V. Vaičaitis

Self-action of Bessel beam in nonlinear medium

The modifications of the angular spectrum of an intense Bessel beam caused by its self-action in nonlinear medium are revealed. The appearance of a central spot and outer ring in its angular spectrum was observed. Experimental results obtained for benzene are in good qualitative agreement with the theoretical predictions.

Cherenkov-type phase matching in bulk KDP crystal

The conical second harmonic emission from a bulk KDP crystal was detected and interpreted in terms of Cherenkov-type phase matching. In contrast to the models describing conical emission from a dense atomic medium, the present model is simple, independent of the intensity of pump and generated radiation and does not require including additional nonlinear effects, such as self-focusing and self-trapping. The experimentally registered parameters are in good agreement with theoretical data. The differences between Cherenkov-type phase matching and Cherenkov-type emission are discussed.

Transverse phase-matching in stimulated Raman scattering by a Bessel beam

Abstract The theoretical analysis of the transverse phase-matching conditions for the first Stokes and first anti-Stokes radiation in stimulated Raman scattering by a Bessel beam is provided. The experimental results of the observation of the first Stokes and first anti-Stokes angular spectra in the acetone pumped by a Bessel beam at λ p =532 nm are presented, and good agreement with the theoretical predictions is obtained.

Self-guiding, supercontinuum generation and damage in bulk materials induced by femtosecond pulses

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.

Second-order Stokes and anti-Stokes coupling in stimulated Raman scattering by Bessel beam

Abstract The theoretical analysis of the longitudinal and transverse phase-matching conditions for coupling of the second-order Stokes and anti-Stokes radiation in stimulated Raman scattering by a Bessel beam is provided. The experimental results of the observation of the second-order Stokes and anti-Stokes components excited by a Bessel beam at λ p =532 nm in acetone are presented. Good qualitative agreement with the theoretical predictions is obtained.

Tunable four-photon picosecond optical parametric oscillator

Abstract Four-photon parametric light oscillation due to synchronous near-resonant two-photon pumping of the 3D sodium level has been investigated under various experimental conditions. By varying the pressure in the sodium cell or utilising a diffraction grating as a dispersive element in the cavity of the four-photon optical parametric oscillator, the tuning range of more than 800 cm −1 has been achieved. The main features of such OPO, including the angular and spectral structure of additional cone, accompanying the parametric oscillation, are presented.

Probing electronic coherences by combined two- and one-photon excitation in atomic vapors

Wave mixing in a three-level quantum system induced by two-photon resonant pump and one-photon resonant coupling at lower transition has been studied theoretically and experimentally. Resonantly enhanced difference-frequency generation efficiency was observed clearly dependent on the time delay and its sign between the femtosecond pump and probe. The role of the atomic coherences on the amplification without inversion during different sequences of interaction in a closed three-level system has been also stressed. These findings were confirmed experimentally in atomic potassium, where strong parametric amplification, induced by the pump delayed with respect to the probe, has been observed for the first time.

Formation of Bessel beams with continuously variable cone angle

A simple device capable to produce Bessel beams with variable cone angle is proposed. It is shown that the cone angle of the Bessel beam produced by the axicone placed inside the telescope depends on the position of the axicone and can be continuously varied. As a result of theoretical investigations a simple formula giving the cone half-angle on the parameters and position of axicone inside the telescope was derived. The possibility of the Bessel beam generation with the help of a proposed device was confirmed experimentally. Good qualitative and quantitative agreement with the theoretical predictions is obtained.

Cherenkov-type phase-matched third harmonic generation in air

The third harmonic of Nd:YAG laser has been generated in air via the four-wave mixing. For the excitation of nonlinear media the second (Gaussian beam) and first (Bessel beam of variable cone angle) harmonics of this laser were used. The spatial spectra of the signal were presented and analysed. It was shown that in this case instead of the full phase matching the longitudinal (Cherenkov-type) phase matching takes place. The generation possibility of low divergence (<1 mrad) signal beam has been demonstrated experimentally.

Time-resolved absorption spectroscopy of hematoporphyrin and its photoproducts

Abstract Irradiation of an aqueous solution of hematoporphyrin causes the formation of the stable photoproduct which may play an important role in cancer therapy. The spectra and kinetic characteristics of the absorption changes of hematoporphyrins and their photoproducts were measured by a picosecond pump and probe absorption spectrophotometer with continuous tunability of pump and probe pulses in the spectral region of 400–1500 nm with an accuracy better than 10 −3 optical units and with a temporal resolution less than 10 ps. Careful examination of the differential absorption spectra showed that the photoproduct is involved in aggregates or exists as a part of the covalently linked oligomer of hematoporphyrins.