1. Department of Chemistry, University of Rochester, Rochester, New York 14627-0216, United States
2. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States
3. Department of Physics, Cornell University, Ithaca, New York 14853, United States
4. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States
5. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, United States
6. The Institute of Optics, University of Rochester, Rochester, New York 14627-0216, United States