1. Department of Physics, University of Rome Tor Vergata and INFN, Via della Ricerca Scientifica 1, 00133 Rome, Italy
2. Energy Technologies and Renewable Sources (TERIN) Department, Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Casaccia Research Centre, 00123 Rome, Italy
3. Institut für Festkörpertheorie und-Optik, Friedrich Schiller Universität, Max Wien Platz 1, 07743 Jena, Germany