1. Russian Research Centre “Kurchatov Institute”, Kurchatov sq. 1, 123182 Moscow, Russia
2. Laboratoire Leon Brillouin, CEA‐CNRS, 91191 Gif‐sur‐Yvette, France
3. Physics Department E21, Technical University of Munich, 85748 Garching, Germany
4. Swiss Light Source, 5232 Villigen PSI, Switzerland
5. DRFMC CEA/Grenoble, 38054 Grenoble Cedex 9, France