1. College of Physics and Optical Engineering, Shenzhen University, Shenzhen 518060, Guangdong, P. R. China
2. Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, Guangdong, P. R. China
3. Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
4. Department of Materials Science and NanoEngineering, Department of Chemistry, and the Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, United States