Affiliation: | (1) Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Sciences, P.O. Box 95, 1113 Sofia, Bulgaria;(2) Department of Physics, Institute of Foreign Students, 27 A. Veltchev Str., 1111 Sofia, Bulgaria;(3) Department of Optics and Spectroscopy, Sofia University, 5 A. Ivanov Blvd., 1126 Sofia, Bulgaria |