5,683 research outputs found
A high resolution spectral atlas of brown dwarfs
We present a UVES/VLT high resolution atlas of three L dwarfs and one T dwarf
system, spectral classes at which most of the objects are brown dwarfs. Our
atlas covers the optical region from H up to the near infrared at 1
m. We present spectral details of ultra-cool atmospheres at very high
resolution () and compare the spectra to model calculations. Our
comparison shows that molecular features from VO and CaH, and atomic features
from Cs and Rb are reasonably well fit by current models. On the other hand,
features due to TiO, CrH, and water, and atomic Na and K reveal large
discrepancies between model calculations and our observations.Comment: 17 pages, 11 figures, accepted by A&A, reduced figure quality for
arXi
Optimal control of light propagation through multiple-scattering media in the presence of noise
We study the control of coherent light propagation through
multiple-scattering media in the presence of measurement noise. In our
experiments, we use a two-step optimization procedure to find the optimal
incident wavefront. We conclude that the degree of optimal control of coherent
light propagation through a multiple-scattering medium is only determined by
the number of photoelectrons detected per single speckle spot. The prediction
of our model agrees well with the experimental results. Our results offer
opportunities for imaging applications through scattering media such as
biological tissue in the shot noise limit
Light propagation and emission in complex photonic media
We provide an introduction to complex photonic media, that is, composite
materials with spatial inhomogeneities that are distributed over length scales
comparable to or smaller than the wavelength of light. This blossoming field is
firmly rooted in condensed matter physics, in optics, and in materials science.
Many stimulating analogies exist with other wave phenomena such as sound and
seismology, X-rays, neutrons. The field has a rich history, which has led to
many applications in lighting, novel lasers, light harvesting, microscopy, and
bio optics. We provide a brief overview of complex photonic media with
different classes of spatial order, varying from completely random to
long-periodically ordered structures, quasi crystalline and aperiodic
structures, and arrays of cavities. In addition to shaping optical waves by
suitable photonic nanostructures, the realization is quickly arising that the
spatial shaping of optical wavefronts with spatial light modulators
dramatically increases the number of control parameters. As a result, it is
becoming possible for instance to literally see through completely opaque
complex media. We discuss a unified view of complex photonic media by means of
a photonic interaction strength parameter. This parameter gauges the
interaction of light with any complex photonic medium, and allows to compare
complex media from different classes for similar applications.Comment: 8 pages, 2 figures, Light Localisation and Lasing: Random and
Quasi-Random Photonic Structures, Eds. M. Ghulinyan and L. Pavesi, (Cambridge
Univ. Press, Cambridge, 2015) Ch. 1, p.
Development of sputtered techniques for thrust chambers
The 0.152 cm thick sputtered and copper deposits were electron beam welded to wrought copper. Tensile specimens were machined from the weld assemblies and tested at room temperature. Tensile strength approached the strength of wrought material. Elongations up to 25% were measured. Sputtered aluminum was used to fill 0.157 cm wide by 0.127 cm deep grooves in thrust chamber spool piece liners. The liners were closed out by sputtering copper from post and hollow cathodes
Numerical simulations of surface convection in a late M-dwarf
Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD)
simulations of time-dependent compressible convection, we have studied the
dynamics and thermal structure of the convective surface layers of a
prototypical late-type M-dwarf (Teff~2800K log(g)=5.0, solar chemical
composition). The RHD models predict stellar granulation qualitatively similar
to the familiar solar pattern. Quantitatively, the granular cells show a
convective turn-over time scale of ~100s, and a horizontal scale of 80km; the
relative intensity contrast of the granular pattern amounts to 1.1%, and
root-mean-square vertical velocities reach 240m/s at maximum. Deviations from
radiative equilibrium in the higher, formally convectively stable atmospheric
layers are found to be insignificant allowing a reliable modeling of the
atmosphere with 1D standard model atmospheres. A mixing-length parameter of
alpha=2.1 provides the best representation of the average thermal structure of
the RHD model atmosphere while alternative values are found when fitting the
asymptotic entropy encountered in deeper layers of the stellar envelope
alpha=1.5, or when matching the vertical velocity field alpha=3.5. The close
correspondence between RHD and standard model atmospheres implies that
presently existing discrepancies between observed and predicted stellar colors
in the M-dwarf regime cannot be traced back to an inadequate treatment of
convection in the 1D standard models. The RHD models predict a modest extension
of the convectively mixed region beyond the formal Schwarzschild stability
boundary which provides hints for the distribution of dust grains in cooler
(brown dwarf) atmospheres.Comment: 19 pages, 16 figures, accepted for publication in A&
Superpixel-based spatial amplitude and phase modulation using a digital micromirror device
We present a superpixel method for full spatial phase and amplitude control
of a light beam using a digital micromirror device (DMD) combined with a
spatial filter. We combine square regions of nearby micromirrors into
superpixels by low pass filtering in a Fourier plane of the DMD. At each
superpixel we are able to independently modulate the phase and the amplitude of
light, while retaining a high resolution and the very high speed of a DMD. The
method achieves a measured fidelity for a target field with fully
independent phase and amplitude at a resolution of pixels per
diffraction limited spot. For the LG orbital angular momentum mode the
calculated fidelity is , using DMD pixels. The
superpixel method reduces the errors when compared to the state of the art Lee
holography method for these test fields by and , with a comparable
light efficiency of around . Our control software is publicly available.Comment: 9 pages, 6 figure
Pathlengths of open channels in multiple scattering media
We report optical measurements of the spectral width of open transmission
channels in a three-dimensional diffusive medium. The light transmission
through a sample is enhanced by efficiently coupling to open transmission
channels using repeated digital optical phase conjugation. The spectral
properties are investigated by enhancing the transmission, fixing the incident
wavefront and scanning the wavelength of the laser. We measure the transmitted
field to extract the field correlation function and the enhancement of the
total transmission. We find that optimizing the total transmission leads to a
significant increase in the frequency width of the field correlation function.
Additionally we find that the enhanced transmission persists over an even
larger frequency bandwidth. This result shows open channels in the diffusive
regime are spectrally much wider than previous measurements in the localized
regime suggest
Evolution of low-mass stars and substellar objects. Contribution to the Galactic mass budget
We briefly summarize our present knowledge of the theory of low-mass stars
and substellar objects and their contribution to the Galactic population.Comment: 9 pages, Latex file, Invited Review VLT Opening Symposium,
Antofagasta (march 1999
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