1. Photonics Theory Group, Tyndall National Institute, Dyke Parade, Cork, Ireland
2. Department of Physics, University College Cork, Cork, Ireland
3. Department of Electrical Engineering and Automation, Aalto University, Espoo 02150, Finland
4. COMP Centre of Excellence in Computational Nanoscience, Aalto University, Espoo 02150, Finland
5. Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom
6. School of Physics and Astronomy, Photon Science Institute, University of Manchester, Manchester M13 9PL, United Kingdom