834 research outputs found
High dispersive and monolithic 100% efficiency grisms
We present a type of grism, a series combination of transmission grating and
prism, in which we reduce the number of diffraction orders and achieve a
configuration with very high angular dispersion. The grism can be fabricated
from a single dielectric material and requires no metallic or dielectric film
layers for high transmission diffraction efficiency. One can reach 100% in the
-1st transmission diffraction order and the equal damage threshold as the
dielectric bulk material. We realized such an element in fused silica with an
efficiency of more then 99%. The bevel backside reflection is reduced by a
statistical antireflective structure, so we measured an efficiency of the
entire grism of 95% at a single wavelength
Demonstration of 3-port grating phase relations
We experimentally demonstrate the phase relations of 3-port gratings by
investigating 3-port coupled Fabry-Perot cavities. Two different gratings which
have the same 1st order diffraction efficiency but differ substantially in
their 2nd order diffraction efficiency have been designed and manufactured.
Using the gratings as couplers to Fabry-Perot cavities we could validate the
results of an earlier theoretical description of the phases at a three port
grating
A Super-Earth caught in a trap
This paper is an extension of the work done by Pierens & Nelson (2008) in
which they have investigated the behaviour of a two-planet system embedded in a
protoplanetary disc. They have put a Jupiter mass gas giant on the internal
orbit and a lower mass planet on the external one. We consider here a similar
problem taking into account a gas giant with masses in the range of 0.5 to 1
Jupiter mass and a Super-Earth as the outermost planet. By changing disc
parameters and planet masses we have succeeded in getting the convergent
migration which allows for the possibility of their resonant locking. However,
in the case in which the gas giant has the mass of Jupiter, before any mean
motion first order commensurability could be achieved, the Super-Earth is
caught in a trap when it is very close to the edge of the gap opened by the
giant planet. This confirms the result obtained by Pierens & Nelson (2008) in
their simulations. Additionally, we have found that, in a very thin disc, an
apsidal resonance is observed in the system if the Super-Earth is captured in
the trap. Moreover, the eccentricity of the small planet remains low, while
that of the gas giant increases slightly due to the imbalance between Lindblad
and corotational resonances. We have also studied analogous systems in which
the gas giant is allowed to take Sub-Jupiter masses. In this case, after
performing an extensive survey over all possible parameters, we have succeeded
in getting the 1:2 mean motion resonant configuration only in a disc with low
aspect ratio and low surface density. However, the resonance is maintained just
for few thousand orbits. Thus, we conclude that for typical protoplanetary
discs the mean motion commensurabilities are rare if the Super-Earth is located
on the external orbit relative to the gas giant. (abridged)Comment: 10 pages, 11 figures, accepted for publication in MNRA
Rapid Detection of Avian Eimeria Species Using Denaturing Gradient Gel Electrophoresis
A denaturing gradient gel electrophoresis (DGGE) assay was developed to rapidly discriminate species of avian Eimeria. Amplification by PCR of the small subunit ribosomal RNA gene (approximately 1,600 nucleotides) with Eimeria genus-specific primers followed by cloning and sequencing allowed us to carry out phylogenetic analyses and identify clone sequences to species level in most cases. Clones were subsequently used to amplify a smaller fragment (approximately 120 nucleotides) suitable for DGGE. The fragments were separated on denaturing gradient gel and bands with unique migration distances were mixed to obtain an identification ladder. The identification ladder and PCR products obtained from DNA extracted from fecal samples from several poultry farms were compared. Applying the DGGE method in this study allowed a rapid differentiation of Eimeria species present in fecal samples collected from poultry farms
Asymmetric transmission of linearly polarized light at optical metamaterials
We experimentally demonstrate a three-dimensional chiral optical metamaterial
that exhibits an asymmetric transmission for forwardly and backwardly
propagating linearly polarized light. The observation of this novel effect
requires a metamaterial composed of three-dimensional chiral metaatoms without
any rotational symmetry. Our analysis is supported by a systematic
investigation of the transmission matrices for arbitrarily complex, lossy media
that allows deriving a simple criterion for asymmetric transmission in an
arbitrary polarization base. Contrary to physical intuition, in general the
polarization eigenstates in such three-dimensional and low-symmetry
metamaterials do not obey fxed relations and the associated transmission
matrices cannot be symmetrized
Superpositions of the Orbital Angular Momentum for Applications in Quantum Experiments
Two different experimental techniques for preparation and analyzing
superpositions of the Gaussian and Laguerre-Gassian modes are presented. This
is done exploiting an interferometric method on the one hand and using computer
generated holograms on the other hand. It is shown that by shifting the
hologram with respect to an incoming Gaussian beam different superpositions of
the Gaussian and the Laguerre-Gaussian beam can be produced. An analytical
expression between the relative phase and the amplitudes of the modes and the
displacement of the hologram is given. The application of such orbital angular
momenta superpositions in quantum experiments such as quantum cryptography is
discussed.Comment: 18 pages, 4 figures. to appear in Journal of Optics
Identifying Deficiencies of Standard Accretion Disk Theory: Lessons from a Mean-Field Approach
Turbulent viscosity is frequently used in accretion disk theory to replace
the microphysical viscosity in order to accomodate the observational need for
in- stabilities in disks that lead to enhanced transport. However, simply
replacing the microphysical transport coefficient by a single turbulent
transport coeffi- cient hides the fact that the procedure should formally arise
as part of a closure in which the hydrodynamic or magnetohydrodynamic equations
are averaged, and correlations of turbulent fluctuations are replaced by
transport coefficients. Here we show how a mean field approach leads quite
naturally two transport coefficients, not one, that govern mass and angular
momentum transport. In particular, we highlight that the conventional approach
suffers from a seemingly inconsistent neglect of turbulent diffusion in the
surface density equation. We constrain these new transport coefficients for
specific cases of inward, outward, and zero net mass transport. In addition, we
find that one of the new transport terms can lead to oscillations in the mean
surface density which then requires a constant or small inverse Rossby number
for disks to maintain a monotonic power-law surface density.Comment: 11 page
3D-MHD simulations of an accretion disk with star-disk boundary layer
We present global 3D MHD simulations of geometrically thin but unstratified
accretion disks in which a near Keplerian disk rotates between two bounding
regions with initial rotation profiles that are stable to the MRI. The inner
region models the boundary layer between the disk and an assumed more slowly
rotating central, non magnetic star. We investigate the dynamical evolution of
this system in response to initial vertical and toroidal fields imposed in a
variety of domains contained within the near Keplerian disk. Cases with both
non zero and zero net magnetic flux are considered and sustained dynamo
activity found in runs for up to fifty orbital periods at the outer boundary of
the near Keplerian disk. Simulations starting from fields with small radial
scale and with zero net flux lead to the lowest levels of turbulence and
smoothest variation of disk mean state variables. For our computational set up,
average values of the Shakura & Sunyaev (1973) parameter in the
Keplerian disk are typically Magnetic field eventually always
diffuses into the boundary layer resulting in the build up of toroidal field
inward angular momentum transport and the accretion of disk material. The mean
radial velocity, while exhibiting large temporal fluctuations is always
subsonic. Simulations starting with net toroidal flux may yield an average
While being characterized by one order of magnitude larger
average , simulations starting from vertical fields with large radial
scale and net flux may lead to the formation of persistent non-homogeneous,
non-axisymmetric magnetically dominated regions of very low density.Comment: Accepted for publication in Ap
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