Rybak A. Effects of micro- and nanoscale structuring in solid media under femtosecond laser irradiation.

The thesis is devoted to the experimental studies of micro- and nanoscale structuring effects in solid media under femtosecond laser irradiation. Femtosecond optical fiber laser based on ytterbium with a multistage amplifier is constructed. Each stage of the amplifier was arranged in such a way that it provides minimal variation of the pulse shape generated in the oscillator. This was achieved by balancing between so-called gain narrowing and self-phase modulation. Pulses with energy of 1 μJ and duration of 100 fs at a repetition rate of 1 MHz were obtained. Polarization features of white supercontinuum (SC) and conical emission (CE) of femtosecond laser filaments in quartz and sapphire crystals, resulting from their positive and negative birefringence, are reported. The SC and CE acquire orthogonal polarization planes as a result of the difference between the group velocities of the ordinary and extraordinary rays. A physical mechanism of the CE generation is proposed, explaining the specific features of its polarization. A cheap and fast technique for the production of high-quality laser-induced periodic surface structures (LIPPS) based on the negative and positive feedback is proposed. A fast and flexible process of fabrication of diffraction-limited microlenses and micromirrors in a chalcohalogenide glass is demonstrated. Each lens is fabricated by a single laser pulse, the energy of which was absorbed by the medium due to two-photon absorption process. The blast wave favoured the removal of material from the ablation spot. Solidification of the residual thin liquid layer formed an optically smooth surface due to the surface tension forces. After the two-photon absorption the residual part of the pulse penetrated into the sample bulk in a filament or multifilament mode without causing any irreversible changes in the material. An array of microlenses was fabricated by scanning the sample surface with a focused laser beam. Covering the microlense surface with a metal film converted it into micromirrors. Computer control of the sequence of laser pulses enables the formation of arrays of microlenses of various geometry. This technique can be used as a basis for the technology of fabrication of microlenses and micromirrors. The productivity of the technology is limited solely by the femtosecond laser pulse repetition rate which, as a rule, is 1 kHz. New technique for orientation of nematic liquid crystals on the material surface is proposed. LIPPS are found to be capable of orienting nematic liquid crystals deposited on a surface with LIPPS. A method of increase of azimutal anchoring energy (AAE) of the surface of a nanostructured titanium layer (NSTL) by deposition the polymer film is demonstrated. NSTL is shown to possess a relatively low AAE while deposition of a polymer film results in a strong AAE increase. The AAE value can be varied in a broad range by variation of at least two parameters (the scanning speed and the pulse energy density on the surface) during the orienting surface processing.