1. Dublin City University, School of Physical Sciences and NCPST, Dublin 9, Ireland
2. Università degli Studi di Padova, LUXOR CNR IFN, Italy
3. School of Physics, University College Dublin, Dublin, Ireland
4. National Research Council-Institute for Photonics and Nanotechnologies CNR IFN, Via Trasea 7, 35131 Padova, Italy