1. Optical
Sciences Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, United States
2. ICFO—The Institute of Photonic Sciences, Avenida Carl Friedrich Gauss, 3, 08860 Castelldefels (Barcelona), Spain
3. Department
of Biomedical Engineering, University of North Carolina and North Carolina State University, EB3, Mail Stop 7115, Raleigh, North Carolina 27695-7115, United States
4. Department
of Chemistry, Howard University, 525 College Street NW, Washington, DC 20059, United States