1. Photonics and Optoelectronics Group, Physics Department and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Amalienstrasse 54, D-80799 Munich, Germany, Physical Chemistry, TU Dresden, Bergstrasse 66b, D-01062 Dresden, Germany, and Semiconductor Photonics Group, Department of Physics, and CRANN Research Institute, Trinity College Dublin, Dublin 2, Ireland