1. School of Optoelectronic Engineering and Grüenberg Research Centre, Nanjing University of Posts and Telecommunications, Nanjing 210023, People's Republic of China
2. College of Electronic Engineering, Guangxi Normal University, Guilin 541004, People's Republic of China
3. Collaborative Innovation Center of Advanced Microstructures and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People's Republic of China