1. Optics Laboratory, Tampere University of Technology, FI-33101 Tampere, Finland
2. NooEL - Nonlinear Optics and OptoElectronics Lab, University “Roma Tre,” IT-00146 Rome, Italy
3. Optoelectronics Research Centre, University of Southampton, SO17 1BJ Southampton, United Kingdom
4. Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079, USA
5. CNR Nanotec, 87036 Rende and Italian Institute of Technology, 16163 Genova, Italy
6. CNR-ISC, Institute for Complex Systems, IT-00185 Rome, Italy