[1] Natl Sun Yat Sen Univ, Dept Phys, Kaohsiung 80424, Taiwan
[2] Natl Sun Yat Sen Univ, Dept Mat & Optoelect Sci, Kaohsiung 80424, Taiwan
[3] Atom Energy Council, Inst Nucl Energy Res, Taoyuan 32546, Taiwan
[4] Peking Univ, Sch Elect & Comp Engn, Shenzhen 518055, Peoples R China
[5] Hubei Univ, Fac Phys & Elect Sci, Wuhan 430062, Hubei, Peoples R China
[6] Stanford Univ, Dept Elect Engn, Stanford, CA 94305 USA