1. Department of Science and High Technology, University of Insubria, Via Valleggio
11, 22100 Como, Italy
2. Institute for Photonics and Nanotechnologies, C.N.R.,
Via Valleggio 11, 22100 Como, Italy
3. Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia, Via Campi 183, 41125 Modena, Italy