1. Faculté des Sciences, Optique Nonlinéaire Thérorique,
Université libre de B ruxelles (U.L.B.), C.P. 231, Campus
Plaine, B-1050 Bruxelles, Belgium
2. Department of Applied Physics and Photonics (IR-TONA), Vrije Universiteit Brussels, Pleinlaan 2, B-1050 Brussels,
Belgium
3. Institute of Solid State Physics, 72
Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria