1. Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland
2. Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
3. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
4. Ginzton Laboratory, Stanford University, Stanford, California 94305, United States