1. Department of Electrical and Electronic Engineering The University of Hong Kong Pok Fu Lam Road Hong Kong SAR P. R. China
2. Key Laboratory of Luminescence and Optical Information Ministry of Education School of Science Beijing Jiaotong University Beijing 100044 P. R. China
3. Department of Materials Science and Engineering University of Washington Seattle WA 98195 USA
4. Laboratory for Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences Ecole Polytechnique Fédérale de Lausanne CH‐1015 Lausanne Switzerland