1. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA
2. Leibniz Institute for Polymer Research, Dresden 01069, Germany
3. Terahertz Materials Analysis Center and Center for III-N technology, C3Nit - Janzén, Department of Physics, Chemistry and Biology, Linköping University, SE 58183 Linköping, Sweden
4. Leibniz-Institut für Kristallzüchtung, Berlin 12489, Germany
5. United States Naval Research Laboratory, Washington, DC 20375, USA
6. Novel Crystal Technology, Inc., Sayama, Saitama-ken 350-1328, Japan