1. Department of Biotechnology and Biophysics, Julius-Maximilians-University Würzburg, Am Hubland, 97074 Würzburg, Germany
2. National Physics Laboratory, Hampton Road, Teddington TW11 0LW, U.K.
3. Laboratory of Molecular Biophysics, Institute for Research in Biomedicine, Baldiri Reixac 10-12, Barcelona, Spain
4. School of Advanced Optical Technologies, Friedrich Alexander University, D-91058 Erlangen, Germany