1. Department of Electrical and Computer Engineering, University of Nebraska—Lincoln, Lincoln, Nebraska 68588-0511, United States
2. Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
3. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
4. Institut de Chimie
de la Matière Condensée de Bordeaux, Avenue du Docteur Albert Schweitzer, Cedex F-33608 Pessac, France