1. Department of Physics and Astrophysics, University of Florence, 50019 Sesto Fiorentino, Italy
2. Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, 50019 Sesto Fiorentino, Italy
3. National Institute of Optics (INO-CNR), 50019 Sesto Fiorentino, Italy
4. European Laboratory for Nonlinear Spectroscopy (LENS), 50019 Sesto Fiorentino, Italy
5. Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56127 Pisa, Italy
6. Department of Physics, University of Pisa, 56127 Pisa, Italy
7. Laboratory of Nanooptics and Plasmonics, Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russian Federation
8. Center for Quantum Science and Technology in Arcetri (QSTAR), 50125 Florence, Italy
9. Laboratory of Nano-Optics, University of Siegen, 57072 Siegen, Germany