1. Photonics Institute, TU Wien, 1040 Vienna, Austria
2. Center for Micro- and Nanostructures, TU Wien, 1040 Vienna, Austria
3. Institute of Solid State Electronics, TU Wien, 1040 Vienna, Austria
4. Central European Institute of Technology, Brno University of Technology, 61200 Brno, Czech Republic
5. Laboratoire de Physique de l'Ecole Normale Supérieure, PSL Research University, 75005 Paris, France