1. Department of Physics and Astronomy; The University of Kansas; Lawrence KS 66045 USA
2. Key Laboratory of Luminescence and Optical Information; Ministry of Education; Institute of Optoelectronic Technology; Beijing Jiaotong University; Beijing 100044 China
3. Department of Electrical and Computer Engineering; University of Nebraska-Lincoln; Lincoln NE 68588 USA
4. Department of Physics; Chemistry and Biology (IFM); Linköping; SE-581 83 Linköping Sweden