1. Department of Physics, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece
2. Department of Chemistry, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom
3. School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom
4. Institute of Experimental Physics I, Justus-Liebig-University Giessen, D-35392 Giessen, Germany