1. Department
of Electronics and Nanoengineering, Aalto University, Tietotie
3, FI-02150 Espoo, Finland
2. Department
of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
3. Institute
of Photonics, University of Eastern Finland, Yliopistokatu 7, FI-80101 Joensuu, Finland
4. College
of Optical Sciences, University of Arizona, 1630 East University Boulevard, Tucson, Arizona 85721, United States