1. School of Physics, Huazhong University of Science and Technology, 1037 Luoyu Rd., Wuhan, Hubei 430074, China
2. Department of Electrical and Computer Engineering, George Mason University, 4400 University Dr., Fairfax, Virginia 22030, USA
3. Engineering Physics Division, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, Maryland 20899, USA