1. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, USA
2. Department of Electrical and Computer Engineering, University of Maryland, College Park, Maryland 20742, USA
3. Physics Department, University of Maryland, College Park, Maryland 20742, USA
4. Fakultät für Physik, Universität Duisburg-Essen, Lotharstr. 1, 47057 Duisburg, Germany