720 research outputs found
Performance characterization of the HiCIAO instrument for the Subaru Telescope
HiCIAO is a near-infrared, high contrast instrument which is specifically designed for searches and studies for extrasolar planets and proto-planetary/debris disks on the Subaru 8.2 m telescope. A coronagraph technique and three differential observing modes, i.e., a dual-beam simultaneous polarimetric differential imaging mode, quad-beam simultaneous spectral differential imaging mode, and angular differential imaging mode, are used to extract faint objects from the sea of speckle around bright stars. We describe the instrument performances verified in the laboratory and during the commissioning period. Readout noise with a correlated double sampling method is 15 e- using the Sidecar ASIC controller with the HAWAII-2RG detector array, and it is as low as 5 e- with a multiple sampling method. Strehl ratio obtained by HiCIAO on the sky combined with the 188-actuator adaptive optics system (AO188) is 0.4 and 0.7 in the H and K-band, respectively, with natural guide stars that have R ~ 5 and under median seeing conditions. Image distortion is correctable to 7 milli-arcsec level using the ACS data as a reference image. Examples of contrast performances in the observing modes are presented from data obtained during the commissioning period. An observation for HR 8799 in the angular differential imaging mode shows a clear detection of three known planets, demonstrating the high contrast capability of AO188+HiCIAO
Bubbly Flows through a Convergent-Divergent Nozzle
Characteristics of bubbly flow with a small void fraction through a vertical, two-dimensional, converging-diverging nozzle are investigated experimentally and numerically. Emphasis is placed on the mechanism for large velocity slip near the nozzle throat, where the pressure gradient is very large. Bubble velocities are measured by taking double-exposure photographs with stroboscopic light sources having a flash duration of a few μ sec. The pressure distribution of the mixture along the nozzle axis is measured by semiconductor pressure transducers. The local liquid velocity is determined through continuity equations of gas and liquid in conjunction with the measured data of pressure distribution and experimental conditions at the nozzle inlet and exit. The power spectrum density of the pressure fluctuations is measured to investigate some instabilty of the bubbly flow, which is believed to be inherent to the velocity slip. It is proved that the numerical results using Wijngaarden's model equations agree well with the experiments. The characteristics of flow instability are explained according to the theoretical predictions of Morioka et al
Revisit of magnetic orders in 1/1 approximant crystals of Tsai-type quasicrystal from theoretical points of view
The various magnetic orders observed in approximant crystals of Tsai-type
quasicrystal are important for deepening our understanding of mysterious
magnetism in quasicrystals. Using an icosahedral cluster model with
inter-cluster interactions, we give an intuitive explanation of the mechanism
of magnetic orders previously reported by classical Monte-Carlo simulations for
an approximant-crystal model describing a Gd-based Tsai-type 1/1 approximant
crystal.Comment: 2 pages, 2 figure
Propagation-invariant vortex Airy beam whose singular point follows its main lobe
We propose and demonstrate a novel vortex Airy beam which is a superposition
of an Airy beam and its laterally sheared beam with a phase shift. This
new-type of vortex Airy beam exhibits stable propagation dynamics, wherein its
singular point closely follows its main lobe, unlike conventional vortex Airy
beams. Notably, the orbital angular mode purity of this new vortex Airy beam is
up to 10% better than that of a conventional vortex Airy beam. We anticipate
that this new type of vortex Airy beam, which combines the characteristics of
an optical vortex and a diffraction-free Airy beam, will facilitate new
directions in applications such as microscopy, material processing and
nonlinear optics
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Atomically phase-matched second-harmonic generation in a 2D crystal.
Second-harmonic generation (SHG) has found extensive applications from hand-held laser pointers to spectroscopic and microscopic techniques. Recently, some cleavable van der Waals (vdW) crystals have shown SHG arising from a single atomic layer, where the SH light elucidated important information such as the grain boundaries and electronic structure in these ultra-thin materials. However, despite the inversion asymmetry of the single layer, the typical crystal stacking restores inversion symmetry for even numbers of layers leading to an oscillatory SH response, drastically reducing the applicability of vdW crystals such as molybdenum disulfide (MoS2). Here, we probe the SHG generated from the noncentrosymmetric 3R crystal phase of MoS2. We experimentally observed quadratic dependence of second-harmonic intensity on layer number as a result of atomically phase-matched nonlinear dipoles in layers of the 3R crystal that constructively interfere. By studying the layer evolution of the A and B excitonic transitions in 3R-MoS2 using SHG spectroscopy, we also found distinct electronic structure differences arising from the crystal structure and the dramatic effect of symmetry and layer stacking on the nonlinear properties of these atomic crystals. The constructive nature of the SHG in this 2D crystal provides a platform to reliably develop atomically flat and controllably thin nonlinear media
The infrared imaging spectrograph (IRIS) for TMT: electronics-cable architecture
The InfraRed Imaging Spectrograph (IRIS) is a first-light instrument for the
Thirty Meter Telescope (TMT). It combines a diffraction limited imager and an
integral field spectrograph. This paper focuses on the electrical system of
IRIS. With an instrument of the size and complexity of IRIS we face several
electrical challenges. Many of the major controllers must be located directly
on the cryostat to reduce cable lengths, and others require multiple bulkheads
and must pass through a large cable wrap. Cooling and vibration due to the
rotation of the instrument are also major challenges. We will present our
selection of cables and connectors for both room temperature and cryogenic
environments, packaging in the various cabinets and enclosures, and techniques
for complex bulkheads including for large detectors at the cryostat wall
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