1. CNR‐SPIN Salerno via Giovanni Paolo II n. 132 Fisciano 84084 Italy
2. Department of Physical and Chemical Science University of L'Aquila, and CNR‐SPIN L'Aquila via Vetoio, Coppito L'Aquila 67100 Italy
3. Physics Department “E. R. Caianiello,” University of Salerno and CNR‐SPIN Salerno via Giovanni Paolo II n. 132 Fisciano 84084 Italy
4. Department of Engineering Physics McMaster University 1280 Main Street West Hamilton Ontario L8S 4L7 Canada