1. CNR/FBK Trento Division, Institute of Photonics and Nanotechnology, Via alla Cascata 56/C, Povo 38100 (TN), Italy, Institut fuer Angewandte Photophysik, George-Bähr-Strasse 1, 01069 Dresden, Germany, and LICRYL-INFM Laboratory-CEMIF.CAL, Department of Physics, University of Calabria, Via P. Bucci, Cubo 33B, 87036 Arcavacata da Rende (CS), Italy