Spectral self-action of THz emission from ionizing two-color laser pulses in gases

The spectrum of terahertz (THz) emission in gases via ionizing two-color femtosecond pulses is analyzed by means of a semi-analytic model and finite-difference-time-domain simulations in 1D and 2D geometries. We show that produced THz signals interact with free electron trajectories and thus influence significantly further THz generation upon propagation, i.e., make the process inherently nonlocal. This self-action plays a key role in the observed strong spectral broadening of the generated THz field. Diffraction limits the achievable THz bandwidth by efficiently depleting the low frequency amplitudes in the propagating field.Comment: 12 pages, 6 figure

Generation of terahertz radiation from ionizing two-color laser pulses in Ar filled metallic hollow waveguides

The generation of THz radiation from ionizing two-color femtosecond pulses propagating in metallic hollow waveguides filled with Ar is numerically studied. We observe a strong reshaping of the low-frequency part of the spectrum. Namely, after several millimeters of propagation the spectrum is extended from hundreds of GHz up to $\sim 150$~THz. For longer propagation distances, nearly single-cycle near-infrared pulses with wavelengths around 4.5~$\mu$m are obtained by appropriate spectral filtering, with an efficiency of up to 0.25~\%

Nonlinear dynamics of trapped beams

Die vorliegende Arbeit befasst sich mit der nichtlinearen Propagation von Licht in schwach führenden Wellenleitern und in der Atmosphäre. Bei der Lichtausbreitung im Wellenleiter leiten wir ein hinreichendes Kriterium für die Stabilität schwach nichtlinearer Wellenleitermoden ab. Der zweite Teil der Arbeit beschäftigt sich mit der Ausbreitung hochintensiver ultrakurzer Laserpulse in der Atmosphäre, insbesondere mit multipler Filamentierung

Directionality of THz emission from photoinduced gas plasmas

Forward and backward THz emission by ionizing two-color laser pulses in gas is investigated by means of a simple semi-analytical model based on Jefimenko's equation and rigorous Maxwell simulations in one and two dimensions. We find the emission in backward direction having a much smaller spectral bandwidth than in forward direction and explain this by interference effects. Forward THz radiation is generated predominantly at the ionization front and thus almost not affected by the opacity of the plasma, in excellent agreement with results obtained from a unidirectional pulse propagation model

Azimuthons in weakly nonlinear waveguides of different symmetries

We show that weakly guiding nonlinear waveguides support stable propagation of rotating spatial solitons (azimuthons). We investigate the role of waveguide symmetry on the soliton rotation. We find that azimuthons in circular waveguides always rotate rigidly during propagation and the analytically predicted rotation frequency is in excellent agreement with numerical simulations. On the other hand, azimuthons in square waveguides may experience spatial deformation during propagation. Moreover, we show that there is a critical value for the modulation depth of azimuthons above which solitons just wobble back and forth, and below which they rotate continuously. We explain these dynamics using the concept of energy difference between different orientations of the azimuthon.Comment: 12 pages, 8 figure

Supercontinuum generation of ultrashort laser pulses in air at different central wavelengths

Supercontinuum generation by femtosecond filaments in air is investigated for different laser wavelengths ranging from ultraviolet to infrared. Particular attention is paid on the role of third-harmonic generation and temporal steepening effects, which enlarge the blue part of the spectrum. A unidirectional pulse propagation model and nonlinear evolution equations are numerically integrated and their results are compared. Apart from the choice of the central wavelength, we emphasize the importance of the saturation intensity reached by self-guided pulses, together with their temporal duration and propagation length as key players acting on both supercontinuum generation of the pump wave and emergence of the third harmonics. Maximal broadening is observed for large wavelengths and long filamentation ranges.Comment: 10 pages, 11 figure

Shaping the longitudinal electric field component of light

International audienceThis paper illustrates examples of shaping the longitudinal electric field component of light which is relevant for tightly focused beams. Given that the latter is not directly accessible via conventional beam shaping techniques we elaborate on the interplay between the transverse polarization and longitudinal electric field components. A Helmholtz decomposition of the transverse electric field components in the transverse plane permits on the one hand to draw insightful analogies with electro-and magnetostatics and with fluid dynamics. On the other hand, it allows to clearly isolate the remaining degree of freedom in the transverse electric field components for a given longitudinal electric field component and with that to generalize the concepts of radial and azimuthal polarization. We discuss degrees of freedom and show how one can exploit the findings to generate novel customized vector beams. Furthermore, we present a thought experiment to study beams containing evanescent waves

Structuring the three electric field components of light

International audienceUnless the beam's transverse electric field components are divergence-free in the two-dimensional transverse plane [1], tightly focused light typically leads to a non-negligible longitudinal electric field component [2], where the terms longitudinal and transverse electric field components refer to the components of the electric field that are parallel or perpendicular, respectively, to the direction of the mean Poynting flux. Having a longitudinal electric field component does not add a new degree of freedom, in the sense that all components of the electric and magnetic fields are still fixed by prescribing two electric field components in a plane. However, it is the electric field component parallel to the direction of the Poynting flux that makes it somewhat special

Filamentation of ultrashort laser pulses in silica glass and KDP crystals: A comparative study

International audienceIonizing 800-nm femtosecond laser pulses propagating in silica glass and in potassium dihydrogen phosphate (KDP) crystal are investigated by means of a unidirectional pulse propagation code. Filamentation in fused silica is compared with the self-channeling of light in KDP accounting for the presence of defect states and electron-hole dynamics. In KDP, laser pulses produce intense filaments with higher clamping intensities up to 200 TW/cm² and longer plasma channels with electron densities above 10¹⁶ cm⁻³. Despite these differences, the propagation dynamics in silica and KDP are almost identical at equivalent ratios of input power over the critical power for self-focusing