1. Institute for Energy Efficiency University of California Santa Barbara Santa Barbara CA 93106 USA
2. State Key Laboratory of Modern Optical Instrumentation College of Optical Science and Engineering Zhejiang University Hangzhou 310027 P. R. China
3. Materials Department University of California Santa Barbara Santa Barbara CA 93106 USA
4. Department of Electrical and Computer Engineering University of California Santa Barbara Santa Barbara CA 93106 USA
5. Department of Electronic Engineering The Chinese University of Hong Kong Shatin Hong Kong P. R. China