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Physics Bulletin

Repulsive light beams

G.H.C. New, W.T. Welford

Published May 1, 1977• 0 citations
Publisher

Abstract

Laser beams of opposite circular polarization which travel along adjacent (but not overlapping) paths in sodium vapour, appear to 'repel' each other when the frequency of the light is just above that of the D 1 transition. This effect, which has recently been demonstrated by A C Tam and W Happer (see Phys. Rev. Lett. 1977 38 278–82), can be understood in terms of the optical pumping of the sodium atoms by the respective radiation fields.

DOI
10.1088/0031-9112/28/5/022

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Laser beams of opposite circular polarization which travel along adjacent (but not overlapping) paths in sodium vapour, appear to 'repel' each other when the frequency of the light is just above that of the D 1 transition. This effect, which has recently been demonstrated by A C Tam and W Happer (see Phys. Rev. Lett. 1977 38 278–82), can be understood in terms of the optical pumping of the sodium atoms by the respective radiation fields.
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.
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 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.
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.
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.
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.