1. Department of Physics and Research Center OPTIMAS
2. University of Kaiserslautern
3. 67663 Kaiserslautern
4. Germany
5. Instituto de Micro y Nanotecnología IMN-CNM
6. CSIC
7. CEI UAM+CSIC
8. Madrid
9. Spain
10. Institute of Physics, Martin-Luther University Halle-Wittenberg