30,724 research outputs found
Plasmon resonances in coupled Babinet complementary arrays in the mid-infrared range
A plasmonic structure with transmission highly tunable in the mid-infrared spectral range is developed. This structure consists of a hexagonal array of metallic discs located on top of silicon pillars protruding through holes in a metallic Babinet complementary film. We reveal with FDTD simulations that changing the hole diameter tunes the main plasmonic resonance frequency of this structure throughout the infrared range. Due to the underlying Babinet physics of these coupled arrays, the spectral width of these plasmonic resonances is strongly reduced, and the higher harmonics are suppressed. Furthermore, we demonstrate that this structure can be easily produced by a combination of the nanosphere lithography and the metal-assisted chemical etching technique
Combining high-dispersion spectroscopy (HDS) with high contrast imaging (HCI): Probing rocky planets around our nearest neighbors
Aims: In this work, we discuss a way to combine High Dispersion Spectroscopy
and High Contrast Imaging (HDS+HCI). For a planet located at a resolvable
angular distance from its host star, the starlight can be reduced up to several
orders of magnitude using adaptive optics and/or coronography. In addition, the
remaining starlight can be filtered out using high-dispersion spectroscopy,
utilizing the significantly different (or Doppler shifted) high-dispersion
spectra of the planet and star. In this way, HDS+HCI can in principle reach
contrast limits of ~1e-5 x 1e-5, although in practice this will be limited by
photon noise and/or sky-background.
Methods: We present simulations of HDS+HCI observations with the E-ELT, both
probing thermal emission from a planet at infrared wavelengths, and starlight
reflected off a planet atmosphere at optical wavelengths. For the infrared
simulations we use the baseline parameters of the E-ELT and METIS instrument,
with the latter combining extreme adaptive optics with an R=100,000 IFS. We
include realistic models of the adaptive optics performance and atmospheric
transmission and emission. For the optical simulation we also assume R=100,000
IFS with adaptive optics capabilities at the E-ELT.
Results: One night of HDS+HCI observations with the E-ELT at 4.8 um (d_lambda
= 0.07 um) can detect a planet orbiting alpha Cen A with a radius of R=1.5
R_earth and a twin-Earth thermal spectrum of T_eq=300 K at a signal-to-noise
(S/N) of 5. In the optical, with a Strehl ratio performance of 0.3, reflected
light from an Earth-size planet in the habitable zone of Proxima Centauri can
be detected at a S/N of 10 in the same time frame. Recently, first HDS+HCI
observations have shown the potential of this technique by determining the
spin-rotation of the young massive exoplanet beta Pictoris b. [abridged]Comment: 9 pages, A&A in press: A movie of the simulation can be found at
http://www.strw.leidenuniv.nl/~snellen/simulation.mpe
SOWAT: Speckle Observations With Alleviated Turbulence
Adaptive optics (AO) systems and image reconstruction algorithms are
indispensable tools when it comes to high-precision astrometry. In this paper,
we analyze the potential of combining both techniques, i.e. by applying image
reconstruction on partially AO corrected short exposures. Therefore we simulate
speckle clouds with and without AO corrections and create synthetic
observations. We apply holographic image reconstruction to the obtained
observations and find that (i) the residual wavefronts decorrelate slowlier and
to a lower limit when AO systems are used, (ii) the same reference stars yield
a better reconstruction, and (iii) using fainter reference stars we achieve a
similar image quality. These results suggest that holographic imaging of
speckle observations is feasible with 2-3 times longer integration times and
3mag fainter reference stars, to obtain diffraction-limited imaging from
low-order AO systems that are less restricted in sky-coverage than typical
high-order AO systems.Comment: 18 pages, 13 figures, and 3 table
Dynamic Base Station Repositioning to Improve Spectral Efficiency of Drone Small Cells
With recent advancements in drone technology, researchers are now considering
the possibility of deploying small cells served by base stations mounted on
flying drones. A major advantage of such drone small cells is that the
operators can quickly provide cellular services in areas of urgent demand
without having to pre-install any infrastructure. Since the base station is
attached to the drone, technically it is feasible for the base station to
dynamic reposition itself in response to the changing locations of users for
reducing the communication distance, decreasing the probability of signal
blocking, and ultimately increasing the spectral efficiency. In this paper, we
first propose distributed algorithms for autonomous control of drone movements,
and then model and analyse the spectral efficiency performance of a drone small
cell to shed new light on the fundamental benefits of dynamic repositioning. We
show that, with dynamic repositioning, the spectral efficiency of drone small
cells can be increased by nearly 100\% for realistic drone speed, height, and
user traffic model and without incurring any major increase in drone energy
consumption.Comment: Accepted at IEEE WoWMoM 2017 - 9 pages, 2 tables, 4 figure
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