1. Univ Rennes, ENSCR, CNRS, ISCR − UMR 6226, F-35000 Rennes, France
2. Univ Rennes, INSA Rennes, CNRS, Institut FOTON - UMR 6082, F-35000 Rennes, France
3. Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom