1. Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA
2. Department of Optical Science and Engineering, Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Fudan University, Shanghai 200433, China
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
4. Inston, Inc., Los Angeles, California 90095, USA