1. EMPA, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Mechanics of Materials and Nanostructures, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland
2. Institut für Photonische Technologien, Albert-Einstein-Strasse 9, D-07745 Jena, Germany, and Max-Planck Institute for Microstructure Physics, Weinberg 2, D-06120 Halle, Germany