Viktor Kadan

Time-resolved imaging of ultrafast laser pulse interaction with transparent materials

A study of femtosecond laser interaction with a range of transparent solids, such as glass, sapphire, and fused silica in femtosecond-to-nanosecond time domain is reported. Time-resolved pump-probe microscopic imaging of femtosecond laser breakdown area in different transparent solids has been done. Both absorption and Femtosecond Time-resolved Optical Polarigraphy (FTOP) microscopy techniques were used. White continuum was used as a probe pulse. Strong filamented plasma absorption initially dominates, being replaced by refraction index variations in filament traces and blast wave at hundreds of picoseconds delays. Dynamics of transient absorption in K8 glass was for the first time precisely linked with the pump pulse passage observed by FTOP. Cylindrical blast wave generation was observed starting from 200 ps delays in K8 glass and silica and its velocity has been measured. Exponential decay time of laser-induced plasma in sapphire, both pure and Ti doped has been found.

Interaction of femtosecond filaments in sapphire

The interaction of two femtosecond filaments has been studied in single-crystal sapphire at small crossing angles 1.2° ÷5°. Regular multifilament pattern (MFP) can be formed in the overlapping area of two beams, which depends on the pulse energy, crossing angle, and phase difference. As a special realization of MFP, formation of a single filament from two different pump beams was for the first time observed. The number of filaments in the MFP depends on the number of the interference maxima in overlapping area, whose power exceeds critical power for self-focusing. We demonstrate control of multifilament pattern, changing the phase shift between the interacting beams. Attraction and repulsion of the filaments is observed in vicinity of the overlapping region for both in- and counter-phase beams depending on the position Z along the propagation axis. Transformation of near- and far-field images of the single filament, formed by two pump beams, and spectra of its axial emission have been studied depending on the filament length. It has been shown that four-wave mixing plays a major role in forming the axial emission spectrum.

Non-linear effects in chalcogenide glasses

Femtosecond filamentation of 800 nm laser pulses in the conditions of strong two-photon absorption was first directly observed in As4Ge30S66 chalcogenide glass. In this paper we also present the experimental results on laser self-written waveguides in arsenic-germanium-sulfide glasses. In contrast, there was no filamentation and spectrum widening in stoichiometric As2S3 glass. It was shown that nonlinear figure of merit was as high as 0.52 and only 0.04 in glassy As4Ge30S66 and As2S3, respectively.