Porras, Miguel A.Gonzalo Fonrodona, IsabelMondello, Alessia2023-06-202023-06-202003-06-171539-375510.1103/PhysRevE.67.066604https://hdl.handle.net/20.500.14352/51361© 2003 The American Physical Society.We study the group velocity of pulsed light beams in vacuum. Gouy's phase associated with the diffraction of transversally limited pulses can create a strong anomalous dispersion in vacuum leading to highly superluminal and negative group velocities. As a consequence, a focusing pulse can diverge beyond the focus before converging into it. The experimental feasibility is discussed.engjournal articlehttp://dx.doi.org/10.1103/PhysRevE.67.066604http://journals.aps.org/open access535PropagationÓptica (Física)2209.19 Óptica Física