1. Department of Physics and Astronomy, Uppsala University 1 , Lägerhyddsvägen 1, S-75237 Uppsala, Sweden
2. Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University 2 , Canberra, ACT 0200, Australia
3. KTH—Royal Institute of Technology, School of Information and Communication Technology 3 , PO Box 229, SE-164 40 Kista, Sweden
4. CNR-IMM MATIS and Department of Physics and Astronomy, University of Catania 4 , Via S. Sofia 64, I-95123 Catania, Italy