title: Pulsed light beams in vacuum with superluminal and negative group velocities.
abstract: 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.
authors.affiliation: Departamento de Física Aplicada, ETSI Minas, Universidad Politécnica de Madrid, Madrid, Spain.
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Porras
authors.forename: Isabel
authors.initials: I
authors.lastname: Gonzalo
authors.forename: Alessia
authors.initials: A
authors.lastname: Mondello