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