1. Institute of Applied Physics, Westfälische Wilhelms‐Universität Münster, 48149 Münster, Germany
2. Institute of Applied Physics, Darmstadt University of Technology, 64289 Darmstadt, Germany
3. Present address: Sophienstr. 1, 07743 Jena, Germany
4. Optique Nonlinéaire Théorique, Université Libre de Bruxelles, 1050 Bruxelles, Belgium
5. Permanent address: Institute of Physics, P.O. Box 57, 11001 Belgrade, Serbia