187 research outputs found
The Galactic Exoplanet Survey Telescope (GEST)
The Galactic Exoplanet Survey Telescope (GEST) will observe a 2 square degree
field in the Galactic bulge to search for extra-solar planets using a
gravitational lensing technique. This gravitational lensing technique is the
only method employing currently available technology that can detect Earth-mass
planets at high signal-to-noise, and can measure the frequency of terrestrial
planets as a function of Galactic position. GEST's sensitivity extends down to
the mass of Mars, and it can detect hundreds of terrestrial planets with
semi-major axes ranging from 0.7 AU to infinity. GEST will be the first truly
comprehensive survey of the Galaxy for planets like those in our own Solar
System.Comment: 17 pages with 13 figures, to be published in Proc. SPIE vol 4854,
"Future EUV-UV and Visible Space Astrophysics Missions and Instrumentation
Laboratory far-infrared spectroscopy of terrestrial sulphides to support analysis of cosmic dust spectra
As an aid in interpreting data from space far-infrared (far-IR) missions, such as the Herschel Space Observatory with its Photodetector Array Camera and Spectrometer, this paper presents spectroscopic studies of selected naturally occurring terrestrial sulphide minerals in the wavelength range 15-250 mu m. The data can also be used to support the return from other, both past and planned, IR space missions, such as the Infrared Space Observatory, Spitzer, SOFIA, SPiCA and Millimetron. In this study, we present far-IR spectra for 11 natural sulphide minerals in the form of dispersed powders of micron particle dimensions. Samples of various sulphides from the American Museum of Natural History mineral collection were selected based on criteria of diversity and potential astrophysical relevancy, based on their identification in Stardust, in stratospheric interplanetary dust particle samples, or in meteorites. Mineral species include digenite, galena, alabandite, sphalerite, wurtzite, covellite, pyrrhotite, pyrite, marcasite, chalcopyrite and stibnite. Most of the sulphides examined possess prominent and characteristic features in the far-IR range. Spectra obtained are compared to those available from previous studies. Far-IR peak frequencies and mass absorption coefficient values are tabulated. Effects of particle size distribution, low temperature, and provenance on IR spectra are demonstrated for selected samples
Infrared surface plasmons on heavily doped silicon
Conductors with infrared plasma frequencies are potentially useful hosts of surface plasmon polaritons (SPP) with sub-wavelength mode confinement for sensing applications. A challenge is to identify such a conductor that also has sharp SPP excitation resonances and the capability to be functionalized for biosensor applications. In this paper we present experimental and theoretical investigations of IR SPPs on doped silicon and their excitation resonances on doped-silicon gratings. The measured complex permittivity spectra for p-type silicon with carrier concentration 6 x 10(19) and 1 x 10(20) cm(-3) show that these materials should support SPPs beyond 11 and 6 mu m wavelengths, respectively. The permittivity spectra were used to calculate SPP mode heights above the silicon surface and SPP propagation lengths. Reasonable merit criteria applied to these quantities suggest that only the heaviest doped material has sensor potential, and then mainly within the wavelength range 6 to 10 mu m. Photon-to-plasmon coupling resonances, a necessary condition for sensing, were demonstrated near 10 mu m wavelength for this material. The shape and position of these resonances agree well with simple analytic calculations based on the theory of Hessel and Oliner (1965)
Dielectric selective mirror for intracavity wavelength selection in far-infrared p-Ge lasers
A robust metal-free intracavity fixed-wavelength selector for the cryogenically cooled far-infrared p-Ge laser is demonstrated. The device is a back mirror consisting of a thin silicon etalon and dielectric SrTiO3 flat. A laser line width of 0.2 cm-1 is achieved, which corresponds to an active cavity finesse of similar to0.15. The wavelength position and spectral purity are maintained over a wide range of laser operating fields. Use of SrTiO3 lowers the laser resonance line frequencies by similar to1 cm-1 compared with expectations for metal mirrors. The effect is due to phase shift, which is determined from far-infrared reflectivity measurements of SrTiO3. A p-Ge laser with such selector is free from danger of electrical breakdown and mirror oxidation during repeatable thermal cycling, which makes it more reliable than previous selection schemes for practical applications
The Gibbs-Thomson formula at small island sizes - corrections for high vapour densities
In this paper we report simulation studies of equilibrium features, namely
circular islands on model surfaces, using Monte-Carlo methods. In particular,
we are interested in studying the relationship between the density of vapour
around a curved island and its curvature-the Gibbs-Thomson formula. Numerical
simulations of a lattice gas model, performed for various sizes of islands,
don't fit very well to the Gibbs-Thomson formula. We show how corrections to
this form arise at high vapour densities, wherein a knowledge of the exact
equation of state (as opposed to the ideal gas approximation) is necessary to
predict this relationship. Exploiting a mapping of the lattice gas to the Ising
model one can compute the corrections to the Gibbs-Thomson formula using high
field series expansions. We also investigate finite size effects on the
stability of the islands both theoretically and through simulations. Finally
the simulations are used to study the microscopic origins of the Gibbs-Thomson
formula. A heuristic argument is suggested in which it is partially attributed
to geometric constraints on the island edge.Comment: 27 pages including 7 figures, tarred, gzipped and uuencoded. Prepared
using revtex and espf.sty. To appear in Phys. Rev.
Temperature dependence of plasmonic terahertz absorption in grating-gate gallium-nitride transistor structures
Strong plasmon resonances have been observed in the terahertz transmission spectra (1-5 THz) of large-area slit-grating-gate AlGaN/GaN-based high-electron-mobility transistor (HEMT) structures at temperatures from 10 to 170 K. The resonance frequencies correspond to the excitation of plasmons with wave vectors equal to the reciprocal lattice vectors of the metal grating, which serves both as a gate electrode for the HEMT and a coupler between plasmons and incident terahertz radiation. Wide tunability of the resonances by the applied gate voltage demonstrates potential of these devices for terahertz applications
Decay of isolated surface features driven by the Gibbs-Thomson effect in analytic model and simulation
A theory based on the thermodynamic Gibbs-Thomson relation is presented which
provides the framework for understanding the time evolution of isolated
nanoscale features (i.e., islands and pits) on surfaces. Two limiting cases are
predicted, in which either diffusion or interface transfer is the limiting
process. These cases correspond to similar regimes considered in previous works
addressing the Ostwald ripening of ensembles of features. A third possible
limiting case is noted for the special geometry of "stacked" islands. In these
limiting cases, isolated features are predicted to decay in size with a power
law scaling in time: A is proportional to (t0-t)^n, where A is the area of the
feature, t0 is the time at which the feature disappears, and n=2/3 or 1. The
constant of proportionality is related to parameters describing both the
kinetic and equilibrium properties of the surface. A continuous time Monte
Carlo simulation is used to test the application of this theory to generic
surfaces with atomic scale features. A new method is described to obtain
macroscopic kinetic parameters describing interfaces in such simulations.
Simulation and analytic theory are compared directly, using measurements of the
simulation to determine the constants of the analytic theory. Agreement between
the two is very good over a range of surface parameters, suggesting that the
analytic theory properly captures the necessary physics. It is anticipated that
the simulation will be useful in modeling complex surface geometries often seen
in experiments on physical surfaces, for which application of the analytic
model is not straightforward.Comment: RevTeX (with .bbl file), 25 pages, 7 figures from 9 Postscript files
embedded using epsf. Submitted to Phys. Rev. B A few minor changes made on
9/24/9
A mid-term astrometric and photometric study of Trans-Neptunian Object (90482) Orcus
From CCD observations of a fixed and large star field that contained the
binary TNO Orcus, we have been able to derive high-precision relative
astrometry and photometry of the Orcus system with respect to background stars.
The RA residuals of an orbital fit to the astrometric data revealed a
periodicity of 9.7+-0.3 days, which is what one would expect to be induced by
the known Orcus companion. The residuals are also correlated with the
theoretical positions of the satellite with regard to the primary. We therefore
have revealed the presence of Orcus' satellite in our astrometric measurements.
The photocenter motion is much larger than the motion of Orcus around the
barycenter, and we show here that detecting some binaries through a carefully
devised astrometric technique might be feasible with telescopes of moderate
size. We also analyzed the system's mid-term photometry to determine whether
the rotation could be tidally locked to the satellite's orbital period. We
found that a photometric variability of 9.7+-0.3 days is clear in our data, and
is nearly coincident with the orbital period of the satellite. We believe this
variability might be induced by the satellite's rotation. There is also a
slight hint for an additional small variability in the 10 hr range that was
already reported in the literature. This short-term variability would indicate
that the primary is not tidally locked and therefore the system would not have
reached a double synchronous state. Implications for the basic physical
properties of the primary and its satellite are discussed. From angular
momentum considerations we suspect that the Orcus satellite might have formed
from a rotational fission. This requires that the mass of the satellite would
be around 0.09 times that of the primary, close to the value that one derives
by using an albedo of 0.12 for the satellite and assuming equal densities for
both objects.Comment: in Press at A&
Quantum cascade laser intracavity absorption spectrometer for trace gas sensing
A mid-infrared intracavity laser absorption spectrometer for trace gas sensing is demonstrated. An external-cavity multi-mode quantum cascade laser with central wavelength 8.0 mu m was combined with a scanning Fabry-Perot interferometer, which analyzed the change of the laser emission spectrum caused by introducing an analyte inside the cavity. The detection mechanism is based on monitoring the laser spectrum dynamics at adiabatically changing laser conditions in long pulse operation mode. Fast acquisition and vapor exchange allow nearly real-time analyte detection. Sensitivity at the level of 1 x 10(-5) cm(-1) was demonstrated based on a weak water vapor absorption line
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