1. Department of Optoelectronic Engineering; Chongqing University; Chongqing 400044 China
2. Chongqing Institute of Green and Intelligent Technology; Chinese Academy of Sciences; Chongqing 400714 China
3. College of Physics and Electronic Engineering; Chongqing Normal University; Chongqing 401331 China
4. Department of Materials Science and Engineering; Stanford University; Stanford CA 94305 USA