1. Department of Physics, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan
2. Department of Physics, National Sun Yat Sen University, No. 70, Lienhai Road, Kaohsiung 80424, Taiwan
3. Research Center for Applied Sciences, Academia Sinica, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan