1. Department of Physics, Baylor University, Waco, Texas 76798, United States
2. Department of Physics and Astronomy, University of California, Irvine, California 92697, United States
3. Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
4. Institute of Nanoengineering and Microsystems, National Tsing-Hua University, Hsinchu 30013, Taiwan
5. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
6. Department of Physics, National Tsing-Hua University, Hsinchu 30013, Taiwan