1. School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
2. CNRS UMI 2958, Georgia Institute of Technology, 2 Rue Marconi, 57070 Metz, France
3. CNRS, Laboratoire de Photonique et de Nanostructures, Route de Nozay, 91460 Marcoussis, France
4. Université de Lorraine, CentraleSupélec, LMOPS, EA4423, 57070 Metz, France
5. School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
6. CNRS, Institut Néel, BP166, 38042 Grenoble Cedex 9, France