1. School of Primary Education, Chongqing Normal University, Chongqing 400700, China
2. School of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, China
3. Department of Electrical and Computer Engineering, University of California, Riverside, CA 92521, USA
4. Chongqing Key Laboratory of Micro&Nano Structure Optoelectronics, School of Physical Science and Technology, Southwest University, Chongqing 400715, China