1. Department of Electrical Engineering, National Central University, Taoyuan City 32054, Taiwan
2. Department of Photonics and Institute of Electro-Optical Engineering, National Yang Ming Chiao Tung University, Hsinchu 30050, Taiwan
3. Photonics Integration System Research Center, Tokyo Institute of Technology, Tokyo 152-8550, Japan; koyama@pi.titech.ac.jp