1. Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China
2. Laboratoire de Photonique et Interfaces, Institut des Sciences et Ingenierie Chimique, Ecole Polytechnique Federale de Lausanne Station 6, 1015 Lausanne, Switzerland
3. Department of Materials Engineering, Monash University, Melbourne, Victoria 3800, Australia