1. Department of Electronic Engineering, Xiamen University, Xiamen 361005, P. R. China
2. Department of Applied Mathematics, Computer Science, University of California, San Diego, USA
3. Department of Physics, NCHU University, Taichung City 402, Taiwan, ROC
4. Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen 361005, P. R. China
5. Department of Physics, Xiamen University, Xiamen 361005, P. R. China