1. School of Physics and Astronomy and the Photon Science Institute, The University of Manchester, Manchester M13 9PL, United Kingdom
2. Laser Quantum GmbH, Max-Stromeyer-Str. 116, 78467 Konstanz, Germany
3. Department of Materials Science and Metallurgy, 27 Charles Babbage Road, University of Cambridge, Cambridge CB3 0FS, United Kingdom