1. Dipartimento di Fisica “E. Fermi,” Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
2. Istituto Nanoscienze-CNR, Piazza S. Silvestro 12, I-56127 Pisa, Italy
3. Istituto Nazionale di Ottica-CNR, Via G. Moruzzi 1, 56124 Pisa, Italy
4. Joint Quantum Institute, Department of Physics, University of Maryland and National Institute of Standards and Technology, College Park, Maryland 20742, USA