1. Department of Physics, Chemistry, Biology, Linköping University, 581 83 Linköping, Sweden
2. Center for III-Nitride Technology, C3NiT-Janzen, Linköping University, 581 83 Linköping, Sweden
3. Department of Electrical and Computer Engineering, NCSU, Raleigh, North Carolina 27695, USA
4. Hitachi ABB Power Grids, Power Grid Research, Forskargränd 7, 721 78 Västerås, Sweden
5. THz Materials Analysis Center (TheMAC), Linköping University, 581 83 Linköping, Sweden