1. Department of Physics, University of Calabria, via Ponte Bucci, Cubo 31C, 87036 Rende, CS, Italy
2. Consiglio Nazionale delle Ricerche - Istituto di Nanotecnologia (CNR-Nanotec), via Ponte Bucci, Cubo 33C, 87036 Rende CS, Italy
3. Physics Department, University of Florence, 50019 Sesto Fiorentino, FI, Italy
4. European Laboratory for Non Linear Spectroscopy, LENS, 50019 Sesto Fiorentino, FI, Italy