1. Photonics Institute, TU Wien, Gußhausstraße 27-29, 1040 Vienna, Austria
2. Center for Micro- and Nanostructures, TU Wien, Floragasse 7, 1040 Vienna, Austria
3. Institute for Quantum Electronics, ETH Zürich, Auguste-Piccard-Hof 1, 8093 Zurich, Switzerland