G. Tamošauskas

Multiple filamentation induced by input-beam ellipticity

We provide what is to our knowledge the first experimental evidence that multiple filamentation (MF) of ultra-short pulses can be induced by input beam ellipticity. Unlike noise-induced MF, which results in complete beam breakup, the MF pattern induced by small input beam ellipticity appears as a result of nucleation of annular rings surrounding the central filament. Moreover, our experiments show that input beam ellipticity can dominate the effect of noise (transverse modulational instability), giving rise to predictable and highly reproducible MF patterns. The results are explained with a theoretical model and simulations.

Time-resolved off-axis digital holography for characterization of ultrafast phenomena in water

We present the application of time-resolved off-axis digital holography for the investigation of refractive index/transmission properties of laser-induced plasma filaments in water. Time evolution of both amplitude- and phase-contrast images of the self-focused beam in water was characterized with temporal resolution better than 50 fs. To the best of our knowledge, this is the first attempt to characterize the propagation of femtosecond laser pulse in nonlinear media using off-axis digital holography.

Time-resolved refractive index and absorption mapping of light-plasma filaments in water

By means of a quantitative shadowgraphic method, we performed a space-time characterization of the refractive index variation and transient absorption induced by a light-plasma filament generated by a 120 fs laser pulse in water. The formation and evolution of the plasma channel in the proximity of the nonlinear focus were observed with a 23 fs time resolution.

Nature of Spatiotemporal Light Bullets in Bulk Kerr Media

We present a detailed experimental investigation which uncovers the nature of light bullets generated from self-focusing in a bulk dielectric medium with Kerr nonlinearity in the anomalous group velocity dispersion regime. By high dynamic range measurements of three-dimensional intensity profiles, we demonstrate that the light bullets consist of a sharply localized high-intensity core, which carries the self-compressed pulse and contains approximately 25% of the total energy, and a ring-shaped spatiotemporal periphery. Subdiffractive propagation along with dispersive broadening of the light bullets in free space after they exit the nonlinear medium indicate a strong space-time coupling within the bullet. This finding is confirmed by measurements of a spatiotemporal energy density flux that exhibits the same features as a stationary, polychromatic Bessel beam, thus highlighting the nature of the light bullets.

Self-guided propagation of femtosecond light pulses in water

We report experimental evidence of self-guided propagation of femtosecond laser pulses in water. A light filament induced by 170-fs, 527-nm pulses has a diameter of 60 microm (at the 1/e2 level) and persists over a distance of 20 mm. The filamentary mode is sustained over a wide range of input power, and the energy excess is converted into conical emission. In the time domain, the pulse trapped in a filamentary mode experiences a number of splittings occurring in the early stage of filament formation.

Combining effect in a multiple-beam-pumped optical parametric amplifier

We demonstrate highly efficient performance (quantum efficiency as great as 80%) of a degenerate three-beam-pumped optical parametric amplifier (OPA). Such an OPA exhibits a parametric combining effect, as the signal gains its energy from all three pump pulses. The efficiency of the parametric amplification process is defined only by a triple phase-matching geometry and is insensitive to the phase relationship of the pump pulses. We also show that OPA operation is not affected by the number of pump pulses in terms of the energy conversion and the temporal characteristics of the amplified pulse.

Spatio-temporal reshaping and X Wave dynamics in optical filaments.

Focus Serial: Frontiers of Nonlinear Optics We investigate ultrashort laser pulse filamentation within the framework of spontaneous X Wave formation. After a brief overview of the filamentation process we study the case of an intense filament co-propagating with a weaker seed pulse. The filament is shown to induce strong Cross-Phase-Modulation (XPM) effects on the weak seed pulse: driven by the pump, the seed pulse undergoes pulse splitting with the daughter pulses slaved to their pump counterparts. They undergo strong spatio-temporal reshaping and are transformed into XWaves traveling at the same group velocities as the pump split-off pulses. In the presence of a gain mechanism such as Four-Wave-Mixing or Stimulated Raman Scattering, energy is then transferred from the pump filament leading to amplification of the seed X Wave and formation of a temporally compressed intensity peak.

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.

Gain bandwidth broadening of the continuum-seeded optical parametric amplifier by use of two pump beams

Abstract We show that the gain bandwidth of the continuum-seeded single-pass optical parametric amplifier may be easily extended using several pump beams amplifying neighboring regions of the seed signal spectrum. Experimentally we demonstrate more than twofold gain bandwidth broadening of type I phase-matching BBO OPA along with subsequent compression of the amplified signal down to the transform limit.

Highly efficient four-wave parametric amplification in transparent bulk Kerr medium

We report on highly efficient four-wave optical parametric amplification in a water cell pumped by an elliptically shaped, ultrashort pulsed laser beam under non-collinear phase-matching configuration. Energy conversion from pump to parametric waves as high as 25 % is obtained owing to the achievement of 1-dimensional spatial-soliton regime, which guarantees high intensity over a large interaction length and ensures high beam quality.