1. Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, Center for Micro-Nanotechnology, School of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China
2. Key Lab of Advanced Optoelectronic Quantum Design and Measurement, Ministry of Education, Beijing Institute of Technology, Beijing 100081, P. R. China
3. School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China
4. Beijing Key Laboratory for Precision Optoelectronic Measurement Instrument and Technology, School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, P. R. China