1. Kavli Institute of Nanoscience, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, Netherlands, National CRI Center for Semiconductor Nanorods, POSTECH, Korea, CEA, INAC/SPSMS/LaTEQS, 17 Rue des Martyrs, 38054 Grenoble, France, CNR-INFM, Laboratorio Nazionale TASC, S.S. 14, Km 163.5, 34012 Trieste, Italy, and Philips Research Laboratories, Professor Holstlaan 4, 5656 AA Eindhoven, Netherlands