1. Photonics
Laboratory, ETH Zürich, 8093 Zürich, Switzerland
2. Departamento
de Física, Universidade Federal de Minas Gerais, Belo Horizonte, MG 31270-901, Brazil
3. School
of Materials and National Graphene Institute, The University of Manchester, Manchester M13 9PL United Kingdom
4. Department
of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland