1. Optical
and Semiconductor Devices Group, Imperial College London, London SW7 2AZ, U.K.
2. Departamento
de Física Teórica de la Materia Condensada and Condensed
Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Madrid 28049, Spain
3. School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
4. School
of
Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, U.K.