1 Department of Information Engineering, Electronics, and Telecommunications, Sapienza University of Rome, Via Eudossiana 18, 00184 Rome, Italy
2 Department of Information Engineering, University of Brescia, Via Branze 38, 25123 Brescia, Italy
3 Université de Limoges, XLIM, UMR CNRS 7252, 123 Avenue A. Thomas, 87060 Limoges, France
4 Physics Department, University of Calabria, I-87036 Arcavacata di Rende, CS, Italy
5 CNR Nanotec-Institute of Nanotechnology, S.S. Cosenza, I-87036 Rende, CS, Italy
6 CNR-INO, Istituto Nazionale di Ottica, Via Campi Flegrei 34, I-80078 Pozzuoli, NA, Italy