1. Kirchhoff Institute for Physics, Heidelberg University, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany
2. Integrated Centre for Applied Physics and Photonic Materials and Centre for Advancing Electronics Dresden (cfaed), Technical University of Dresden, Nöthnitzer Straße 61, 01187 Dresden, Germany
3. School of Biochemical Engineering, University of Birmingham, Edgbaston, Birmingham, West Midlands B15 2TT, United Kingdom