Nonlinear beam propagation
Introduction
It is well known that intense laser beams propagating in nonlinear media
can self-focus. This phenomenon occurs in media in which
the refractive index increases with increasing intensity. Therefore, the center part
of a beam experiences a phase retardation with respect to the wings which causes the
beam to focus. This effect can even result in the
destruction of the material. We performed detailed experimental and theoretical
investigations on the mechanism and strength of the lensing effect in sodium
vapor with circular input polarization (quasi-scalar situation). For high
particle densities 'phase singularities' can be spontaneously generated
due to astigmatic lensing. These so called 'optical vortices' have a phase
structure similar to hydrodynamical vortices and received a lot of attention
during the last years due to their possible role in decorrelating optical
patterns. If the input beam is linearly polarized, Tam
and Happer observed in 1977 that it can split into two daughter beam
of opposite circular polarization. Since then, much effort was put into
an understanding of the instability, one even involving the hypothesis
of a 'fifth force' (see Naik and Pradhan
1981 for details). In a series of careful experiments and subsequent
theoretical analysis it was shown that the symmetry breaking is due to
small magnetic moments which induce a lateral shift of the two polarization
components in opposite directions. This shift is mediated by a small longitudinal
component of the dielectric polarization of the medium. Recently a general
treatment of the instability in terms of a coupled nonlinear wave equation
was given.
Recent activity concerns nonlinear beam shaping effects in ensembles of cold, laser-cooled atoms. The experiments are performed in a magneto-optical trap with rubidium atoms in collaboration with R. Kaiser, G. Labeyrie, and G.L. Lippi (Nice, France).
The team
-
Thorsten Ackemann
-
Matthias Pesch
-
Collaboration with: Prof. Dr. G. L.
Lippi (lippi@inln.cnrs.fr), Dr. R. Kaiser, Dr. G. Labeyrie, Institut Non Linéaire de
Nice, France
- Former group members: Dipl.-Phys. Christian Arntzen, Dr. Andreas Aumann, Dr. Andreas Gahl, Dipl.-Phys. Christian Grebner,
Dr. L.M. Hoffer, Dipl.-Phys. Eberhard Kriege, Dr. Yu.A. Logvin, Dr. Michael Möller, Dipl.-Phys. Jens Seipenbusch, Dipl.-Phys. Christoph Vorgerd
 List of publications
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