354 research outputs found
Effect of input pulse chirp on nonlinear energy deposition and plasma excitation in water
We analyze numerically and experimentally the effect of the input pulse chirp
on the nonlinear energy deposition from J fs-pulses at nm to
water. Numerical results are also shown for pulses at nm, where linear
losses are minimized, and for different focusing geometries. Input chirp is
found to have a big impact on the deposited energy and on the plasma
distribution around focus, thus providing a simple and effective mechanism to
tune the electron density and energy deposition. We identify three relevant
ways in which plasma features may be tuned.Comment: 9 pages, 7 figure
Propagating Pattern Selection and Causality Reconsidered
International audiencePattern selection, occurring when a nonuniform state of a nonlinear dissipative system propagates into an initially unstable, homogeneous basic state is reconsidered by application of the causality principle. In particular, the nonlinear marginal stability criterion that determines the selection of a nonlinear front solution is replaced by an exact general necessary condition that has never been considered before. The demonstration is based on the causal signaling problem derived in the context of plasma physics
On the 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. Sub-diffractive 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 spatiotemporal energy density flux that exhibits the same
features as stationary, polychromatic Bessel beam, thus highlighting the
physical nature of the light bullets
Nonlinear photoionization of transparent solids: a nonperturbative theory obeying selection rules
We provide a nonperturbative theory for photoionization of transparent
solids. By applying a particular steepest-descent method, we derive analytical
expressions for the photoionization rate within the two-band structure model,
which consistently account for the related to the parity of
the number of absorbed photons ( or ). We demonstrate the crucial
role of the interference of the transition amplitudes (saddle-points), which in
the semi-classical limit, can be interpreted in terms of interfering quantum
trajectories. Keldysh's foundational work of laser physics [Sov. Phys. JETP 20,
1307 (1965)] disregarded this interference, resulting in the violation of
. We provide an improved Keldysh photoionization theory and
show its excellent agreement with measurements for the frequency dependence of
the two-photon absorption and nonlinear refractive index coefficients in
dielectrics
X and Y waves in the spatiotemporal Kerr dynamics of a self-guided light beam
The nonlinear stage of development of the spatiotemporal instability of the
monochromatic Townes beam in a medium with self-focusing nonlinearity and
normal dispersion is studied by analytical and numerical means. Small
perturbations to the self-guided light beam are found to grow into two giant,
splitting Y pulses featuring shock fronts on opposite sides. Each shocking
pulse amplifies a co-propagating X wave, or dispersion- and diffraction-free
linear wave mode of the medium, with super-broad spectrum.Comment: 9 pages, 9 figure
Phase-Insensitive Scattering of Terahertz Radiation
The nonlinear interaction between Near-Infrared (NIR) and Terahertz pulses is
principally investigated as a means for the detection of radiation in the
hardly accessible THz spectral region. Most studies have targeted second-order
nonlinear processes, given their higher efficiencies, and only a limited number
have addressed third-order nonlinear interactions, mainly investigating
four-wave mixing in air for broadband THz detection. We have studied the
nonlinear interaction between THz and NIR pulses in solid-state media
(specifically diamond), and we show how the former can be frequency-shifted up
to UV frequencies by the scattering from the nonlinear polarisation induced by
the latter. Such UV emission differs from the well-known electric-field-induced
second harmonic (EFISH) one, as it is generated via a phase-insensitive
scattering, rather than a sum- or difference-frequency four-wave-mixing
process
Light-filament dynamics and the spatiotemporal instability of the Townes profile
The origin of the spatiotemporal filament dynamics of ultrashort pulses in nonlinear media, including axial-conical emission coupling, temporal splitting, and X waves, is explained by the spatiotemporal instability
of spatially localized nonlinear modes. Our experiments support this interpretation
Effect of light polarization on plasma distribution and filament formation
We show that, for 200 fs light pulses at 790 nm, the formation of filaments
is strongly affected by the laser light polarization . Filamentation does not
exist for a pure circularly polarized light, propagating in vacuum before
focusing in air, while there is no difference for focusing the light in air or
vacuum for linearly polarized light.Comment: 4pages 2 figure
Filamentation in Kerr media from pulsed Bessel beams
In contrast with filamentation of ultrashort laser pulses with standard Gaussian beams in Kerr media, three different types of Bessel filaments are obtained in air or in water by focusing ultrashort laser pulses with an axicon. We thoroughly investigate the different regimes and show that the beam reshapes as a nonlinear Bessel beam which establishes a conical energy flux from the low intensity tails toward the high intensity peak. This flux efficiently sustains a high contrast long-distance propagation and easily generates a continuous plasma channel in air
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