[1] Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 109, 1113 Sofia, Bulgaria
[2] Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Blvd, 1784 Sofia, Bulgaria
[3] Laser-Laboratorium, Hans-Adolf-Krebs-Weg 1, D-37077 Goetingen, Germany