1 University of Palermo, Department of Engineering, Palermo, Italy
2 National Institute of Optics-National Research Council, Brescia, Italy
3 University of Brescia, Department of Information Engineering, Brescia, Italy
4 ITMO University, School of Physics and Engineering, Saint-Petersburg, Russia
5 ETH Zurich, Institute for Quantum Electronics, Department of Physics, Optical Nanomaterial Group, Zurich, Switzerland
6 Technical University of Denmark, Department of Electrical and Photonics Engineering, Kongens Lyngby, Denmark