1. Institute of Applied Physics, Technische Universität Braunschweig, Mendelssohnstraße 2, 38106 Braunschweig, Germany
2. Laboratory for Emerging Nanometrology, Braunschweig, Germany
3. Institute of Optoelectronics, University of Ulm, Albert-Einstein-Allee 45, 89081 Ulm, Germany