1. National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 200083 Shanghai, China
2. Department of Microtechnology and Nanoscience, Chalmers University of Technology, S-41296 Göteborg, Sweden
3. State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 200050 Shanghai, China
4. Physics Department, Shanghai University, 200444 Shanghai, China