1. Institute of Electronics, Computer and Telecommunication Engineering, National Research Council, IEIIT-CNR, Via De Marini 6, 16149 Genova, Italy
2. Italian Institute of Technology, Via Morego 30, 16163 Genova, Italy
3. Nanomed Labs, Department of Physics, University of Genova, via Dodecaneso 33, 16146 Genova, Italy
4. Department of Applied Physics and COMP Center of Excellence, Aalto University School of Science, P.O. Box 11100, FIN-00076 Aalto, Espoo, Finland and Department of Physics, Brown University, Providence, Rhode Island 02912-1843, USA