Improving the astrometric performance of VLTI-PRIMA

In the summer of 2011, the first on-sky astrometric commissioning of PRIMA-Astrometry delivered a performance of 3 m'' for a 10 '' separation on bright objects, orders of magnitude away from its exoplanet requirement of 50 {\mu}'' ~ 20 {\mu}'' on objects as faint as 11 mag ~ 13 mag in K band. This contribution focuses on upgrades and characterizations carried out since then. The astrometric metrology was extended from the Coud\'e focus of the Auxillary Telescopes to their secondary mirror, in order to reduce the baseline instabilities and improve the astrometric performance. While carrying out this extension, it was realized that the polarization retardance of the star separator derotator had a major impact on both the astrometric metrology and the fringe sensors. A local compensation of this retardance and the operation on a symmetric baseline allowed a new astrometric commissioning. In October 2013, an improved astrometric performance of 160 {\mu}'' was demonstrated, still short of the requirements. Instabilities in the astrometric baseline still appear to be the dominating factor. In preparation to a review held in January 2014, a plan was developed to further improve the astrometric and faint target performance of PRIMA Astrometry. On the astrometric aspect, it involved the extension of the internal longitudinal metrology to primary space, the design and implementation of an external baseline metrology, and the development of an astrometric internal fringes mode. On the faint target aspect, investigations of the performance of the fringe sensor units and the development of an AO system (NAOMI) were in the plan. Following this review, ESO decided to take a proposal to the April 2014 STC that PRIMA be cancelled, and that ESO resources be concentrated on ensuring that Gravity and Matisse are a success. This proposal was recommended by the STC in May 2014, and endorsed by ESO.Comment: 12 pages, 9 figures, 2 tables, Proceeding of SPIE conference in Montrea

An overview of the mid-infrared spectro-interferometer MATISSE: science, concept, and current status

MATISSE is the second-generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L and M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 um, exploring angular scales as small as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range from R ~ 30 to R ~ 5000. Here, we present one of the main science objectives, the study of protoplanetary disks, that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT and NAOMI). We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. We also discuss the current status of the MATISSE instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first light at Paranal.Comment: SPIE Astronomical Telescopes and Instrumentation conference, June 2016, 11 pages, 6 Figure

Bi-stability of contact angle and its role in tuning the morphology of self-assisted GaAs nanowires

We demonstrate the existence of two stable contact angles for the gallium droplet on top of self-assisted GaAs nanowires grown by MBE on patterned silicon substrates. Contact angle around 130 degrees fosters a continuous increase in the nanowire radius, while 90 degrees allows for the nanowire thinning, followed by the stable growth of ultra-thin tops. We develop a model that explains the observed morphological evolution under the two different scenarios

Facet-driven formation of axial and radial In(Ga)As clusters in GaAs nanowires

Embedding quantum dots in nanowires (NWs) constitutes one promising building block for quantum photonic technologies. Earlier attempts to grow InAs quantum dots on GaAs nanowires were based on the Stranski-Krastanov growth mechanism. Here, we propose a novel strain-driven mechanism to form 3D In-rich clusters on the NW sidewalls and also on the NW top facets. The focus is on ternary InGaAs nanowire quantum dots which are particularly attractive for producing single photons at telecommunication wavelengths. In(Ga)As clusters were realized on the inclined top facets and also on the {11-2} corner facets of GaAs NW arrays by depositing InAs at a high growth temperature (630 degrees C). High-angle annular dark-field scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy confirms that the observed 3D clusters are indeed In-rich. The optical functionality of the as-grown samples was verified using optical technique of cathodoluminescence. Emission maps close to the NW tip shows the presence of optically active emission centers along the NW sidewalls. Our work illustrates how facets can be used to engineer the growth of localized emitters in semiconducting NWs

Spatial Modulation of Vibrational and Luminescence Properties of Monolayer MoS2 Using a GaAs Nanowire Array

The integration of transition-metal dichalcogenides (TMDs) with non-planar substrates such as nanopillars provides a way to spatially modify the optical properties mainly through the localized strain. Similar studies to date have utilized insulating SiO2 nanopillars. Here, we combine monolayer MoS2 with free standing GaAs nanowires (NWs), in views of coupling their semiconducting properties. We find that monolayer MoS2 exhibits three different configurations: pierced, wrapped and tent-like. We demonstrate how to identify the configurations by optical microscopy and elucidate the impact on the vibrational and luminescence characteristics by confocal spectroscopy mapping. In particular, we highlight the increase of intensity and shift due to the photonic properties of nanowires and increase in dielectric screening associated with the GaAs NW. This work signifies the first step towards the use of vertical III-V NW arrays as a versatile platform for spatially engineering the optical properties of TMDs