1. Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique, Xi'an Jiaotong University; Xi'an 710049 China
2. Department of Applied Physics; School of Science, Xi'an Jiaotong University; Xi'an 710049 China
3. Science Program, Texas A & M University at Qatar; P.O. Box 23874 Doha Qatar
4. Department of Physics; University of Arkansas; Fayetteville Arkansas 72701 USA
5. National Laboratory of Solid State Microstructures and School of Physics, Nanjing University; Nanjing 210093 China