1. School of Optics and Photonics; Beijing Institute of Technology; Beijing 100081 China
2. Department of Physics; University of Paderborn; Warburger Straße 100 D-33098 Paderborn Germany
3. Institute of Physics; The Chinese Academy of Sciences; Beijing 100191 China
4. Beijing National Laboratory for Condensed Matter Physics; Beijing 100191 China
5. Laser Micro/Nano-Fabrication Laboratory; School of Mechanical Engineering; Beijing Institute of Technology; Beijing 100081 China