1. Department of Applied Physics and COMP Center of Excellence, Aalto University School of Science, P.O. Box 11000, FI-00076 Aalto, Espoo, Finland
2. VTT Technical Research Centre of Finland Ltd., P.O. Box 1000, FI-02044 VTT, Finland
3. Department of Physics, Brown University, P.O. Box 1843, Providence, Rhode Island 02912-1843, USA