1. Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, USA
2. Institute of Optoelectronic Material and Technology, South China Normal University, Guangzhou 510631, China
3. Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
4. Graduate School and Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka 819-0395, Japan
5. School of Science, Northwestern Polytechnical University, Xi'an 710072, China
6. Nissan Chemical Industries, Ltd., 488-6, Suzumi-cho, Funabashi 274-0052, Japan