This proceeding details the experimental investigation and scheme used to generate vortex beams at THz and far-infrared frequencies. Main experimental and numrical results are presented by M. Ivanov et al.1, 2 We investigate the generation of broadband terahertz (THz) pulses between 10 and 40 THz with a phase singularity from two-color femtosecond laser-induced gas-plasmas. Experimental scheme allows for independent control of the amplitude, phase, polarization, and longitudinal and transverse positions of fundamental and second harmonics. When the second harmonic beam has a vortex charge the THz beam acquires an azimuthal phase modulation as well. Moreover, it turns out that also the THz intensity becomes modulated along the azimuthal angle. The phase structure of the singular THz beam is indirectly revealed by astigmatic transformation of the beam by a cylindrical mirror and additionally confirmed by the vortex THz beam behaviour upon change of relative phase between the fundamental and the second harmonic.
We report on the refractive index grating formation by filamentary propagation of femtosecond pulses in fused
silica. The relevant exposure and work cycles are considered both experimentally and through numerical study,
involving a model of light filaments supported by conical wave, capable to capture permanent glass refraction
The dynamics of multiple pulse laser-induced damage in the form of cracking or nonlinear coloration in bulk materials (fused silica and borosilicate K8 glass) was studied under the irradiation by femtosecond pulses at 800 nm wavelength. A Ti:sapphire chirped pulse amplification system with ~130-fs pulse duration and ~1-mJ pulse energy at 1-kHz repetition rate was used in the experiment. Self-guided propagation of femtosecond pulses over greater than 1-cm lengths accompanied by intensive supercontinuum generation was observed and studied in an interaction geometry where the laser beam was focused in the middle of the thick (~4 cm) sample. The pulse energy value at which self-guided propagation and supercontinuum generation in fused silica was observed was ~60 times lower than the laser-induced damage threshold. The nonlinear coloration in K8 glass was present at pulse energy values which exceeded the threshold for self-guided propagation. Numerical simulations involving self-focusing, temporal dispersion and multiphoton absorption were found to be in good agreement with the experimental results.
The experimental results of an investigation of the self- action of Bessel beam propagating in the benzene are presented. The typical modifications of the far-field intensity distribution of Bessel beam caused by its self- action are revealed. A good agreement with computer simulation results is obtained.
The sensitizing hematoporphyrins and related species used in photosensitized tumor therapy (PTT) were revealed to be complex mixtures of monomers, dimers (mainly dihematoporphyrin ether or ester) and aggregates. Time resolved absorption spectroscopy was proved to be helpful for differentiation between these components. The spectral and temporal characteristics of the absorption changes ((Delta) A) of Hp, HpD and PS is aqueous and ethanol solutions were measured by picosecond pump-and-probe techniques. The (Delta) A spectrum of Hp in ethanol solution is formed by the bleaching of steady-state bands and the absorption from the excited S1 state. The (Delta) A of Hp relaxes exponentially with time constant (tau) equals 10 divided by 17 ns (approximately equal to the fluorescence decay time of monomeric Hp). The (Delta) A of HpD in ethanol shows biexponential decay with time constants (tau) equals 10 divided by 17 and (tau) 1 equals 1.5 divided by 4 ns. We suppose that lifetime 1.5 divided by 4 ns reflects the excitation energy relaxation in covalently linked oligomers and (tau) equals 10 divided by 16 ns is the lifetime of excited S1 state of HpD monomers. It should be noted that the (tau) 1 value depends on the existing different oligomers in HpD due to the HpD preparation conditions. The measured (Delta) A kinetics in aqueous Hp and HpD solution showed the different character. It can be fitted with the three exponents with time constants of (tau) 2 equals 100 divided by 200 ps (short component), (tau) 1 equals 1.5 divided by 4 ns (long component) and (tau) about 15 ns (monomeric component). The main changes in excitation relaxation in aqueous solutions is caused by the presence of equilibrium aggregates of porphyrins. We attribute the estimated lifetimes (tau) 2 equals 100 divided by 200 ps and (tau) 1 equals 1.5 divided by 4 ns in aqueous solutions to equilibrium aggregates and/or covalently linked dimers and oligomers of HpD, respectively.