1. Laboratoire de Photonique et de Nanostructures (LPN-CNRS); Route de Nozay; 91460 Marcoussis; France
2. Institut des Sciences Moléculaires; CNRS; University Paris Sud; Bâtiment 350; 91405 Orsay Cedex; France
3. Institut d'Electronique; de Microelectronique et de Nanotechnologie (IEMN), CNRS, University Lille I, Villeneuve d'Ascq; France
4. State Key Laboratory of Modern Optical Instrumentation; Centre for Integrated Optoelectronics; Department of Optical Engineering; Zhejiang University; Hangzhou 310027; P. R. China