1. Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410012, People's Republic of China
2. Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Erwin-Schrödinger-Straße 46, 67663 Kaiserslautern, Germany
3. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore
4. Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA