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Journal of Optics B Quantum and Semiclassical Optics

Lateral beam splitting in sodium vapour: magneto-optical origin and polarization competition effects

A. Gahl, Andreas Aumann, M Möller, Wulfhard K. Lange

Published Jan 1, 1999• 4 citations
Publisher

Abstract

An almost linearly polarized laser beam focused into a sodium vapour cell may split into spatially separated components of opposite circular polarization. The symmetry of the emerging structures is controlled by static magnetic fields of the order of the Earth's field. The mechanism of the decomposition of the light field into its two spin components is clarified by numerical simulations revealing the evolution of the spatial distribution of the light field during the propagation through the sodium vapour. In this context, the experimentally observed occurrence of a negative differential power transmission can be explained.

DOI
10.1088/1464-4266/1/1/022

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An almost linearly polarized laser beam focused into a sodium vapour cell may split into spatially separated components of opposite circular polarization. The symmetry of the emerging structures is controlled by static magnetic fields of the order of the Earth's field. The mechanism of the decomposition of the light field into its two spin components is clarified by numerical simulations revealing the evolution of the spatial distribution of the light field during the propagation through the sodium vapour. In this context, the experimentally observed occurrence of a negative differential power transmission can be explained.
An almost linearly polarized laser beam focused into a sodium vapor cell may split into different patterns ranging from two spots to a spot and a ring of opposite circular polarization. The patterns are controlled by static magnetic fields of the order of the earth's field. The behavior is explained, allowing for a longitudinal component of the dielectric polarization in the paraxial wave equation. Numerical results reveal the delicate balance between light shift, influence of the magnetic field, and the creation of ground-state orientation by the competing polarization components.
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 occurrence of polarization instabilities in alkaline vapours and their coupling with spatial degrees of freedom are considered. We discuss experiments in which a laser beam is transmitted through a vapour cell without external feedback or with feedback by a single mirror. In both cases a lateral separation of the light field into domains of orthogonal circular polarization is found. Without feedback, relatively simple structures like rings or two spots are obtained. With feedback, more complicated polarization patterns result from spontaneous symmetry breaking. We emphasize the crucial role of the polarization ellipticity of the input beam. Changing this control parameter we observe a transition from positive to negative hexagons in one of the circular polarization components, while simultaneously the opposite transition occurs in the other one. The transition is mediated by square patterns occurring for linear input polarization. This metamorphosis is governed by symmetry principles and therefore provides an example of the ease of influencing the symmetry properties of systems consisting of multilevel atoms interacting with a vector light field.
An elliptically polarized laser beam tuned to the D1 transition of atomic sodium is sent through a vapor of sodium atoms that has been collisionally broadened by a buffer gas. The beam, initially Gaussian, is observed to break up into a narrower, pure circularly polarized spot surrounding an annulus of the opposite circular polarization. The sense of circular polarization of the central output spot corresponds to the weaker polarization component of the input field, and the polarization of the annular output beam corresponds to the stronger polarization component of the input field. A detailed comparison is made between the behavior observed and a recently developed theory of self-induced spatial separation of circularly polarized components.
Polarization rotation effects in atomic sodium vapor are studied both experimentally and theoretically. Two pulsed dye laser beams propagate collinearly through a cell containing sodium vapor. The direction of polarization of a linearly polarized dye laser beam of frequency ${\ensuremath{\omega}}_{1}$ is found to be rotated by a circularly polarized beam of frequency ${\ensuremath{\omega}}_{2}$. This rotation is due to the difference in refractive index at ${\ensuremath{\omega}}_{1}$ for right and left circularly polarized light which is produced by the circularly polarized beam. When ${\ensuremath{\omega}}_{1}+{\ensuremath{\omega}}_{2}$ is tuned near the $3S\ensuremath{-}5S$ two-photon transition of atomic sodium, this difference is primarily due to the dispersion associated with the two-photon transition. Other sources of polarization rotation which were identified include (i) real transfers of population to excited atomic levels due to collisionally induced transitions and (ii) optically induced atomic energy-level shifts. The theory includes both the effects of two-photon dispersion and optically induced energy shifts. The application of the two-photon polarization rotation effect as a fast, optically controlled shutter or modulator and as a sensitive detection technique for two-photon spectroscopy are briefly discussed.