[1] Univ Naples Federico II, Dipartimento Sci Fis, CNR, INFM Coherentia, I-80126 Naples, Italy
[2] Politecn Milan, Dipartimento Fis, CNR, INFM Coherentia & Soft, I-20133 Milan, Italy
[3] European Synchrotron Radiat Facil, F-38043 Grenoble, France
[4] Univ Roma Tor Vergata, CNR, INFM Coherentia, I-00133 Rome, Italy
[5] Univ Roma Tor Vergata, Dipartimento Ingn Meccan, I-00133 Rome, Italy