1. Hasselt University, Martelarenlaan 42, 3500 Hasselt, Belgium
2. IMO-IMOMEC, Wetenschapspark 1, 3590 Diepenbeek, Belgium
3. Department of Physics, Heinrich-Heine Universität Düsseldorf, 40225 Düsseldorf, Germany
4. Center for Nano- and Bio-Photonics, Ghent University, 9052 Gent, Belgium
5. Physics and Chemistry of Nanostructures, Ghent University, 9000 Gent, Belgium
6. Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200 F, 3001 Leuven, Belgium