1. National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences & Jiangsu Key Laboratory of Artificial Functional Materials & Collaborative Innovation Center of Advanced Microstructures, Nanjing University 1 , Nanjing 210093, China
2. Functional Materials and Acousto-Optic Instruments Institute, School of Instrumentation Science and Engineering, Harbin Institute of Technology 2 , Harbin 150080, China