1. Institute for Experimental and
Applied Physics, University of Kiel, Leibnizstr.
19, D-24118 Kiel, Germany
2. Department of Physics and Research
Center OPTIMAS, University of Kaiserslautern, Erwin Schroedinger Str. 46, D-67663 Kaiserslautern, Germany
3. Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany