1. Department of Electrical Engineering and Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, Arizona 85281, Instituto de Investigacións Tecnolóxicas, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain, and Fachbereich Physik, Universität Duisburg-Essen, D-47048 Duisburg, Germany