1. Department of Health Sciences and Technology, ETH Zürich, 8092 Zürich, Switzerland
2. European Laboratory for Non-Linear Spectroscopy, LENS, 50019 Florence, Italy
3. Department of Physic and Geology, University of Perugia, 06123 Perugia, Italy
4. CNR-INO, National Research Council-National Institute of Optics, 50125 Florence, Italy
5. Department of Chemistry, Biology and Biotechnology, University of Perugia, 06123 Perugia, Italy
6. Department of Materials, ETH Zürich, 8093 Zürich, Switzerland