1. Laboratory of Photonics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland
2. Institute of Physics, Faculty of Physics, Astronomy, and Informatics, Nicolaus Copernicus University, Grudziadzka 5/7, 87-100 Torun, Poland
3. Institute of Photonics, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland
4. Nanophotonics and Metrology Laboratory (NAM), Swiss Federal Institute of Technology, Lausanne (EPFL), 1015 Lausanne, Switzerland