1. The Institute of Photonic Sciences (ICFO) and ICREA, Mediterranean Technology Park, Avenue del Canal Olímpic s/n, 08860 Castelldefels (Barcelona), Spain
2. Photonics for Nanoapplications Laboratory, Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation (NHRF), 48 Vassileos Constantinou Avenue, 11635 Athens, Greece
3. Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK
4. Department of Electrical and Computer Engineering, National Technical University of Athens, 15780 Athens, Greece