1. School of Optoelectronics; Beijing Institute of Technology; Beijing 100081 China
2. Department of Physics; University of Paderborn; Warburger Straße 100 D-33098 Paderborn Germany
3. Laser Micro/Nano-Fabrication Laboratory; School of Mechanical Engineering; Beijing Institute of Technology; Beijing 100081 China
4. State Key Laboratory of Precision Measurement Technology and Instruments; Department of Precision Instrument; Tsinghua University; Beijing 100084 China