1. Institute of Applied Physics, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
2. Institute of Physics, Martin Luther University Halle-Wittenberg (MLU), 06120 Halle, Germany
3. NICE Solar Energy GmbH, 74523 Schwäbisch Hall, Germany
4. Light Technology Institute, KIT, 76131 Karlsruhe, Germany