Yakup Boran

Pressure optimization of high harmonic generation in a differentially pumped Ar or H2 gas jet

We experimentally studied the dependence of high harmonic generation in argon and molecular hydrogen on pressure changes in a gas jet that cause variations of the phase matching conditions and absorption. The study was performed at a peak laser intensity of ∼1.5 × 10(14) W/cm(2). To enable measurements over a wide range of pressures, we employed differential pumping with an additional cell (∼20 cm(3) volume) enclosing the gas jet. By increasing the pressure in the gas jet up to a maximum of 1.5 bars with argon or 0.5 bars with hydrogen, we observed an increase in the high harmonic (HH) yield until an optimum pressure of 0.2 bars was reached for Ar, beyond which the output began decreasing. For H2, we observed an increase of the HH output up to the maximum pressure of 0.5 bars. This pressure-dependence study allowed us to achieve a tenfold enhancement in the high harmonic yield at the optimum pressure.

Extension of filament propagation in water with Bessel-Gaussian beams

We experimentally studied intense femtosecond pulse filamentation and propagation in water for Bessel-Gaussian beams with different numbers of radial modal lobes. The transverse modes of the incident Bessel-Gaussian beam were created from a Gaussian beam of a Ti:sapphire laser system by using computer generated hologram techniques. We found that filament propagation length increased with increasing number of lobes under the conditions of the same peak intensity, pulse duration, and the size of the central peak of the incident beam, suggesting that the radial modal lobes may serve as an energy reservoir for the filaments formed by the central intensity peak.

Nonlinear mixing of optical vortices with fractional topological charge in Raman sideband generation

We studied the nonlinear parametric interaction of femtosecond fractionally-charged optical vortices in a Raman-active medium. Propagation of such beams was described using the Kirchhoff–Fresnel integrals by embedding a non-integer 2π phase step in a Gaussian beam profile. When using fractionally-charged pump or Stokes beams, we observed the production of new topological charge and phase discontinuities in the Raman field. These newly generated fractionally-charged Raman vortex beams were found to follow the same orbital angular momentum algebra derived by Strohaber et al (2012 Opt. Lett. 37 3411) for integer vortex beams.

Control of femtosecond filamentation by revivals of nonadiabatic molecular alignment

We realized control of filamentation and white light generation by using the laser induced nonadiabatic molecular alignment, as a means for transient modification of the ionizing medium. In the pump-probe experimental setup [1], the pump pulse induces periodical rotational revivals due to quantum-mechanical discreteness of the rotational eigenfrequencies of the molecules; then the delayed probe pulse experiences a propagating wake of index modification created by the pump pulse. This results in the modification of the filamentation process and the white-light signal.