1. Department of Physics and Applied Optics Beijing Area Major Laboratory; Beijing Normal University; Beijing 100875 P. R. China
2. Department of Physics; National Tsing-Hua University; Hsinchu 30013 Taiwan
3. Department of Photonics; National Chiao-Tung University; Hsinchu 30010 Taiwan
4. National Synchrotron Radiation Research Center; Hsinchu 30076 Taiwan