1. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB 1 , Bellaterra 08193, Barcelona, Spain
2. Department of Electrical and Photonics Engineering, DTU Electro, Technical University of Denmark 2 , Building 343, DK-2800 Kgs. Lyngby, Denmark
3. NanoPhoton - Center for Nanophotonics, Technical University of Denmark 3 , Ørsteds Plads 345A, DK-2800 Kgs. Lyngby, Denmark
4. ICREA - Institució Catalana de Recerca i Estudis Avançats 4 , 08010 Barcelona, Spain