1. Terahertz Center, University of Regensburg, 93040 Regensburg, Germany
2. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
3. Ioffe Institute, 194021 St. Petersburg, Russia
4. CENTERA, Institute of High Pressure Physics PAS, 01142 Warsaw, Poland
5. International Center for Materials Nanoarchitectonics, National Institute of Material Science, 1-1 Namiki, Tsukuba 305-0044, Japan
6. Research Center for Functional Materials, National Institute of Material Science, 1-1 Namiki, Tsukuba 305-0044, Japan