1. MOE Key Laboratory of Materials Physics and Chemistry in Extraordinary ConditionsShaanxi Key Laboratory of Macromolecular Science and TechnologySchool of ScienceNorthwestern Polytechnical University Xi'an 710072 P. R. China
2. College of Physics and Optoelectronic EngineeringShenzhen University Shenzhen 518060 P. R. China
3. Department of Applied PhysicsThe Hong Kong Polytechnic University Hong Kong 999077 P. R. China