1. École Polytechnique Fédérale de Lausanne (EPFL), School of Engineering, Optics Laboratory, 1015 Lausanne, Switzerland
2. Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125, USA
3. École Polytechnique Fédérale de Lausanne (EPFL), Institute of Physics of Condensed Matter (IPMC), Laboratoire des Nanostructures et Nouveaux Matériaux Electroniques (LNNME), 1015 Lausanne, Switzerland