1. a INFM Unità di Napoli and Dipartimento di Scienze Fisiche, Università di Napoli “Federico II” , Via Cinzia Monte S. Angelo, 80126 , Napoli , ITALY
2. b School of Physics, Department of Microelectronics and Nanoscience, Chalmers University of Technology , 412 96, Göteborg , SWEDEN
3. c IROE CNR , Via Panciatichi 64, 50127 , Firenze , ITALY
4. d Division of Laser Systems , National Defense Research Establishment , P.O. Box 1165, 581 11 , Linköping , SWEDEN
5. e Department of Physics , Measurement Technology, Linköping University , 581 83 , Linköping , SWEDEN