1. School of Physics and Key Laboratory of MEMS of the Ministry of Education Southeast University Nanjing 211189 P. R. China
2. Department of Optoelectronic Science Harbin Institute of Technology at Weihai Weihai 264209 P. R. China
3. Beijing Key Laboratory of Optoelectronic Functional Materials & MicroNano Devices Renmin University of China Beijing 100872 P. R. China
4. National Laboratory of Solid State Microstructures School of Electronic Science and Engineering and Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing 210093 P. R. China