1. Center for Fluorescence Spectroscopy, Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, 725 West Lombard Street, Baltimore, Maryland 21201, United States
2. Institute of Photonics, Department of Optics and Optical Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China
3. Department of Applied Science and Technology, Polytechnic University of Turin, Corso Duca degli Abruzzi 24, Turin 10129, Italy
4. Department of Electronic Systems, Norwegian University of Science and Technology, O. S. Bragstads Plass 2b, Trondheim 7034, Norway