1. Center for Micro‐ and Nanotechnologies, Technical University Ilmenau, 98693 Ilmenau, Germany
2. A.F. Ioffe Physico‐Technical Institute of RAS, 26 Polytechnicheskaya Str., 19402 St. Petersburg, Russia
3. Foundation for Research and Technology‐Hellas, P.O. Box 1527, Heraklion, Crete, 71110, Greece