1. Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education; Wuhan University; Wuhan 430072 China
2. Department of Physics and Materials Science; City University of Hong Kong; Tat Chee Avenue Kowloon Hong Kong SAR China
3. National Laboratory for Infrared Physics; Shanghai Institute of Technical Physics; Chinese Academy of Sciences; Shanghai 200083 China
4. Institute of Optoelectronic Display; Fuzhou University; Fuzhou 350002 China