title: Nonlinear X waves in second-harmonic generation: experimental results.
abstract: X waves, spatiotemporal generalization of the monochromatic Bessel- (or Durnin-) type beams, are known in linear acoustic, microwave and optics for their unique property of defeating both spatial and temporal spreadings. Recently, we brought to the attention that X-type waves are also the key to understand the spatiotemporal dynamics observed in the nonlinear (high intensity) regime. Indeed, X waves represent the normal-propagation mode for a wide class of parametric interactions described by hyperbolic nonlinear models featuring spatial self-focusing and temporal self-broadening. Here, we provide a complete and detailed description of the experiment in which the spontaneous appearance of X waves has been observed. The experiment concerns frequency doubling of a 170-fs, 50-microm standard laser wave packet in a 22-mm lithium triborate crystal, tuned for second-harmonic generation with positive phase mismatch, positive group-velocity dispersion, and large group-velocity mismatch. Conventional beam-profile and autocorrelation measurements at the crystal output face show evidence of spatiotemporal self-trapping. The characterization of the free-space propagation reveals sub-Gaussian diffraction and pulse broadening, consistent with the presence of angular dispersion. Space-resolved autocorrelations indicate the generation of an X-type profile.
authors.affiliation: INFM and Department of Chemical, Physical and Mathematical Sciences, University of Insubria, Via Valleggio 11, 22100 Como, Italy.
authors.forename: O
authors.initials: O
authors.lastname: Jedrkiewicz
authors.forename: J
authors.initials: J
authors.lastname: Trull
authors.forename: G
authors.initials: G
authors.lastname: Valiulis
authors.forename: A
authors.initials: A
authors.lastname: Piskarskas
authors.forename: C
authors.initials: C
authors.lastname: Conti
authors.forename: S
authors.initials: S
authors.lastname: Trillo
authors.forename: P
authors.initials: P
authors.lastname: Di Trapani