1. Faculty of Physics, Department of Atomic Physics, Sofia University "St. Kliment Ohridski", 5 James Boucher Blvd., 1164 Sofia, Bulgaria
2. Faculty of Physics, Department of Solid State Physics and Microelectronics, Sofia University "St. Kliment Ohridski", 5 James Boucher Blvd., 1164 Sofia, Bulgaria
3. Faculty of Physics, Department of Optics, Sofia University "St. Kliment Ohridski", 5 James Boucher Blvd. 1164 Sofia, Bulgaria