1. Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Ch. du. Musée 3, CH-1700 Fribourg, Switzerland, Adolphe Merkle Institute, University of Fribourg, Ch. du. Musée 3, CH-1700 Fribourg, Switzerland, Department of Engineering Physics, Helsinki University of Technology, P.O. Box 5100, 02015 TKK, Finland, and Néstlé Research Center, Vers-Chez-Les-Blanc, 1000 Lausanne 26, Switzerland