1. Faculty of Physics and Material Sciences Center; Philipps-Universität Marburg; Marburg D-35032 Germany
2. Peter Grünberg Institut (PGI-9), Forschungszentrum Jülich; Jülich D-52425 Germany
3. Terahertz Systems Technology, Department of Electrical Engineering and Information Technology; Technical University Darmstadt; Rundeturmstrasse. 12 Darmstadt D-64283 Germany