1. Department of Applied Physics, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland
2. Department of Mechanics and Materials Engineering, Vilnius Gediminas Technical University, J. Basanavicius Str. 28, 03224 Vilnius, Lithuania
3. Oxford Instruments Technologies Oy, Tekniikantie 12, 02650 Espoo, Finland
4. Institute of Photonics, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland