1. Experimental Solid State Group, Physics Department, Imperial College London, London SW7 2AZ, U.K.
2. IMEC, Kapeldreef 75, 3001 Leuven, Belgium
3. INPAC-Institute for Nanoscale Physics and Chemistry, Nanoscale Superconductivity and Magnetism and Pulsed Fields Group, K. U. Leuven Celestijnenlaan 200 D, B-3001 Leuven, Belgium
4. ESAT-TELEMIC, K. U. Leuven, B-3001 Leuven, Belgium
5. Laboratory for Nanophotonics, Department of Physics and Astronomy, M.S. 61, Rice University, Houston, Texas 77005-1892