1. Department of Electrical
and Computer Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0407, United States
2. Materials Science and Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0418, United States
3. Department
of Physics, National Tsing-Hua University, Hsinchu 30013, Taiwan
4. Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens
Lyngby, Denmark