1. Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom
2. Physics Department, Lancaster University, Lancaster LA1 4YB, United Kingdom
3. Experimentelle Physik VI, TU Dortmund, Otto-Hahn-Strasse 4, D-44227 Dortmund, Germany
4. Department of Physics and Center for Applied Photonics, University of Konstanz, Universitätstraße 10, 78457 Konstanz, Germany
5. Department of Materials Science and Metallurgy, 27 Charles Babbage Rd., Cambridge CB3 0FS, United Kingdom
6. School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom