1. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20740, United States
2. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States
3. U.S. Naval Research Laboratory, Washington, D.C. 20375, United States
4. Helmholtz-Zentrum Dresden-Rossendorf, P.O. Box 510119, 01314 Dresden, Germany
5. Center for Nanophysics and Advanced Materials, University of Maryland, College Park, Maryland 20742, United States
6. Fakultät für Physik, Universität Duisburg-Essen, 47057 Duisburg, Germany