1. Photonics Laboratory, ETH Zürich, 8093 Zürich, Switzerland
2. Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
3. School of Materials Science and Engineering, State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, Sun Yat-sen University, Guangzhou 510275, Guangdong, P.R. China
4. Faculty of Physics, Universität Wien, 1090 Wien, Austria