1. School of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China
2. School of Physics, Beijing Institute of Technology, Beijing 100081, China
3. Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
4. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
5. Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China
6. Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0511, USA