1. Photonics Research Group, INTEC Department, Ghent University-imec, Technologiepark-Zwijnaarde 15, B-9052 Zwijnaarde, Belgium
2. Center for Nano- and Biophotonics (NB-Photonics), Ghent University, Technologiepark-Zwijnaarde 15, B-9052 Zwijnaarde, Belgium
3. Institut für Physik, Universität Augsburg, Universitätsstraße 1, D-86135 Augsburg, Germany