25 research outputs found
Design process of a vertical backward facing step experiment for forced- and mixed convection low prandtl number flows
Broadly tunable femtosecond near- and mid-IR source by direct pumping of an OPA with a 41.7 MHz Yb:KGW oscillator
We generate over half a watt of tunable near-IR (1380-1830 nm) and several hundred milliwatts in the mid-IR (2.4-4.2 µm) as well as milliwatt level mid-IR (4.85-9.33 µm) femtosecond radiation by pumping an optical parametric amplifier directly with a 7.4 W Yb:KGW oscillator at 41.7 MHz repetition rate. We use 5 mm PPLN and 2 mm GaSe as downconversion crystals and seed this process by a supercontinuum from a tapered fiber. The system is extremely simple and very stable and could replace more complex OPOs as tunable light sources
Maximum relative height of elastic interfaces in random media
The distribution of the maximal relative height (MRH) of self-affine
one-dimensional elastic interfaces in a random potential is studied. We analyze
the ground state configuration at zero driving force, and the critical
configuration exactly at the depinning threshold, both for the random-manifold
and random-periodic universality classes. These configurations are sampled by
exact numerical methods, and their MRH distributions are compared with those
with the same roughness exponent and boundary conditions, but produced by
independent Fourier modes with normally distributed amplitudes. Using Pickands'
theorem we derive an exact analytical description for the right tail of the
latter. After properly rescaling the MRH distributions we find that corrections
from the Gaussian independent modes approximation are in general small, as
previously found for the average width distribution of depinning
configurations. In the large size limit all corrections are finite except for
the ground-state in the random-periodic class whose MRH distribution becomes,
for periodic boundary conditions, indistinguishable from the Airy distribution.
We find that the MRH distributions are, in general, sensitive to changes of
boundary conditions.Comment: 14 pages, 10 figure
The association between physical activity and healthcare costs in children – results from the GINIplus and LISAplus cohort studies
Wavelength-Dependent Third-Harmonic Generation in Plasmonic Gold Nanoantennas: Quantitative Determination of the d‑Band Influence
Plasmonic
gold nanoantennas are highly efficient nanoscale nonlinear
light converters. The nanoantennas provide large resonant light interaction
cross sections as well as strongly enhanced local fields. The actual
frequency conversion, however, takes places inside the gold volume
and is thus ultimately determined by the microscopic gold nonlinearity,
which has been found to significantly surpass common bulk nonlinear
materials. While the influence of the nanoantenna geometry and hence
the plasmonic resonance has been studied in great detail, only little
attention has been paid to the microscopic material nonlinearity.
Here we show that the microscopic third-order nonlinearity of gold
is in fact a resonant one by virtue of interband transitions between
the d- and sp-bands. Utilizing a large set of resonant nanoantennas
and a fiber-feedback optical parametric oscillator as a broadband-tunable
light source, we show that the radiated third-harmonic signals significantly
increase at the onset of interband transitions, namely, as soon as
the third harmonic becomes resonant with allowed interband transitions.
With the help of an anharmonic oscillator model and independent reference
measurements on a gold film we can unambiguously demonstrate that
the observed third-harmonic increase is related to a strongly wavelength-dependent
microscopic third-order gold nonlinearity, which is additionally underlined
by quantitative agreement between simulation and measurement. This
additional tuning parameter allows further manipulation and optimization
of nonlinear nanoscale systems and thus renders the investigation
of other plasmonic materials, especially with interband transitions
located in the ultraviolet range, highly intriguing