1. Center for Nanophotonics, AMOLF, Science Park 104, Amsterdam 1098 XG, The Netherlands
2. Institute of Photoelectronic Thin Film Devices and Technology, College of Electronic Information and Optical Engineering, Nankai University, Tianjin, 300071 China
3. Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, CH-1015 Switzerland