47,638 research outputs found
Inferring telescope polarization properties through spectral lines without linear polarization
We present a technique to determine the polarization properties of a
telescope through observations of spectral lines that have no intrinsic linear
polarization signals. For such spectral lines, any observed linear polarization
must be induced by the telescope optics. We apply the technique to observations
taken with the SPINOR at the DST and demonstrate that we can retrieve the
characteristic polarization properties of the DST at three wavelengths of 459,
526, and 615 nm. We determine the amount of crosstalk between the intensity
Stokes I and the linear and circular polarization states Stokes Q, U, and V,
and between Stokes V and Stokes Q and U. We fit a set of parameters that
describe the polarization properties of the DST to the observed crosstalk
values. The values for the ratio of reflectivities X and the retardance tau
match those derived with the telescope calibration unit within the error bars.
Residual crosstalk after applying a correction for the telescope polarization
stays at a level of 3-10%. We find that it is possible to derive the parameters
that describe the polarization properties of a telescope from observations of
spectral lines without intrinsic linear polarization signal. Such spectral
lines have a dense coverage (about 50 nm separation) in the visible part of the
spectrum (400-615 nm), but none were found at longer wavelengths. Using
spectral lines without intrinsic linear polarization is a promising tool for
the polarimetric calibration of current or future solar telescopes such as
DKIST.Comment: 22 pages, 24 figures, accepted for publication in A&
Arrest of flow and emergence of activated processes at the glass transition of a suspension of particles with hard sphere-like interactions
By combining aspects of the coherent and self intermediate scattering
functions, measured by dynamical light scattering on a suspension of hard
sphere-like particles, we show that the arrest of particle number density
fluctuations spreads from the position of the main structure factor peak.
Taking the velocity auto-correlation function into account we propose that as
density fluctuations are arrested the system's ability to respond to diffusing
momentum currents is impaired and, accordingly, the viscosity increases. From
the stretching of the coherent intermediate scattering function we read a
quantitative manifestation of the undissipated thermal energy, the source of
those, ergodicity restoring, processes that short-circuit the sharp transition
to a perfect glass.Comment: 9 pages, 4 figure
On the Born-Oppenheimer approximation of diatomic molecular resonances
We give a new reduction of a general diatomic molecular Hamiltonian, without
modifying it near the collision set of nuclei. The resulting effective
Hamiltonian is the sum of a smooth semiclassical pseudodifferential operator
(the semiclassical parameter being the inverse of the square-root of the
nuclear mass), and a semibounded operator localised in the elliptic region
corresponding to the nuclear collision set. We also study its behaviour on
exponential weights, and give several applications where molecular resonances
appear and can be well located.Comment: 22 page
A Lloyd-model generalization: Conductance fluctuations in one-dimensional disordered systems
We perform a detailed numerical study of the conductance through
one-dimensional (1D) tight-binding wires with on-site disorder. The random
configurations of the on-site energies of the tight-binding
Hamiltonian are characterized by long-tailed distributions: For large
, with . Our
model serves as a generalization of 1D Lloyd's model, which corresponds to
. First, we verify that the ensemble average is proportional to the length of the wire for all values of
, providing the localization length from . Then, we show that the probability distribution
function is fully determined by the exponent and
. In contrast to 1D wires with standard
white-noise disorder, our wire model exhibits bimodal distributions of the
conductance with peaks at and . In addition, we show that
is proportional to , for , with , in
agreement to previous studies.Comment: 5 pages, 5 figure
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Optimized Multimode Interference Fiber Based Refractometer in A Reflective Interrogation Scheme
A fiber based refractometer in a reflective interrogation scheme is investigated and optimized. A thin gold film was deposited on the tip of a coreless fiber section, which is spliced with a single mode fiber. The coreless fiber is a multimode waveguide, and the observed effects are due to multimode interference. To investigate and optimize the structure, the multimode part of the sensor is built with 3 different lengths: 58 mm, 29 mm and 17 mm. We use a broadband light source ranging from 1475 nm to 1650 nm and we test the sensors with liquids of varying refractive indices, from 1.333 to 1.438. Our results show that for a fixed wavelength, the sensor sensitivity is independent of the multimode fiber length, but we observed a sensitivity increase of approximately 0.7 nm/RIU for a one-nanometer increase in wavelength
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