1. Department of Information Engineering, University of Brescia 1 , Via Branze 38, Brescia 25123, Italy
2. National Institute of Optics - National Research Council (INO-CNR) 2 , Via Branze 45, Brescia 25123, Italy
3. Department of Engineering, University of Palermo 3 , Viale delle Scienze ed. 9, Palermo 90128, Italy
4. ETH Zurich, Department of Physics, Institute for Quantum Electronics, Optical Nanomaterial Group 4 , Auguste-Piccard-Hof 1, 8093 Zurich, Switzerland
5. Department of Electrical and Photonics Engineering, Technical University of Denmark 5 , Ørsteds Plads, Building 345A, DK-2800 Kongens Lyngby, Denmark