85 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
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
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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 spacetime 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
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
Self-pinching of pulsed laser beams in a plasma filament
Competing nonlinear optical effects that act on femtosecond laser pulses propagating in a self-generated plasma filament may give rise to a pronounced radial deformation of the beam, similar to the z-pinch contraction of pulsed high-current discharges. This self-pinching locally increases the photon density. The process is further identified as the first stage in the recently observed self-compression of femtosecond laser pulses propagating in filaments. Self-pinching also explains the complicated spatio-temporal shapes generally observed in filament compression experiments
THz and far-infrared radiation from ionizing multi-color pulses
International audienceResearch on intense terahertz (THz) electromagnetic sources has received an increasing attention owing to numerous applications, for example, in time-domain spectroscopy, biomedical imaging or security screening. Here, THz emission in gases via ionizing multi-color femtosecond pulses is analyzed by means of semi-analytical models and finite-difference-time-domain simulations in 1D and 2D geometries. 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. Moreover, we show that produced THz signals interact with free electron trajectories andthus 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 in forward direction. Diffraction limits the achievable THz bandwidth by efficiently depleting the low frequency amplitudes in the propagating field
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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 is thus almost not affected by the opacity of the plasma, in excellent agreement with results obtained from a unidirectional pulse propagation model
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
Tailoring THz radiation by controlling tunnel photoionization events in gases
Applications ranging from nonlinear terahertz spectroscopy to remote sensing require broadband and intense THz radiation which can be generated by focusing two-color laser pulses into a gas. In this setup, THz radiation originates from the buildup of the electron density in sharp steps of attosecond duration due to tunnel ionization, and subsequent acceleration of free electrons in the laser field. We show that the spectral shape of the THz pulses generated by this mechanism is determined by superposition of contributions from individual ionization events. This provides a straightforward analogy with linear diffraction theory, where the ionization events play the role of slits in a grating. This analogy offers simple explanations for recent experimental observations and opens new avenues for THz pulse shaping based on temporal control of the ionization events. We illustrate this novel technique by tailoring the spectral width and position of the resulting radiation using multi-color pump pulses
Filamentary pulse self-compression: The impact of the cell windows
Self-compression of multi-millijoule laser pulses during filamentary propagation is usually explained by the interplay of self-focusing and defocusing effects, causing a substantial concentration of energy on the axis of the propagating optical pulse. Recently, it has been argued that cell windows may play a decisive role in the self-compression mechanism. As such windows have to be used for media other than air their presence is often unavoidable, yet they present a sudden non-adiabatic change in dispersion and nonlinearity that should lead to a destruction of the temporal and spatial integrity of the light bullets generated in the self-compression mechanism. We now experimentally prove that there is in fact a self-healing mechanism that helps to overcome the potentially destructive consequences of the cell windows. We show in two carefully conducted experiments that the cell window position decisively influences activation or inhibition of the self-healing mechanism. A comparison with a windowless cell shows that presence of this mechanism is an important prerequisite for the exploitation of self-compression effects in windowed cells filled with inert gases
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