1. ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels, Barcelona, Spain
2. Department
of Optics, FEMTO-ST Institute, UMR CNRS 6174, University of Franche-Comté, 25000 Besançon, France
3. IBM Almaden Research Center, San Jose, California 95120, United States
4. Interface
Physics Group, Physics Institute, University of Münster, 48149 Münster, Germany
5. ICREA-Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain