1. Department of Physics, University of Rome—Tor Vergata and Istituto Nazionale Scienze e Tecnologie dei Material (INSTM), Via della Ricerca Scientifica, 1-00133 Rome, Italy
2. Department of Energetics, University of Rome—la Sapienza, and CNISM, Via A. Scarpa 16, 00161 Rome, Italy
3. CNR-IFN, Institute for Photonics and Nanotechnologies, Via Cineto Romano, 00156 Rome, Italy