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The possibility of the influence of thermal effects on the formation of color centers during laser femtosecond filamentation in a LiF crystal has been investigated theoretically. The conditions are formulated under which the influence of thermal effects can be noticeable.

Line-by-line two-dimensional step small-scale scanning irradiation of a transparent cubic LiF crystal was carried out by intense linearly polarized femtosecond laser radiation in the multiple filamentation mode. As a result, isolated longitudinal extended tracks consisting of induced color centers were formed in the medium. It was found that no transverse periodicity associated with the scanning step is observed in the arrangement of tracks formed by laser filaments. This is because inhomogeneities that stimulate filamentation are not contained in the laser beam itself but are formed randomly when the medium interacts with the first laser pulses and are supported and amplified by subsequent pulses. The efficiency of color center formation in crystals at normal laser beam incidence on the cube face depends periodically on the azimuth angle θ between the electric vector and the cube edge on the face, with the period of π/2. It was found that azimuthal dependences for defect formation (maximum at θ = π/4) and for carrier photogeneration (maximum at θ = 0) are in the antiphase. Calculations showed that the processes of self-focusing and filamentation controlled by the components of the third-order nonlinear susceptibility tensor are most effective at the orientation where θ = π/4. The experiment showed that at such an orientation, the critical power and the length of self-focusing decrease, and therefore, the density of the number of filaments in the beam section increases and, as a result, the average concentration of the color centers created by laser filaments increases. [ABSTRACT FROM AUTHOR]