1. Department of Applied Physics, KTH Royal Institute of Technology, SE-10691 Stockholm, Sweden
2. Department of Chemistry, University of Turku, FI-20014 Turku, Finland
3. Department of Life Technologies/Biotechnology, University of Turku, FI-20520 Turku, Finland
4. Division of Biophotonics, Federal Institute for Materials Research and Testing (BAM), Richard-Willstätter-Str. 11, 12489 Berlin, Germany