1. SUPA and Department of Physics, University of Strathclyde, G4 0NG Glasgow, United Kingdom
2. Kelvin Nanotechnology, University of Glasgow, G12 8LS Glasgow, United Kingdom
3. Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
4. National Institute of Standards and Technology, Boulder, Colorado 80305, USA
5. FEMTO-ST, CNRS, 26 Chemin de l'Epitaphe, 25030 Besançon, France