1. Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
2. Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, United States
3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Center for Nanophotonics, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands