[1] Politecn Milan, Dept Phys, I-20133 Milan, Italy
[2] Fdn Res & Technol Hellas IESL FORTH, Inst Elect Struct & Laser, GR-71110 Iraklion, Crete, Greece
[3] Politecn Milan, Dept Phys, Consiglio Nazl Ric, Inst Photon & Nanotechnol, I-20133 Milan, Italy
[4] Univ Crete, Dept Chem, GR-71003 Iraklion, Crete, Greece