1. Terahertz Center University of Regensburg 93040 Regensburg Germany
2. CENTERA Laboratories Institute of High Pressure Physics Polish Academy of Sciences PL-01142 Warsaw Poland
3. Ioffe Institute 194021 St. Petersburg Russia
4. Rzhanov Institute of Semiconductor Physics 630090 Novosibirsk Russia
5. Laboratoire Charles Coulomb University of Montpellier and CNRS F‐34095 Montpellier France