1. Department of Physical Chemistry University of Geneva Geneva, 30 Quai Ernest‐Ansermet Geneva 4 CH‐1211 Switzerland
2. Laboratoire Léon Brillouin UMR12 CEA‐CNRS Université Paris‐Saclay CEA‐Saclay Gif‐sur‐Yvette Cedex 91191 France
3. CSEM Centre Suisse d'Electronique et de Microtechnique SA Neuchâtel 2002 Switzerland