1. National Laboratory for Infrared Physics; Shanghai Institute of Technical Physics; Chinese Academy of Sciences; 500 Yu Tian Road Shanghai 200083 China
2. School of Materials Science and Engineering; Xiangtan University; Xiangtan Hunan 411105 China
3. Department of Physics and Key Laboratory of Artificial Micro- and Nano-Structures of Ministry of Education; Wuhan University; Wuhan 430072 China
4. Department of Microelectronics; Fudan University; 220 Handan Road Shanghai 200433 China