140 research outputs found
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 ~THz. For longer propagation distances, nearly
single-cycle near-infrared pulses with wavelengths around 4.5~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
3D numerical simulations of THz generation by two-color laser filaments
Terahertz (THz) radiation produced by the filamentation of two-color pulses over long distances in argon is
numerically investigated using a comprehensive model in full space-time resolved geometry.
We show that the dominant physical mechanism for THz generation in the filamentation regime at
clamping intensity is based on quasi-dc plasma currents. The calculated THz spectra for different pump
pulse energies and pulse durations are in agreement with previously reported experimental observations.
For the same pulse parameters, near-infrared pump pulses at 2~m are shown to generate a more
than one order of magnitude larger THz yield than pumps centered at 800 nm
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