1. Key Laboratory of Artificial Micro- and; Nano-structures of Ministry of Education of China; School of Physics and Technology; Wuhan University; Wuhan 430072 People's Republic of China
2. Key Laboratory of Advanced Transducers and Intelligent Control System; Ministry of Education and Shanxi Province; College of Physics and Optoelectronics; Taiyuan University of Technology; Taiyuan 030024 People's Republic of China