1. School of Physics, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116024, P. R. China
2. School of Microelectronics, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116024, P. R. China
3. School of Optoelectronic Engineering and Instrumentation Science, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116024, P. R. China