1. Institute of Physics, University of Oldenburg, 26129 Oldenburg, Germany
2. Department of Physics, University of Regensburg, 93053 Regensburg, Germany
3. Institute of Physics, University of Greifswald, 17489 Greifswald, Germany
4. Regensburg Center for Ultrafast Nanoscopy, University of Regensburg, 93053 Regensburg, Germany