1. Department of Materials Science and Engineering University of California Los Angeles CA 90095 USA
2. California NanoSystems Institute University of California Los Angeles CA 90095 USA
3. State Key Laboratory of Modern Optical Instrumentation College of Optical Science and Engineering Zhejiang University Hangzhou 310027 China
4. Department of Electronic and Information Engineering The Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong