1. School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China
2. Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique, School of Electronic Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China
3. Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, School of Electronic Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China
4. Department of Physics & Materials Science, City University of Hong Kong, Kowloon, Hong Kong