1. 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, Faculty of Electronic and Information Engineering Xi'an Jiaotong University Xi'an Shaanxi 710049 China
2. Department of Physics University of Arkansas Fayetteville Arkansas 72701 USA
3. National Laboratory of Solid State Microstructures and School of Physics Nanjing University Nanjing Jiangsu 210093 China