1. Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215, USA.
2. Photonics Center, Boston University, Boston, MA 02215, USA.
3. Department of Chemistry, University of Massachusetts, Lowell, MA 01854, USA.
4. Department of Chemistry, Boston University, Boston, MA 02215, USA.
5. Department of Biomedical Engineering, Boston University, Boston, MA 02215, USA.
6. Department of Chemistry and Biomedical Sciences, Linnaeus University, SE-39182 Kalmar, Sweden.