1. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA.
2. IBM Thomas J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA.
3. Laboratoire de Nanotechnologie et d’Instrumentation Optique, Institut Charles Delaunay CNRS-UMR 6281, Université de Technologie de Troyes, Troyes 10004, France.
4. Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USA.