1. Institute for Solid State Physics, Friedrich Schiller University Jena 1 , Jena, Germany
2. Leibniz Institute of Photonic Technology (IPHT) 2 , Jena, Germany
3. Institute of Electronic Materials II, RWTH Aachen University 3 , Aachen, Germany
4. Peter Grünberg Institut (PGI-7), Forschungszentrum Juelich GmbH 4 , Juelich, Germany