as the input power was varied, we were able to determine that n2 ≃ − 10 − 7 cm2/W. Self-deflection angles as large as 5.9 mrad or eight times the diffraction angle were observed, which is more than enough for some useful applications such as resonatorless optical bistability, optical switching, and interconnecting, nonlinear optical coupling, and radiation protection, the latter of which we have demonstrated.