170 research outputs found
Multiple-octave spanning mid-IR supercontinuum generation in bulk quadratic nonlinear crystals
Bright and broadband coherent mid-IR radiation is important for exciting and
probing molecular vibrations. Using cascaded nonlinearities in conventional
quadratic nonlinear crystal like lithium niobate, self-defocusing near-IR
solitons have been demonstrated that led to very broadband supercontinuum
generation in the visible, near-IR and short-wavelength mid-IR. Here we conduct
an experiment where a mid-IR crystal pumped in the mid-IR gives multiple-octave
spanning supercontinua. The crystal is cut for noncritical interaction, so the
three-wave mixing of a single mid-IR femtosecond pump source leads to highly
phase-mismatched second-harmonic generation. This self-acting cascaded process
leads to the formation of a self-defocusing soliton at the mid-IR pump
wavelength and after the self-compression point multiple octave-spanning
supercontinua are observed (covering 1.6-m). The results were recorded
in a commercially available crystal LiInS pumped in the 3-m range,
but other mid-IR crystals can readily be used as well.Comment: submitted to APL Photonic
Ultrafast nonlinear dynamics of thin gold films due to an intrinsic delayed nonlinearity
Using long-range surface plasmon polaritons light can propagate in metal
nano-scale waveguides for ultracompact opto-electronic devices. Gold is an
important material for plasmonic waveguides, but although its linear optical
properties are fairly well understood, the nonlinear response is still under
investigation. We consider propagation of pulses in ultrathin gold strip
waveguides, modeled by the nonlinear Schr\"odinger equation. The nonlinear
response of gold is accounted for by the two-temperature model, revealing it as
a delayed nonlinearity intrinsic in gold. The consequence is that the measured
nonlinearities are strongly dependent on pulse duration. This issue has so far
only been addressed phenomenologically, but we provide an accurate estimate of
the quantitative connection as well as a phenomenological theory to understand
the enhanced nonlinear response as the gold thickness is reduced. In comparison
with the previous works, the analytical model for the power-loss equation has
been improved, and can be applied now to cases with a high laser peak power. We
show new fits to experimental data from literature and provide updated values
for the real and imaginary part of the nonlinear susceptibility of gold for
various pulse durations and gold layer thicknesses. Our simulations show that
the nonlinear loss is inhibiting efficient nonlinear interaction with low-power
laser pulses. We therefore propose to design waveguides suitable for the
mid-IR, where the ponderomotive instantaneous nonlinearity can dominate over
the delayed hot-electron nonlinearity and provide a suitable plasmonics
platform for efficient ultrafast nonlinear optics.Comment: J. Opt., in pres
Experimental observation of long-wavelength dispersive wave generation induced by self-defocusing nonlinearity in BBO crystal
We experimentally observe long-wavelength dispersive waves generation in a
BBO crystal. A soliton was formed in normal GVD regime of the crystal by a
self-defocusing and negative nonlinearity through phase-mismatched quatradic
interaction. Strong temporal pulse compression confirmed the formation of
soliton during the pulse propagation inside the crystal. Significant dispersive
wave radiation was measured in the anomalous GVD regime of the BBO crystal.
With the pump wavelengths from 1.24 to 1.4 m, tunable dispersive waves are
generated around 1.9 to 2.2 m. The observed dispersive wave generation is
well understood by simulations.Comment: in preparatio
On type I cascaded quadratic soliton compression in lithium niobate: Compressing femtosecond pulses from high-power fiber lasers
The output pulses of a commercial high-power femtosecond fiber laser or
amplifier are typically around 300-500 fs with a wavelength around 1030 nm and
10s of J pulse energy. Here we present a numerical study of cascaded
quadratic soliton compression of such pulses in LiNbO using a type I phase
matching configuration. We find that because of competing cubic material
nonlinearities compression can only occur in the nonstationary regime, where
group-velocity mismatch induced Raman-like nonlocal effects prevent compression
to below 100 fs. However, the strong group velocity dispersion implies that the
pulses can achieve moderate compression to sub-130 fs duration in available
crystal lengths. Most of the pulse energy is conserved because the compression
is moderate. The effects of diffraction and spatial walk-off is addressed, and
in particular the latter could become an issue when compressing in such long
crystals (around 10 cm long). We finally show that the second harmonic contains
a short pulse locked to the pump and a long multi-ps red-shifted detrimental
component. The latter is caused by the nonlocal effects in the nonstationary
regime, but because it is strongly red-shifted to a position that can be
predicted, we show that it can be removed using a bandpass filter, leaving a
sub-100 fs visible component at nm with excellent pulse quality.Comment: 14 pages, 10 figures, 1 table, submitted to PR
The anisotropic Kerr nonlinear refractive index of the beta-barium borate (\beta-BaB2O4) nonlinear crystal
We study the anisotropic nature of the Kerr nonlinear response in a
beta-barium borate (\beta-BaB2O4, BBO) nonlinear crystal. The focus is on
determining the relevant cubic tensor components that affect
interaction of type I cascaded second-harmonic generation. Various experiments
in the literature are analyzed and we correct the data from some of the
experiments for contributions from cascading as well as for updated material
parameters. We find that the Kerr nonlinear tensor component responsible for
self-phase modulation in cascading is considerably larger than what has been
used to date. We evaluate the impact of using such a cubic anisotropic response
in ultrafast cascading experiments.Comment: Updated version, comments on experiments from the literature welcom
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
Poor-man's model of hollow-core anti-resonant fibers
We investigate various methods for extending the simple analytical capillary
model to describe the dispersion and loss of anti-resonant hollow-core fibers
without the need of detailed finite-element simulations across the desired
wavelength range. This poor-man's model can with a single fitting parameter
quite accurately mimic dispersion and loss resonances and anti-resonances from
full finite-element simulations. Due to the analytical basis of the model it is
easy to explore variations in core size and cladding wall thickness, and should
therefore provide a valuable tool for numerical simulations of the ultrafast
nonlinear dynamics of gas-filled hollow-core fibers.Comment: In preparatio
Directional supercontinuum generation: the role of the soliton
In this paper we numerically study supercontinuum generation by pumping a
silicon nitride waveguide, with two zero-dispersion wavelengths, with
femtosecond pulses. The waveguide dispersion is designed so that the pump pulse
is in the normal-dispersion regime. We show that because of self-phase
modulation, the initial pulse broadens into the anomalous-dispersion regime,
which is sandwiched between the two normal-dispersion regimes, and here a
soliton is formed. The interaction of the soliton and the broadened pulse in
the normal-dispersion regime causes additional spectral broadening through
formation of dispersive waves by non-degenerate four-wave mixing and
cross-phase modulation. This broadening occurs mainly towards the second
normal-dispersion regime. We show that pumping in either normal-dispersion
regime allows broadening towards the other normal-dispersion regime. This
ability to steer the continuum extension towards the direction of the other
normal-dispersion regime beyond the sandwiched anomalous-dispersion regime
underlies the directional supercontinuum notation. We numerically confirm the
approach in a standard silica microstructured fiber geometry with two
zero-dispersion wavelengths
- …