1. Center for Optics Research and Engineering, Shandong University, Qingdao 266237, P. R. China
2. Key Laboratory of Laser & Infrared System (Shandong University), Ministry of Education, Shandong University, Qingdao 266237, P. R. China
3. Institute of Novel Semiconductors, Shandong University, Jinan 250100, P. R. China
4. School of Information Science and Engineering, Shandong University, Qingdao 266237, P. R. China