1. Materials Science and Engineering Department and Chemical and Biomolecular Engineering Department, Cornell University, Ithaca, New York 14853-1501, Department für Physik and CeNS, Ludwig-Maximilians-Universität, München, Germany, IFN Istituto di Fotonica e Nanotecnologie Sezione di Trento CNR-ITC, Povo Trento, Italy, and Department of Physics, The University of Vermont, Burlington, Vermont 05405