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Effect of group velocity dispersion on femtosecond filamentation of Bessel – Gaussian beams

Laser colouration technique is employed to investigate the structures of colour centres induced in an isotropic LiF crystal by Bessel – Gaussian and Gaussian beams in the single-pulse filamentation regime. Femtosecond laser pulses at radiation wavelengths corresponding to zero (1250 nm) and anomalous (1900 nm) group velocity dispersions were focused into the sample using an axicon or a spherical lens. As shown experimentally and numerically, with increase in radiation energy the length of the structure of colour centres (or the plasma channel length) increases in the domain of zero group velocity dispersion but remains invariable under conditions of anomalous dispersion, when there emerge light bullets. The path length of the light bullets amounts to about 200 μm and does not depend on the pulse energy and the way of focusing, which confirms their robustness.