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