1. State Key Laboratory of Modern Optical Instrumentation College of Optical Science and Engineering Zhejiang University Hangzhou 310027 P. R. China
2. Key Laboratory of Artificial Structures and Quantum Control Shanghai Center for Complex Physics School of Physics and Astronomy Shanghai Jiao Tong University Shanghai 200240 P. R. China
3. School of Physics Southeast University Nanjing 211189 P. R. China
4. Department of Physics Pennsylvania State University University Park PA 16802 USA