1. University of Catania,D. I. E. E. I., Dipartimento di Ingegneria Elettrica Elettronica e Informatica,Catania,Italy,95125
2. CNR-IMM,Catania,Italy,95123
3. Institut FEMTO-ST, Université Franche-Comté,Besançon,France,25030
4. Institut FEMTO-ST, ENSMM,Besançon,France,25030