1 Dipartimento di Fisica "E. Pancini", Università degli Studi di Napoli "Federico II" and INFN, Napoli, Italy. Electronic address: mettivier@na.infn.it.
2 Centre For Medical and Radiation Physics, University of Wollongong, Wollongong, Australia.
3 Optical Sciences Centre, Department of Physics and Astronomy, School of Science, Swinburne University of Technology, Melburne, Australia.
4 Univ. Bordeaux, CNRS, LP2I, UMR 5797, F-33170 Gradignan, France.