1. ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain
2. California Institute of Technology, Pasadena, California 91125, United States
3. Instituto de Óptica—CSIC, Serrano 121, 28006 Madrid, Spain
4. Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, U.K.