1. Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Department of Electrical and Computer Engineering, University of Washington, Seattle, Washington 98195, United States
3. Department of Physics, University of Washington, Seattle, Washington 98195, United States
4. Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan