1. TU Wien─Institute of Solid State Electronics & Center for Micro- and Nanostructures, Gußhausstraße 25-25a, Vienna 1040, Austria
2. University of Naples Federico II, Corso Umberto I 40, Napoli 80138, Italy
3. CNR-INO─Istituto Nazionale di Ottica, Largo Fermi, 6, Firenze, FI 50125, Italy
4. CNR-INO─Istituto Nazionale di Ottica, Via Carrara, 1, Sesto Fiorentino, Florence 50019, Italy
5. LENS─European Laboratory for Non-Linear Spectroscopy, Via Carrara, 1, Sesto Fiorentino, Florence 50019, Italy
6. nanoplus Nanosystems and Technologies GmbH, Oberer Kirschberg 4, Gerbrunn 97218, Germany
7. Julius-Maximilians-Universität Würzburg─Physikalisches Institut, Lehrstuhl für Technische Physik, Am Hubland, Würzburg 97074, Germany
8. INFN─Istituto Nazionale di Fisica Nucleare, Via Sansone, 1, Sesto Fiorentino, Florence 50019, Italy