[1] CNR, Nello Carrara Inst Appl Phys, MDF Lab, I-50019 Sesto Fiorentino, FI, Italy
[2] Univ Trent, Dept Phys, CSMFO Grp, I-38050 Trento, Italy
[3] CNR, Inst Photon & Nanotechnol, Trento Sect, CSMFO Grp, I-38050 Trento, Italy
[4] Univ Bologna, Elect Comp Sci & Syst Dept, I-40100 Bologna, Italy