1. Laboratory for Optics and Biosciences, CNRS UMR 7645, Ecole Polytechnique, 91128 Palaiseau Cedex, France, INSERM Unité 696, 91128 Palaiseau Cedex, France, LENS, University of Florence, 50019 Sesto Fiorentino, Italy, and BioMoCeTi, CNRS UMR 7033, University Pierre & Marie Curie, Genopole Campus 1, 5 Rue Henri Desbrueres, 91030 Evry Cedex, France