Generation of Femtosecond Vortex Beams, Self-channeling, and Filamentation in Gaseous Media

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Filamentation, i.e. self-trapping of powerful femtosecond laser beams has seen a very rapid development in the past five years to the point that several applications like LIDAR and pulse compression have emerged. The seemingly simple phenomenon involves complex spatial and temporal reshaping of the femtosecond laser pulses. The background beam surrounding the filament and containing the major part of the optical energy is known to play a pivotal role in the dissipative effects shaping the beam in space and time, however, the exact mechanisms are not known, other aspects like self-stabilization of the filament intensity (clamping) are much easier to comprehend. At laser powers exceeding the critical power for filamentation by two or more orders of magnitude, stochastic multiple filamentation and optical turbulence is observed. Little is known about the mutual interaction of the filaments and their interaction with the background beam.We propose to investigate filamentation in the presence of phase dislocations. One of the key goals is to generate ordered structures of filaments by using optical vortices in different configurations, e.g., in lattice-like structures. This will make multiple filamentation repeatable and thus more easily accessible to systematic experimental study. A question to address in this regard is whether ordered patterns, and if which, support filament formation and range. In addition, filamentation in conjunction with regular patterns of phase dislocations appears to be an interesting problem by itself. This can be inferred merely from the fact that vortices influence the background beam thus providing control parameters for experimentation and a test bed for theoretical modeling. In fact, as multiple filamentation is inevitably linked to symmetry breaking, it seems to be likely that phase dislocations play a role in multiple filamentation anyway and thus are worth being investigated.We also propose to probe the dynamics of filamentation by refracting a femtosecond probe beam off the filament. Comparing experimental results for the case of isolated and ordered multiple filaments will provide illuminating information not only on the dynamics of filamentation itself, but also on the role of the interaction of filaments.While most of the experiments will use a standard 10-mJ Ti:Sapphire femtosecond laser system, the availability of femtosecond laser sources in the mid-infrared provides additional opportunities for systematic investigations. Moreover, the Jena TW laser system can be used to demonstrate novel effects at high powers.The project takes advantage of the expertise and research infrastructure rounded up in the DFG Research Unit 532. Particularly important are the microstructuring capabilities at the Institute of Applied Physics (Prof. Tünnermann) providing unique opportunities for realizing customized optics for femtosecond vortex generation. Equally important will be the theoretical support through members of the Institute of Solid-State Theory and Optics (Prof. Lederer). In addition, we hope that cross links of ordered filament structures to regular wave guide structures (Profs. Nolte und Pertsch) will be identified.