[1] Natl Taiwan Univ, Dept Phys, Taipei 106, Taiwan
[2] Acad Sinica, Res Ctr Appl Sci, Taipei 11529, Taiwan
[3] Acad Sinica, Taiwan Int Grad Program, Nanosci & Technol Program, Taipei 115, Taiwan
[4] Natl Chiao Tung Univ, Dept Photon, Hsinchu 30010, Taiwan