1. School of Electrical and Electronic Engineering, University of Manchester, Manchester M60 1QD, U.K., VTT Information Technology, P.O. Box 1208, FIN-02044 VTT, Finland, Technische Physik, Universität Würzburg, 97074 Würzburg, Germany, Departamento Física Aplicada, Universidad de Salamanca, Salamanca, Spain, and Solid State Physics and Nanometer Consortium, Lund University, S-221 00 Lund, Sweden