1. Laboratory for Mechanics of Materials and Nanostructures, Empa Materials Science and Technology, Feuerwerkstrasse 39, 3602 Thun, Switzerland, Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany, Institute of Solid State Physics, University of Jena, Max-Wien-Platz 1, 07743 Jena, Germany, Institute of Photonic Technology, AlbertEinsteinStrasse 9, 07745 Jena, Germany, and Institute of Microtechnology, University of Neuchatel, A.-L. Breguet 2, 2000 Neuchatel, Switzerland