1. Nanophotonic Systems Laboratory, Eidgenössische Technische Hochschule (ETH) Zürich, Zürich 8092, Switzerland
2. Department of Physics and Astronomy, University of Exeter, Exeter EX44QL, United Kingdom
3. ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona 08860, Spain
4. ICREA-Institució Catalana de Recerca i Estudis Avançats, Passeig Lluís Companys 23, Barcelona 08010, Spain