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