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.

• Received 26 February 2002

DOI:https://doi.org/10.1103/PhysRevE.67.066604