1. Department of Electronics and Nanoengineering, Aalto University, FI-00076 Aalto, Finland
2. Physics, Faculty of Science and Engineering, Åbo Akademi University, FI-20500 Turku, Finland
3. Institute of Photonics, University of Eastern Finland, FI-80101 Joensuu, Finland
4. Engineered Nanosystems Group, School of Science, Aalto University, FI-00076 Aalto, Finland