1. Applied Mineralogy Institute, Moskovska Str. 6, BG-1000 Sofia, Bulgaria (T.K.K.); Friedrich-Schiller University, Physics Department, Max-Wien Platz 1, DDR-6900 Jena, German Democratic Republic (V.P.P.); and Sofia University, Faculty of Physics, Department of Quantum Electronics, Anton Ivanov Blvd. 5, BG-1126 Sofia, Bulgaria (J.B.N.)