1. Institute of Physics, Laboratory for Ultrafast Microscopy and Electron Scattering (LUMES), École Polytechnique Fédérale de Lausanne, Station 6, CH-1015 Lausanne, Switzerland.
2. SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK.
3. ICFO–Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain.
4. ICREA–Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, 08010 Barcelona, Spain.