1. State Key Laboratory on Integrated Optoelectronics; College of Electronic Science and Engineering; Jilin University; Changchun 130012 China
2. National Laboratory of Solid State Microstructures and Collaborative Innovation Center of Advanced Microstructures; School of Physics; Nanjing University; Nanjing 210093 China
3. Department of Chemistry and Physics; Louisiana State University; Shreveport LA 71115 USA