1. Chongqing Key Laboratory of Photo‐Electric Functional Materials College of Physics and Electronic Engineering Chongqing Normal University Chongqing 401331 P. R. China
2. School of Microelectronics University of Science and Technology of China Hefei 230029 P. R. China
3. School of Automation Chongqing University of Posts and Telecommunications Chongqing 400065 P. R. China
4. College of Physics Chongqing University Chongqing 401331 P. R. China