1. Institute of Chemical Sciences of Rennes, CNRS-University of Rennes 1, UMR 6226, Rennes F-35042, France
2. Institute of Physics of Rennes, CNRS-University of Rennes 1, UMR 6251, Rennes F-35042, France
3. Laboratoire Léon Brillouin (CEA-CNRS), CEA-Saclay, Université Paris-Saclay, Gif-sur-Yvette F-91191, France