1. Institute of Photonics and Nanotechnology - CNR Trento Division, Via alla Cascata 56/C, Povo 38123 (TN) Italy, Institut fuer Angewandte Photophysik, George-Bähr-Strasse 1, 01069 Dresden, Germany, LICRYL-INFM Laboratory-CEMIF.CAL, Department of Physics, University of Calabria, Via P. Bucci, Cubo 33B, 87036 Arcavacata di Rende (CS) Italy, and IMEM-CNR, Istituto Materiali per Elettronica e Magnetismo Parco Area delle Scienze, 37/a 43100 Parma, Italy