1. Department of Electrical and Computer Engineering, Boston University, Boston, Massachusetts 02215, United States
2. Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States
3. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
4. Photonics Center, Boston University, Boston, Massachusetts 02215, United States
5. Department of Physics, Boston University, Boston, Massachusetts 02215, United States