1. Key Laboratory
of Low-Dimensional Materials and Application Technology and Faculty
of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan 411105, China
2. Faculty
of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan 411105, China
3. National
Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China