title: Femtosecond interferometry of propagation of a laminar ionization front in a gas.
abstract: We use optical interferometry to investigate ultrafast ionization induced by an intense, ultrashort laser pulse propagating in a helium gas. Besides standard phase shift information, our interferograms show a localized region of fringe visibility depletion (FVD) that moves along the laser propagation axis at luminal velocity. We find that such a loss of visibility can be quantitatively explained by the ultrafast change of refractive index due to the field ionization of the gas in the laser pulse width. We demonstrate that by combining the post facto phase shift distribution with the probe pulse transit effect in the ionizing region, the analysis of the observed FVD yields significant information on the ultrafast dynamics of propagation of the ionization front in the gas.
authors.affiliation: Intense Laser Irradiation Laboratory, IPCF-Area della Ricerca CNR, Via Moruzzi, 1 56124 Pisa, Italy and Istituto Nazionale di Fisica Nucleare-INFN, Pisa, Italy.
authors.forename: L A
authors.initials: LA
authors.lastname: Gizzi
authors.forename: M
authors.initials: M
authors.lastname: Galimberti
authors.forename: A
authors.initials: A
authors.lastname: Giulietti
authors.forename: D
authors.initials: D
authors.lastname: Giulietti
authors.forename: P
authors.initials: P
authors.lastname: Koester
authors.forename: L
authors.initials: L
authors.lastname: Labate
authors.forename: P
authors.initials: P
authors.lastname: Tomassini
authors.forename: Ph
authors.initials: P
authors.lastname: Martin
authors.forename: T
authors.initials: T
authors.lastname: Ceccotti
authors.forename: P
authors.initials: P
authors.lastname: De Oliveira
authors.forename: P
authors.initials: P
authors.lastname: Monot