1. Microelectronics Research Group (MRG), Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology-Hellas (FORTH), P.O. Box 1385, GR-70013 Heraklion, Crete, Greece
2. Department of Physics, University of Crete, P.O. Box 2208, GR-71003 Heraklion, Crete, Greece