1. Centre for Photonics and Photonic Materials and Centre for Nanoscience and Nanotechnology; Department of Physics; University of Bath; Claverton Down Bath BA2 7AY United Kingdom
2. Universita' di Roma “La Sapienza”; Dip. di Scienze di Base e Applicate per l'Ingegneria (SBAI); Via A. Scarpa 16 00161 Roma Italy