| Abstract [eng] |
In the disertation, the self-focusing phenomenon of intense femtosecond light pulses in media with cubic nonlinearity is investigated and the origin of formation of light filaments is revealed. In this work, new measurement techniques are introduced and applied experimentally, which allowed high resolution temporal, spatial and spectral mapping of light wave-packet dynamics during the nonlinear propagation in transparent media, and enabled to observe and make accurate quantitative evaluation of the ultrafast change of medium properties. By means of high temporal (20 fs) and spatial (1 μm) resolution laser 3D mapping technique, it was shown that the initial Gaussian wave packet during self-action in Kerr media redistributes its energy in a way that in spatio-temporal domain the wave packet takes a complex X-type intensity distribution. This transformation is universal and is determined by temporal and spatial spectral broadening (as a consequence of self-focusing and self phase modulation), conical emission (as a consequence of four-wave mixing) and nonlinear losses caused by multiphoton absorption. By means of imaging spectrometer technique it was shown that in the medium with normal group velocity dispersion the far-field angular spectrum of the wave packet takes a characteristic X shape, whereas in anomalous group velocity dispersion regime – a characteristic O shape. In both cases the localization of the wave packet is observed, however the quantitative differences of the angular spectra are determined by the phase-matching conditions of the four-wave mixing. By means of high temporal (23 fs) and spatial (1,5 μm) resolution quantitative shadowgraphic method, free electron plasma induced by light filament in water was fully characterized. The dynamics of the refractive index along the filament core was investigated. Local refractive index changes were measured: positive change associated with the Kerr effect and and negative change associated with plasma defocusing. Free electron density in the plasma channel was determined. The obtained data may help to estimate laser-induced long-term structural changes in the material and may be applied for fabrication of various microoptical elements in bulk media. |
