1. Department of Physics, Chemistry and Biology, Linköping University, 581 83 Linköping, Sweden
2. Center for III-Nitride Technology, C3NiT-Janzén, Linköping University, 581 83 Linköping, Sweden
3. State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, Peking University, Beijing, China
4. Center for SMART Green Innovation Research, Chiba University, Chiba 263-8522, Japan
5. Terahertz Materials Analysis Center, THeMAC, Linköping University, 581 83 Linköping, Sweden