1. Photonics Institute, TU Wien, Gusshausstrasse 27-387, A-1040 Vienna, Austria
2. Swiss Terahertz Research-Zurich, Techpark, 8005 Zurich, Switzerland and Park Innovaare, 5234 Villigen, Switzerland
3. Key Laboratory of Terahertz Optoelectronics, Beijing Advanced Innovation Center for Imaging Technology CNU, Beijing 100048, China
4. Institute of Control Systems (IRS), Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany
5. Institute of Solid State Physics, TU Wien, 1040 Vienna, Austria
6. Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania