147 research outputs found
Laboratory validation of the dual-zone phase mask coronagraph in broadband light at the high-contrast imaging THD-testbed
Specific high contrast imaging instruments are mandatory to characterize
circumstellar disks and exoplanets around nearby stars. Coronagraphs are
commonly used in these facilities to reject the diffracted light of an observed
star and enable the direct imaging and spectroscopy of its circumstellar
environment. One important property of the coronagraph is to be able to work in
broadband light.
Among several proposed coronagraphs, the dual-zone phase mask coronagraph is
a promising solution for starlight rejection in broadband light. In this paper,
we perform the first validation of this concept in laboratory.
First, we recall the principle of the dual-zone phase mask coronagraph. Then,
we describe the high-contrast imaging THD testbed, the manufacturing of the
components and the quality-control procedures. Finally, we study the
sensitivity of our coronagraph to low-order aberrations (inner working angle
and defocus) and estimate its contrast performance. Our experimental broadband
light results are compared with numerical simulations to check agreement with
the performance predictions.
With the manufactured prototype and using a dark hole technique based on the
self-coherent camera, we obtain contrast levels down to between 5
and 17 in monochromatic light (640 nm). We also reach contrast
levels of between 7 and 17 in broadband
( nm, nm and %), which demonstrates the excellent chromatic performance of the dual-zone
phase mask coronagraph.
The performance reached by the dual-zone phase mask coronagraph is promising
for future high-contrast imaging instruments that aim at detecting and
spectrally characterizing old or light gaseous planets.Comment: 9 pages, 16 figure
Interface-engineered hole doping in Sr2IrO4/LaNiO3 heterostructure
The relativistic Mott insulator Sr2IrO4 driven by large spin-orbit
interaction is known for the Jeff = 1/2 antiferromagnetic state which closely
resembles the electronic structure of parent compounds of superconducting
cuprates. Here, we report the realization of hole-doped Sr2IrO4 by means of
interfacial charge transfer in Sr2IrO4/LaNiO3 heterostructures. X-ray
photoelectron spectroscopy on Ir 4f edge along with the X-ray absorption
spectroscopy at Ni L2 edge confirmed that 5d electrons from Ir sites are
transferred onto Ni sites, leading to markedly electronic reconstruction at the
interface. Although the Sr2IrO4/LaNiO3 heterostructure remains non-metallic, we
reveal that the transport behavior is no longer described by the Mott variable
range hopping mode, but by the Efros-Shklovskii model. These findings highlight
a powerful utility of interfaces to realize emerging electronic states of the
Ruddlesden-Popper phases of Ir-based oxides.Comment: 9 pages including 3 figures and reference
Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor. II. Concept validation with ZELDA on VLT/SPHERE
Warm or massive gas giant planets, brown dwarfs, and debris disks around
nearby stars are now routinely observed by dedicated high-contrast imaging
instruments on large, ground-based observatories. These facilities include
extreme adaptive optics (ExAO) and state-of-the-art coronagraphy to achieve
unprecedented sensitivities for exoplanet detection and spectral
characterization. However, differential aberrations between the ExAO sensing
path and the science path represent a critical limitation for the detection of
giant planets with a contrast lower than a few at very small
separations (<0.3\as) from their host star. In our previous work, we proposed a
wavefront sensor based on Zernike phase contrast methods to circumvent this
issue and measure these quasi-static aberrations at a nanometric level. We
present the design, manufacturing and testing of ZELDA, a prototype that was
installed on VLT/SPHERE during its reintegration in Chile. Using the internal
light source of the instrument, we performed measurements in the presence of
Zernike or Fourier modes introduced with the deformable mirror. Our
experimental and simulation results are consistent, confirming the ability of
our sensor to measure small aberrations (<50 nm rms) with nanometric accuracy.
We then corrected the long-lived non-common path aberrations in SPHERE based on
ZELDA measurements. We estimated a contrast gain of 10 in the coronagraphic
image at 0.2\as, reaching the raw contrast limit set by the coronagraph in the
instrument. The simplicity of the design and its phase reconstruction algorithm
makes ZELDA an excellent candidate for the on-line measurements of quasi-static
aberrations during the observations. The implementation of a ZELDA-based
sensing path on the current and future facilities (ELTs, future space missions)
could ease the observation of the cold gaseous or massive rocky planets around
nearby stars.Comment: 13 pages, 12 figures, A&A accepted on June 3rd, 2016. v2 after
language editin
Graphene formed on SiC under various environments: Comparison of Si-face and C-face
The morphology of graphene on SiC {0001} surfaces formed in various
environments including ultra-high vacuum, 1 atm of argon, and 10^-6 to 10^-4
Torr of disilane is studied by atomic force microscopy, low-energy electron
microscopy, and Raman spectroscopy. The graphene is formed by heating the
surface to 1100 - 1600 C, which causes preferential sublimation of the Si
atoms. The argon atmosphere or the background of disilane decreases the
sublimation rate so that a higher graphitization temperature is required, thus
improving the morphology of the films. For the (0001) surface, large areas of
monolayer-thick graphene are formed in this way, with the size of these areas
depending on the miscut of the sample. Results on the (000-1) surface are more
complex. This surface graphitizes at a lower temperature than for the (0001)
surface and consequently the growth is more three-dimensional. In an atmosphere
of argon the morphology becomes even worse, with the surface displaying
markedly inhomogeneous nucleation, an effect attributed to unintentional
oxidation of the surface during graphitization. Use of a disilane environment
for the (000-1) surface is found to produce improved morphology, with
relatively large areas of monolayer-thick graphene.Comment: 22 pages, 11 figures, Proceedings of STEG-2 Conference; eliminated
Figs. 4 and 7 from version 1, for brevity, and added Refs. 18, 29, 30, 31
together with associated discussio
Emergent electric field control of phase transformation in oxide superlattices.
Electric fields can transform materials with respect to their structure and properties, enabling various applications ranging from batteries to spintronics. Recently electrolytic gating, which can generate large electric fields and voltage-driven ion transfer, has been identified as a powerful means to achieve electric-field-controlled phase transformations. The class of transition metal oxides provide many potential candidates that present a strong response under electrolytic gating. However, very few show a reversible structural transformation at room-temperature. Here, we report the realization of a digitally synthesized transition metal oxide that shows a reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature. In superlattices comprised of alternating one-unit-cell of SrIrO3 and La0.2Sr0.8MnO3, we find a reversible phase transformation with a 7% lattice change and dramatic modulation in chemical, electronic, magnetic and optical properties, mediated by the reversible transfer of oxygen and hydrogen ions. Strikingly, this phase transformation is absent in the constituent oxides, solid solutions and larger period superlattices. Our findings open up this class of materials for voltage-controlled functionality
Review of high-contrast imaging systems for current and future ground- and space-based telescopes I. Coronagraph design methods and optical performance metrics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in
September 2017 in Leiden, the Netherlands gathered a diverse group of 25
researchers working on exoplanet instrumentation to stimulate the emergence and
sharing of new ideas. In this first installment of a series of three papers
summarizing the outcomes of the OOC workshop, we present an overview of design
methods and optical performance metrics developed for coronagraph instruments.
The design and optimization of coronagraphs for future telescopes has
progressed rapidly over the past several years in the context of space mission
studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based
telescopes. Design tools have been developed at several institutions to
optimize a variety of coronagraph mask types. We aim to give a broad overview
of the approaches used, examples of their utility, and provide the optimization
tools to the community. Though it is clear that the basic function of
coronagraphs is to suppress starlight while maintaining light from off-axis
sources, our community lacks a general set of standard performance metrics that
apply to both detecting and characterizing exoplanets. The attendees of the OOC
workshop agreed that it would benefit our community to clearly define
quantities for comparing the performance of coronagraph designs and systems.
Therefore, we also present a set of metrics that may be applied to theoretical
designs, testbeds, and deployed instruments. We show how these quantities may
be used to easily relate the basic properties of the optical instrument to the
detection significance of the given point source in the presence of realistic
noise.Comment: To appear in Proceedings of the SPIE, vol. 1069
SPICES: Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems
SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary
Systems) is a five-year M-class mission proposed to ESA Cosmic Vision. Its
purpose is to image and characterize long-period extrasolar planets and
circumstellar disks in the visible (450 - 900 nm) at a spectral resolution of
about 40 using both spectroscopy and polarimetry. By 2020/22, present and
near-term instruments will have found several tens of planets that SPICES will
be able to observe and study in detail. Equipped with a 1.5 m telescope, SPICES
can preferentially access exoplanets located at several AUs (0.5-10 AU) from
nearby stars (25 pc) with masses ranging from a few Jupiter masses to Super
Earths (2 Earth radii, 10 M) as well as circumstellar
disks as faint as a few times the zodiacal light in the Solar System
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