1. College of Optoelectronic Engineering, Shenzhen University, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen 518060, P. R. China
2. School of Technical Physics, Xidian University, Xi'an 710071, P. R. China
3. Department of Bioengineering and COMSET, Clemson University, Clemson SC 29634, USA