1. Department of Physics, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon 999077, Hong Kong
2. Research Center for Advanced Optics and Photoelectronics, Department of Physics, College of Science, Shantou University, Shantou, Guangdong 515063, People's Republic of China
3. Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, USA
4. Institute for Functional Materials and Devices, Lehigh University, Bethlehem, Pennsylvania 18015, USA