1. Laboratory of Condensed Matter Spectroscopy and Opto-Electronic Physics, and Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Department of Physics, and Institute of Solar Energy; Shanghai Jiao Tong University; Shanghai 200240 China
2. School of Science; Huaihai Institute of Technology; Lianyungang 222005 Jiangsu Province China
3. School of Materials Science and Engineering; Shanghai Jiao Tong University; Shanghai 200240 China
4. National Laboratory for Infrared Physics; Shanghai Institute of Technical Physics, Chinese Academy of Sciences; Shanghai 200083 China