1. Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology-Hellas (FORTH), 71110 Heraklion, Crete, Greece, Physics Department, University of Patras, 22500 Rio, Patras, Greece, National Nanotechnology Laboratory (NNL) of CNR, Distretto Technologico ISUFI, Via Arnesano, I-73100 Lecce, Italy, and Physics Department, University of Crete, 71003 Heraklio, Crete, Greece