1. School of Physical Science and Technology, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo, Zhejiang 315211, P. R. China
2. Department of Electrical and Electronic Engineering, Southern University of Science and Technology, No. 1088 Xueyuan Road, Nanshan District, Shenzhen, Guangdong 518055, P. R. China