2 Laser Dynamics

Laser Dynamics 12 Laser Dynamics S. Dangel, A. Schneider, R. Holzner A main part of our research activities, besides the laser related projects described in the section on surface physics, is the investigation of nonlinear spatio-temporal e ects caused by the interaction of resonant laser beams with sodium atoms in the gas phase. A circularly polarized laser beam propagating through sodium vapor can optically pump sodium atoms into a nonabsorbing ground state (optical pumping). This strongly nonlinear mechanism which occurs even at very low light intensities causes an intensity dependent refractive index gradient along as well as transverse to the laser beam propagation direction which may give rise to a number of nonlinear spatio-temporal intensity and polarization pattern creating processes. Probably the rst observations of optical pumping of sodium atoms as beeing the cause for laser beam shape modi cations were self-focusing, self-defocusing and self-trapping of the beams [1]. Since then a large number of optical pumping related e ects have been observed by various research groups. A collection of e ects which we have observed in experiments and described by appropriate models [2, 3, 4, 5] are displayed in g.12.1. one beam