1. Laboratory of Nano-Optics, University of Siegen 1 , 57072 Siegen, Germany
2. Cμ-Research Center of Micro- and Nanochemistry and (Bio)Technology, University of Siegen 2 , 57068 Siegen, Germany
3. Istituto Nazionale di Fisica Nucleare, Sezione di Firenze 3 , 50019 Sesto Fiorentino, Italy
4. Micro- and Nanoanalytics Group, University of Siegen 4 , 57076 Siegen, Germany
5. Institute for Materials, Ruhr-University Bochum 5 , 44801 Bochum, Germany
6. Department of Physics and Astronomy, University of Florence 6 , 50019 Sesto Fiorentino, Italy
7. National Institute of Optics (INO), National Research Council (CNR) 7 , 50125 Florence, Italy
8. Diamond Materials GmbH 8 , 79108 Freiburg, Germany