1. COBRA Research Institute, Eindhoven University of Technology (TU/e), PO Box 513, 5600 MB Eindhoven, The Netherlands
2. Institute of Quantum Electronics and Photonics (IPEQ), Ecole Polytechnique Fédérale de Lausanne, Station 3, CH-1015 Lausanne, Switzerland
3. Istituto di Fotonica e Nanotecnologie (IFN), CNR, via Cineto Romano 42, 00156 Roma, Italy
4. Institute of Condensed Matter Physics (IPMC), Ecole Polytechnique Fédérale de Lausanne, Station 3, CH-1015 Lausanne, Switzerland