1. School of Optoelectronic Science and Engineering University of Electronic Science and Technology of China Chengdu 611731 P. R. China
2. School of Physics University of Electronic Science and Technology of China Chengdu 611731 P. R. China
3. Key Laboratory of Information Materials of Sichuan Province and School of Preparatory Education Southwest Minzu University Chengdu 610041 P. R. China