1. Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Ch. du Musée 3, CH-1700 Fribourg, Switzerland, Laboratory for Polymer Chemistry, Swiss Federal Institute of Technology, Department of Materials, ETH-Zurich, HCI J 541, CH-8093 Zurich, Switzerland, Helsinki University of Technology, Physics Laboratory, Helsinki 02015, Finland, and Nestlé Research Center, Vers-Chez-les-Blancs, 1000, Lausanne 26, Switzerland