1. Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Acad. G. Bontchev Str. 109, 1113 Sofia, Bulgaria
2. Institute of Solid-State Physics, Bulgarian Academy of Sciences, 72, Tzarigradsko Chaussee Blvd, 1784 Sofia, Bulgaria
3. Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research “Demokritos” 15341 Agia Paraskevi, Greece
4. Faculty of Chemistry and Pharmacy, Sofia University, 1 James Bourchier Blvd., 1164 Sofia, Bulgaria
5. Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria
6. Department of Electrophysics, National Yang Ming Chiao Tung University, 30010 Hsinchu, Taiwan