1. Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109, USA
2. Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA
3. Technische Physik, Physikalisches Institut and Würzburg-Dresden Cluster of Excellence ct.qmat, Universität Würzburg, 97074 Würzburg, Germany
4. Institute of Physics, University of Oldenburg, 26129 Oldenburg, Germany