1. Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria, Bulgaria
2. Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Sofia, Bulgaria
3. Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
4. 1 Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Sofia, Bulgaria
2 Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
5. 1 Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, Sofia, Bulgaria
2 South-West Univerity “Neofit Rilski” Blagoevgrad, Bulgaria
6. Institute of Solid State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
7. Department of Photonics, National Chiao Tung University, Hsinchu 30010, Taiwan