1. IMEC, Kapeldreef 75, 3001 Leuven, Belgium, 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, Experimental Solid State Group, Physics Department, Imperial College, London SW7 2AZ, U.K., and Laboratory for Nanophotonics, Department of Physics and Astronomy, M.S. 61, Rice University, Houston, Texas 77005-1892