E. Gaižauskas

Self-reconstruction of light filaments.

By observing how a light filament generated in water reconstructs itself after hitting a beam stopper in the presence and in the absence of a nonlinear medium, we describe the occurrence of an important linear contribution to reconstruction that is associated with the conical nature of the wave. A possible scenario by which conical wave components are generated inside the medium by the distributed stopper or reflector created by nonlinear losses or plasma is 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.

On the Role of Conical Waves in Self-focusing and Filamentation of Femtosecond Pulses with Nonlinear Losses

This chapter concerns with the experimental observations and theoretical investigations on the propagation of intense femtosecond pulses in water and fused silica. It emphasizes spontaneous transformation of a beam into a conical (Bessel-like) wave during the filamentary propagation in media with nonlinear losses. This transformation constitutes an interpretation of the energy reservoir surrounding the high intensity central core of the filament. The adopted model is shown as being able to explain related phenomena such as the formation of multiple filaments and that of X-waves, observed experimentally in both water and fused silica.

Towards broadband terahertz generation due to coherent phonon–polariton excitations in centrosymmetric media

In this work, four-wave difference-frequency generation in centrosymmetric media doped with three-level quantum systems is investigated theoretically. Terahertz radiation generation in the coherent regime of interaction, where the durations of optical pulses are shorter than the relaxation time of coherences induced in two- and one-photon resonant transitions, is analyzed. It is shown that the coherent regime of four-wave frequency mixing enhances the efficiency of radiation generation in the terahertz frequency range. Quantum confined systems (e.g., atoms and molecules in nanoparticles) should be considered as possible media for practical implementation of the method.

Studies of femtosecond pulse filamentation in glasses

Self-focusing and filamentation of nearly-Gaussian femtosecond laser pulses propagating in photosensitive silicate glass was investigated. The filamentation was visualized by the precipitation of NaF nano-crystallites along the beam pass after post-exposure treatment of photosensitive glass, which provides a direct proof that ionization of silicate matrix takes place along the filament propagation lines. Theoretical model of the multi-filamentation based on the propagation of a Gaussian beam with elliptical transverse intensity profile modulated by a spatial noise in a medium with multi-photon absorption is proposed. Self-action of the femtosecond Gaussian-Bessel pulses in borosilicate glass was observed at high fluence. This model reproduces qualitatively the dotted damage lines observed after the beam propagation in borosilicate glass.

Fabrication of the Refractive Index Gratings in Optical Glasses by Filamentary Propagation of Femtosecond Laser Pulses

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 index changes.

Rapid microfabrication of transparent materials using a filamented beam of the IR femtosecond laser

Glass drilling and welding applications realized with the help of femtosecond lasers attract industrial attention , however, desired tasks may require systems employing high numerical aperture (NA) focusing conditions, low repetition rate lasers and complex fast motion translation stages. Due to the sensitivity of such systems, slight instabilities in parameter values can lead to crack formations, severe fabrication rate decrement and poor quality overall results. A microfabrication system lacking the stated disadvantages was constructed and demonstrated in this report. An f-theta lens was used in combination with a galvanometric scanner, in addition, a water pumping system that enables formation of water films of variable thickness in real time on the samples. Water acts as a medium for filament formation, which in turn decreases the focal spot diameter and increases fluence and axial focal length . This article demonstrates the application of a femtosecond (280fs) laser towards two different micromachining techniques: rapid cutting and welding of transparent materials. Filament formation in water gives rise to strong ablation at the surface of the sample, moreover, the water, surrounding the ablated area, adds increased cooling and protection from cracking. The constructed microfabrication system is capable of drilling holes in thick soda-lime and hardened glasses. The fabrication time varies depending on the diameter of the hole and spans from a few to several hundred seconds. Moreover, complex-shape fabrication was demonstrated. Filament formation at the interface of two glass samples was also used for welding applications. By varying repetition rate, scanning speed and focal position optimal conditions for strong glass welding via filamentation were determined.

Harmonic generation via excitation of surface states formed from spatially separated electrons and holes in nanocomposites

We search for efficient schemes of second and terahertz harmonic generation in nanocomposites consisted of metal-oxide semiconductor quantum dots incorporated into a dielectric matrix, when the quantum dots are in resonance and the dielectric matrix is out of resonance with femtosecond light pulse. It’s established that large efficiency of frequency up-conversion is possible to attain, which may be for the optimal quantum dot concentration in above mentioned nanocomposites by 70% higher than in pure nonlinear dielectric matrix.

Spatio-temporal localization of powerful femtosecond pulses in Kerr media and light bullet regimes

Possible scenarios of high-intense vortex (and Gaussian) pulsed beam propagation in Kerr media and light bullet (LB) formation conditions are considered. The system of modified nonlinear Schroedinger equation for the complex envelope of the electric field and kinetic equation for the electron plasma density is exploited. Two-scale variational analysis is combined with direct numerical simulations based on finite-difference methods. Hamiltonian approach allows to reveal LB formation conditions. It is shown that the LB parameters correspond to minimum of potential energy when the whole balance of competing processes occurs. Numerical experiment confirms the results obtained on the base of variational analysis, demonstrating at the same time softer conditions for LB formation. It is emphasized that the linear and nonlinear dynamics of spatial and temporal radii obey the coupled oscillator theory.

Microfabrication of transparent materials using filamented femtosecond laser beams

Glass drilling realized with the help of femtosecond lasers attract industrial attention, however, desired tasks may require systems employing high numerical aperture (NA) focusing conditions, low repetition rate lasers and complex fast motion translation stages. Due to the sensitivity of such systems, slight instabilities in parameter values can lead to crack formations, severe fabrication rate decrement and poor quality overall results. A microfabrication system lacking the stated disadvantages was constructed and demonstrated in this report. An f-theta lens was used in combination with a galvanometric scanner, in addition, a water pumping system that enables formation of water films of variable thickness in real time on the samples. Water acts as a medium for filament formation, which in turn decreases the focal spot diameter and increases fluence and axial focal length. This article demonstrates the application of a femtosecond (280fs) laser towards rapid cutting of different transparent materials. Filament formation in water gives rise to strong ablation at the surface of the sample, moreover, the water, surrounding the ablated area, adds increased cooling and protection from cracking. The constructed microfabrication system is capable of drilling holes in thick soda-lime, hardened glasses and sapphire. The fabrication time varies depending on the diameter of the hole and spans from a few to several hundred seconds. Moreover, complex-shape fabrication was demonstrated.