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We investigate the formation of X waves during filamentation in Kerr media. From the standard model developed for femtosecond filamentation in liquids, solids, and gases, the influence of several physical effects and parameters is numerically studied in the strongly nonlinear regime where group velocity dispersion alone is insufficient to arrest collapse. The collapse is shown to be arrested by multiphoton absorption and plasma defocusing, but not by dispersion. The postcollapse dynamics takes the form of a pulse splitting, which induces large gradients in the near field and seeds the formation of X waves, appearing both in the near and far fields. We discuss the universal features of the X-wave patterns, among which the long arms in the far field that follow the linear dispersive properties of the medium [Conti, Phys. Rev. Lett. 90, 170406 (2003); Kolesik, Phys. Rev. Lett. 92, 253901 (2004)] and are accompanied by a strong modulated axial emission.

By observing how a light filament generated in water reconstructs itself after hitting a beam stopper in the presence and in the absence of a nonlinear medium, we describe the occurrence of an important linear contribution to reconstruction that is associated with the conical nature of the wave. A possible scenario by which conical wave components are generated inside the medium by the distributed stopper or reflector created by nonlinear losses or plasma is presented.

Filamentation of Bessel-Gauss pulses propagating in borosilicate glass is found to produce damage lines extending over hundreds of micrometers and consisting of discrete, equidistant damage spots. These discrete damage traces are explained by self-regeneration of Gauss-Bessel beams during the propagation and are potentially applicable in laser microfabrication of transparent materials.Light filaments formed by laser beams propagating in transparent media present an interesting case of spatial, and possibly, temporal localization of electromagnetic radiation at extremely high power densities sustainable over significant propagation lengths. The potential areas of applications of such light channeling range from remote sensing and LIDAR to laser microscopy and microfabrication. Generation and dynamics of light filaments is explained using a wide range of physical models. Some of them adopt moving focus model [1] which implies that filamentation is merely an optical illusion occurring when time-integrated detection is used. Others describe filaments as self-channelled beams [2, 3] whose stationarity is supported by a balance between Kerr-induced self-focusing and plasma-induced defocusing; a genuine soliton-like propagation, however, is destroyed by additional physical effects [4]. The dynamic spatial replenishment model [5] treats filamentation as a cyclic defocusing and focusing due to the dynamic interplay between the Kerr and plasma effects. According to the recently proposed "filamentation without self-channelling" model [6,7], multiphoton absorption alone can dynamically balance the self-focusing, thus leading to filamentation. It was shown numerically [8] that the absorption transforms the initial Gaussian beam toward the Gauss-Bessel (GB) beam, which is a modified solution of the free-space Helmholz equation [9]. Consequently, filament represents a narrow, high-intensity central part of a GB beam, whose propagation losses are replenished from the low-intensity side lobes containing the main part of the beam's energy. This model is strongly supported by recent experimental and theoretical results [7,10,11] demonstrating extreme robustness (self-healing) of the filaments in reconstructing their intensity after encountering microscopic obstacles.In most of the studies reported so far filamentation was seeded using laser beams with Gaussian transverse profiles. Having in mind the GB nature of filaments, the possibility of directly launching a powerful Bessel beam into the material and thus circumventing the internal transformation from Gaussian toward GB beam, is intriguing. The internal transformation is governed by the materials' nonlinear response, which may limit the obtainable peak intensity of the GB beam. An external GB beam (e.g., generated using an axicon) may have power sufficient for inducing and sustaining extensive damage along its entire propagation path. The possibility to fabricate extended lines in transparent solids almost instantaneously is beneficial for laser microfabrication. Here we show ...

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Measurements of the spatio-temporal and far-field profiles of ultrashort laser pulses experiencing conical emission, continuum generation, and beam filamentation in a Kerr medium outline the spontaneous formation of wave packets with X -like features, thus supporting recent numerical results [M. Kołesik, E. Wright, and J. Moloney, Phys. Rev. Lett. 92, 253901 (2004)]. Numerical simulations show good agreement with experimental data.

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