1. Department of Electronic and Electrical Engineering, Trinity College, Dublin 2, Ireland
2. School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
3. Laboratoire de Mathématiques et Physique (LAMPS), Université de Perpignan Via Domitia, F-66860 Perpignan, France
4. Kotel'nikov Institute of Radio Engineering and Electronics of the Russian Academy of Sciences, Vvedenskii Square 1, Fryazino, Moscow Region 141120, Russia