1. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
2. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
3. Department of Chemistry and Division of Materials Science and Engineering, Boston University, Boston, Massachusetts 02215, United States
4. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
5. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
6. Department of Physics, Tohoku University, Sendai 980-8578, Japan
7. The Photonics Center, Boston University, Boston, Massachusetts 02215, United States