1. Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität, Amalienstr. 54, 80799 München, Germany, Institute for Technical and Macromolecular Chemistry, Universität Hamburg, 20146 Hamburg, Germany, Theoretical Division and Center for Integrated Nanotechnologies (CINT), Los Alamos National Laboratory (LANL), Los Alamos, New Mexico 87545, and Department of Physics, University of Utah, Salt Lake City, Utah 84112