1. INSERM U451, Laboratoire d'Optique Appliquée, Ecole Polytechnique-ENSTA, 91761 Palaiseau Cedex, France, Department for Mathematics and Physics, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark, Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen, Denmark, and Department of Biochemistry, School of Medical Sciences, University of Bristol, University Walk, Bristol BS8 1TD, United Kingdom