1. Optoelectronic Division, Engineering Department, University of Sannio, C.so Garibaldi 107, 82100 Benevento, Italy
2. Dipartimento Tecnologie Fisiche e Nuovi Materiali, Ente per le Nuove Tecnologie, l'Energia e l'Ambiente (ENEA), SS.7, Appia, km 714, 72100 Brindisi, Italy
3. Institute for Composite and Biomedical Materials, National Research Council, P.le Enrico Fermi 1, 80055 Portici, Italy