1. Hunan Key Laboratory of Super Microstructure and Ultrafast ProcessSchool of Physics and ElectronicsCentral South University Changsha Hunan 410083 P. R. China
2. School of Physics and Information TechnologyShanxi Normal University Xi'an 710119 China
3. State Key Laboratory of Advanced Design and Manufacturing for Vehicle BodyCollege of Mechanical and Vehicle EngineeringHunan University Changsha 410082 P. R. China
4. Department of Physics and AstronomyUniversity of Rochester Rochester NY 14627 USA
5. School of Electronic Science & Engineering and Collaborative Innovation Centre of Advanced MicrostructuresNanjing University Nanjing 210093 China