1. Physikalisches Institut, Fakultät für Mathematik und Physik, Albert-Ludwigs-Universität, Freiburg 79104, Germany
2. Department of Physics and Materials Science, School of Physical and Biological Sciences, Maseno University, 333, 40105 Maseno, Kenya
3. Freiburg Center for Interactive Materials and Bio-inspired Technologies (FIT), Albert-Ludwigs-Universität, Freiburg 79110, Germany