66,504 research outputs found
Design and construction of an optical test bed for LISA imaging systems and tilt-to-length coupling
The laser interferometer space antenna (LISA) is a future space-based interferometric gravitational-wave detector consisting of three spacecraft in a triangular configuration. The interferometric measurements of path length changes between satellites will be performed on optical benches in the satellites. Angular misalignments of the interfering beams couple into the length measurement and represent a significant noise source. Imaging systems will be used to reduce this tilt-to-length coupling.
We designed and constructed an optical test bed to experimentally investigate tilt-to-length coupling. It consists of two separate structures, a minimal optical bench and a telescope simulator. The minimal optical bench comprises the science interferometer where the local laser is interfered with light from a remote spacecraft. In our experiment, a simulated version of this received beam is generated on the telescope simulator. The telescope simulator provides a tilting beam, a reference interferometer and an additional static beam as a phase reference. The tilting beam can either be a flat-top beam or a Gaussian beam. We avoid tilt-to-length coupling in the reference interferometer by using a small photo diode placed at an image of the beam rotation point. We show that the test bed is operational with an initial measurement of tilt-to-length coupling without imaging systems.
Furthermore, we show the design of two different imaging systems whose performance will be investigated in future experiments
Effect of van der Waals forces on the stacking of coronenes encapsulated in a single-wall carbon nanotube and many-body excitation spectrum
We investigate the geometry, stability, electronic structure and optical
properties of C24H12 coronenes encapsulated in a single-wall (19,0) carbon
nanotube. By an adequate combination of advanced electronic-structure
techniques, involving weak and van derWaals interaction, as well as many-body
effects for establishing electronic properties and excitations, we have
accurately characterized this hybrid carbon nanostructure, which arises as a
promising candidate for opto-electronic nanodevices. In particular, we show
that the structure of the stacked coronenes inside the nanotube is
characterized by a rotation of every coronene with respect to its neighbors
through van derWaals interaction, which is of paramount importance in these
systems. We also suggest a tentative modification of the system in order this
particular rotation to be observed experimentally. A comparison between the
calculated many-body excitation spectrum of the systems involved reveals a
pronounced optical red-shift with respect to the coronene-stacking gas-phase.
The origin of this red-shift is explained in terms of the confinement of the
coronene molecules inside the nanotube, showing an excellent agreement with the
available experimental evidence
Effect of transient pinning on stability of drops sitting on an inclined plane
We report on new instabilities of the quasi-static equilibrium of water drops
pinned by a hydrophobic inclined substrate. The contact line of a statically
pinned drop exhibits three transitions of partial depinning: depinning of the
advancing and receding parts of the contact line and depinning of the entire
contact line leading to the drop's translational motion. We find a region of
parameters where the classical Macdougall-Ockrent-Frenkel approach fails to
estimate the critical volume of the statically pinned inclined drop
Coupling between membrane tilt-difference and dilation: a new ``ripple'' instability and multiple crystalline inclusions phases
A continuum Landau theory for the micro-elasticity of membranes is discussed,
which incorporates a coupling between the bilayer thickness variation and the
difference in the two monolayers' tilts. This coupling stabilizes a new phase
with a rippled micro-structure. Interactions among membrane inclusions combine
a dilation-induced attraction and a tilt-difference-induced repulsion that
yield 2D crystal phases, with possible coexistence of different lattice
spacings for large couplings. Inclusions favoring crystals are those with
either a long-convex or a short-concave hydrophobic core.Comment: EURO LaTeX, 6 pages, 4 figures, to be published in Europhys. Let
Magnetic flux generation and transport in cool stars
The Sun and other cool stars harbouring outer convection zones manifest
magnetic activity in their atmospheres. The connection between this activity
and the properties of a deep-seated dynamo generating the magnetic flux is not
well understood. By employing physical models, we study the spatial and
temporal characteristics of the observable surface field for various stellar
parameters. We combine models for magnetic flux generation, buoyancy
instability, and transport, which encompass the entire convection zone. The
model components are: (1) a thin-layer alpha-Omega dynamo at the base of the
convection zone; (2) buoyancy instabilities and the rise of flux tubes through
the convection zone in 3D, which provides a physically consistent determination
of emergence latitudes and tilt angles; and (3) horizontal flux transport at
the surface. For solar-type stars and rotation periods longer than about 10
days, the latitudinal dynamo waves generated by the deep-seated alpha-Omega
dynamo are faithfully reflected by the surface distribution of magnetic flux.
For rotation periods of the order of two days, however, Coriolis acceleration
of rising flux loops leads to surface flux emergence at much higher latitudes
than the dynamo waves at the bottom of the convection zone reach. A similar
result is found for a K0V star with a rotation period of two days. In the case
of a rapidly rotating K1 subgiant, overlapping dynamo waves lead to noisy
activity cycles and mixed-polarity fields at high latitudes.Comment: 14 pages, 14 figures. Accepted for publication in Astronomy &
Astrophysic
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