1. College of Physical Science and Technology & Microelectronics Industry Research Institute, Yangzhou University, Yangzhou 225002, P. R. China
2. Institute of Applied Physics and Materials Engineering, University of Macau, Taipa, Macao SAR 999078, P. R. China
3. Department of Physics, Umeå University, Umeå SE-90187, Sweden
4. College of Optoelectronic Engineering, Chongqing University of Post and Telecommunications, Chongqing 400065, People’s Republic of China