1. Nanoscience Laboratory, Department of Physics, University of Trento, Via Sommarive 14, 38123 Trento, Italy
2. Centre for Sensors and Devices, Fondazione Bruno Kessler, Via Sommarive 18, 38123 Trento, Italy
3. Department of Photonics Engineering, DTU Fotonik, Technical University of Denmark, Roskilde 4000, Denmark