19,999 research outputs found
Coherent responses of resonance atom layer to short optical pulse excitation
Coherent responses of resonance atom layer to short optical pulse excitation
are numerically considered. The inhomogeneous broadening of one-photon
transition, the local field effect, and the substrate dispersion are involved
into analysis. For a certain intensity of incident pulses a strong coherent
interaction in the form of sharp spikes of superradiation is observed in
transmitted radiation. The Lorentz field correction and the substrate
dispersion weaken the effect, providing additional spectral shifts. Specific
features of photon echo in the form of multiple responses to a double or triple
pulse excitation is discussed.Comment: only PDF,15 page
Hydro-micromechanical modeling of wave propagation in saturated granular media
Biot's theory predicts the wave velocities of a saturated poroelastic
granular medium from the elastic properties, density and geometry of its dry
solid matrix and the pore fluid, neglecting the interaction between constituent
particles and local flow. However, when the frequencies become high and the
wavelengths comparable with particle size, the details of the microstructure
start to play an important role. Here, a novel hydro-micromechanical numerical
model is proposed by coupling the lattice Boltzmann method (LBM) with the
discrete element method (DEM. The model allows to investigate the details of
the particle-fluid interaction during propagation of elastic waves While the
DEM is tracking the translational and rotational motion of each solid particle,
the LBM can resolve the pore-scale hydrodynamics. Solid and fluid phases are
two-way coupled through momentum exchange. The coupling scheme is benchmarked
with the terminal velocity of a single sphere settling in a fluid. To mimic a
pressure wave entering a saturated granular medium, an oscillating pressure
boundary condition on the fluid is implemented and benchmarked with
one-dimensional wave equations. Using a face centered cubic structure, the
effects of input waveforms and frequencies on the dispersion relations are
investigated. Finally, the wave velocities at various effective confining
pressures predicted by the numerical model are compared with with Biot's
analytical solution, and a very good agreement is found. In addition to the
pressure and shear waves, slow compressional waves are observed in the
simulations, as predicted by Biot's theory.Comment: Manuscript submitted to International Journal for Numerical and
Analytical Methods in Geomechanic
Carrier field shock formation of long wavelength femtosecond pulses in dispersive media
We numerically demonstrate the formation of carrier field shocks in various
dispersive media for a wide variety of input conditions using two different
electric field propagation models. In addition, an investigation of the impact
of numerous physical effects on carrier wave shock is performed. It is shown
that in many cases a field shock is essentially unavoidable and therefore
extremely important in the propagation of intense long wavelength pulses in
weakly dispersive nonlinear media such as noble gases, air, and single-crystal
diamond. The results presented here are expected to have a significant impact
in the field of ultrashort nonlinear optics, attosecond pulse generation, and
wavepacket synthesis where the use of mid-IR wavelengths is becoming
increasingly more important.Comment: 14 pages, 17 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
Ultrashort filaments of light in weakly-ionized, optically-transparent media
Modern laser sources nowadays deliver ultrashort light pulses reaching few
cycles in duration, high energies beyond the Joule level and peak powers
exceeding several terawatt (TW). When such pulses propagate through
optically-transparent media, they first self-focus in space and grow in
intensity, until they generate a tenuous plasma by photo-ionization. For free
electron densities and beam intensities below their breakdown limits, these
pulses evolve as self-guided objects, resulting from successive equilibria
between the Kerr focusing process, the chromatic dispersion of the medium, and
the defocusing action of the electron plasma. Discovered one decade ago, this
self-channeling mechanism reveals a new physics, widely extending the frontiers
of nonlinear optics. Implications include long-distance propagation of TW beams
in the atmosphere, supercontinuum emission, pulse shortening as well as
high-order harmonic generation. This review presents the landmarks of the
10-odd-year progress in this field. Particular emphasis is laid to the
theoretical modeling of the propagation equations, whose physical ingredients
are discussed from numerical simulations. Differences between femtosecond
pulses propagating in gaseous or condensed materials are underlined. Attention
is also paid to the multifilamentation instability of broad, powerful beams,
breaking up the energy distribution into small-scale cells along the optical
path. The robustness of the resulting filaments in adverse weathers, their
large conical emission exploited for multipollutant remote sensing, nonlinear
spectroscopy, and the possibility to guide electric discharges in air are
finally addressed on the basis of experimental results.Comment: 50 pages, 38 figure
Research on nonlinear and quantum optics at the photonics and quantum information group of the University of Valladolid
We outline the main research lines in Nonlinear and Quantum Optics of the Group of Photonics and Quantum Information at the University of Valladolid. These works focus on Optical Solitons, Quantum Information using Photonic Technologies and the development of new materials for Nonlinar Optics. The investigations on optical solitons cover both temporal solitons in dispersion managed fiber links and nonparaxial spatial solitons as described by the Nonlinear Helmholtz Equation. Within the Quantum Information research lines of the group, the studies address new photonic schemes for quantum computation and the multiplexing of quantum data. The investigations of the group are, to a large extent, based on intensive and parallel computations. Some associated numerical techniques for the development of the activities described are briefly sketched
Designing microstructured polymer optical fibers for cascaded quadratic soliton compression of femtosecond pulses
The dispersion of index-guiding microstructured polymer optical fibers is
calculated for second-harmonic generation. The quadratic nonlinearity is
assumed to come from poling of the polymer, which in this study is chosen to be
the cyclic olefin copolymer Topas. We found a very large phase mismatch between
the pump and the second-harmonic waves. Therefore the potential for cascaded
quadratic second-harmonic generation is investigated in particular for soliton
compression of fs pulses. We found that excitation of temporal solitons from
cascaded quadratic nonlinearities requires an effective quadratic nonlinearity
of 5 pm/V or more. This might be reduced if a polymer with a low Kerr nonlinear
refractive index is used. We also found that the group-velocity mismatch could
be minimized if the design parameters of the microstructured fiber are chosen
so the relative hole size is large and the hole pitch is on the order of the
pump wavelength. Almost all design-parameter combinations resulted in cascaded
effects in the stationary regime, where efficient and clean soliton compression
can be found. We therefore did not see any benefit from choosing a fiber design
where the group-velocity mismatch was minimized. Instead numerical simulations
showed excellent compression of nm 120 fs pulses with nJ pulse
energy to few-cycle duration using a standard endlessly single-mode design with
a relative hole size of 0.4.Comment: 11 pages, 8 figures, submitted to JOSA
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