Pulsed light beams in vacuum with superluminal and negative group   velocities
  Gouy's phase 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. A simple experiment is proposed. 
M. A. Porras (Departamento de Fisica Aplicada. Universidad Politecnica   de Madrid), I. Gonzalo (Departamento de Optica. Uni...