[1] Natl Tsing Hua Univ, Dept Power Mech Engn, Hsinchu 30013, Taiwan
[2] Natl Taiwan Normal Univ, Dept Phys, Taipei 116, Taiwan
[3] Natl Appl Res Labs, Instrument Technol Res Ctr, Hsinchu 300, Taiwan
[4] Natl Tsing Hua Univ, Inst Nanoengn & Microsyst, Hsinchu 300, Taiwan