1. Institute for Electrical Measurement Science and Fundamental Electrical Engineering and Laboratory for Emerging Nanometrology (LENA), Hans-Sommer-Str. 66, Braunschweig 38106, Germany
2. EMAT, University of Antwerp, Groenenborgerlaan 171, Antwerp B-2020, Belgium
3. CINBIO, Universidade de Vigo, Vigo 36310, Spain
4. Institute of Applied Physics, TU Braunschweig, Mendelssohnstraße 2, Braunschweig 38106, Germany
5. Departamento de Física Aplicada, Universidade de Vigo, Vigo 36310, Spain