1. Laboratory for Optics and Biosciences, INSERM U451-CNRS UMR 7645, Ecole Polytechnique-ENSTA, 91128 Palaiseau Cedex, France, and Solid State Chemistry Group, Condensed Matter Physics Laboratory, CNRS UMR 7643, Ecole Polytechnique, 91128 Palaiseau Cedex, France