1. National Institute of Optics, National Research Council (CNR/INO), Adriano Gozzini Laboratory, via Giuseppe Moruzzi 1, 56124 Pisa, Italy
2. Enrico Fermi Department of Physics, University of Pisa, largo Bruno Pontecorvo 3, 56127 Pisa, Italy
3. Department of Physics, Lund University, P.O. Box 118, SE-22100 Lund, Sweden
4. Department of Energy, Politecnico di Milano, 20133 Milano, Italy
5. CEA/DSM/IRAMIS/LIDYL, CEA Saclay, 91191 Gif-sur-Yvette, France