1. Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States
2. Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States
3. Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan
4. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
5. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States
6. Department of Chemistry, Rice University, Houston, Texas 77005, United States