[1] Natl Taiwan Univ, Dept Elect Engn, Taipei 10617, Taiwan
[2] Natl Taiwan Univ, Inst Electroopt Engn, Taipei 10617, Taiwan
[3] Univ Florida, Dept Phys, Gainesville, FL 32611 USA
[4] Univ Calif Santa Barbara, Dept Elect & Comp Engn, Santa Barbara, CA 93106 USA