2,871 research outputs found
On the coherence/incoherence of electron transport in semiconductor heterostructure optoelectronic devices
This paper compares and contrasts different theoretical approaches based on incoherent electron scattering transport with experimental measurements of optoelectronic devices formed from semiconductor heterostructures. The Monte Carlo method which makes no a priori assumptions about the carrier distribution in momentum or phase space is compared with less computationally demanding energy-balance rate equation models which assume thermalised carrier distributions. It is shown that the two approaches produce qualitatively similar results for hole transport in p-type Si1-xGex/Si superlattices designed for terahertz emission. The good agreement of the predictions of rate equation calculations with experimental measurements of mid- and far-infrared quantum cascade lasers, quantum well infrared photodetectors and quantum dot infrared photodetectors substantiate the assumption of incoherent scattering dominating the transport in these quantum well based devices. However, the paper goes on to consider the possibility of coherent transport through the density matrix method and suggests an experiment that could allow coherent and incoherent transport to be distinguished from each other
High-performance 3D waveguide architecture for astronomical pupil-remapping interferometry
The detection and characterisation of extra-solar planets is a major theme
driving modern astronomy, with the vast majority of such measurements being
achieved by Doppler radial-velocity and transit observations. Another technique
-- direct imaging -- can access a parameter space that complements these
methods, and paves the way for future technologies capable of detailed
characterization of exoplanetary atmospheres and surfaces. However achieving
the required levels of performance with direct imaging, particularly from
ground-based telescopes which must contend with the Earth's turbulent
atmosphere, requires considerable sophistication in the instrument and
detection strategy. Here we demonstrate a new generation of photonic
pupil-remapping devices which build upon the interferometric framework
developed for the {\it Dragonfly} instrument: a high contrast waveguide-based
device which recovers robust complex visibility observables. New generation
Dragonfly devices overcome problems caused by interference from unguided light
and low throughput, promising unprecedented on-sky performance. Closure phase
measurement scatter of only has been achieved, with waveguide
throughputs of . This translates to a maximum contrast-ratio
sensitivity (between the host star and its orbiting planet) at
(1 detection) of (when a conventional
adaptive-optics (AO) system is used) or (for typical
`extreme-AO' performance), improving even further when random error is
minimised by averaging over multiple exposures. This is an order of magnitude
beyond conventional pupil-segmenting interferometry techniques (such as
aperture masking), allowing a previously inaccessible part of the star to
planet contrast-separation parameter space to be explored
First starlight spectrum captured using an integrated photonic micro-spectrograph
Photonic technologies have received growing consideration for incorporation
into next-generation astronomical instrumentation, owing to their miniature
footprint and inherent robustness. In this paper we present results from the
first on-telescope demonstration of a miniature photonic spectrograph for
astronomy, by obtaining spectra spanning the entire H-band from several stellar
targets. The prototype was tested on the 3.9 m Anglo-Australian telescope. In
particular, we present a spectrum of the variable star Pi 01 Gru, with observed
CO molecular absorption bands, at a resolving power R = 2500 at 1600 nm.
Furthermore, we successfully demonstrate the simultaneous acquisition of
multiple spectra with a single spectrograph chip by using multiple fibre
inputs.Comment: 5 Pages, 4 Figures; A&A, Volume 544 (2012
Artificial Incoherent Speckles Enable Precision Astrometry and Photometry in High-Contrast Imaging
State-of-the-art coronagraphs employed on extreme adaptive optics enabled instruments are constantly improving the contrast detection limit for companions at ever-closer separations from the host star. In order to constrain their properties and, ultimately, compositions, it is important to precisely determine orbital parameters and contrasts with respect to the stars they orbit. This can be difficult in the post-coronagraphic image plane, as by definition the central star has been occulted by the coronagraph. We demonstrate the flexibility of utilizing the deformable mirror in the adaptive optics system of the Subaru Coronagraphic Extreme Adaptive Optics system to generate a field of speckles for the purposes of calibration. Speckles can be placed up to 22.5 λ/D from the star, with any position angle, brightness, and abundance required. Most importantly, we show that a fast modulation of the added speckle phase, between 0 and π, during a long science integration renders these speckles effectively incoherent with the underlying halo. We quantitatively show for the first time that this incoherence, in turn, increases the robustness and stability of the adaptive speckles, which will improve the precision of astrometric and photometric calibration procedures. This technique will be valuable for high-contrast imaging observations with imagers and integral field spectrographs alike
SafeWeb: A Middleware for Securing Ruby-Based Web Applications
Web applications in many domains such as healthcare and finance must process sensitive data, while complying with legal policies regarding the release of different classes of data to different parties. Currently, software bugs may lead to irreversible disclosure of confidential data in multi-tier web applications. An open challenge is how developers can guarantee these web applications only ever release sensitive data to authorised users without costly, recurring security audits.
Our solution is to provide a trusted middleware that acts as a “safety net” to event-based enterprise web applications by preventing harmful data disclosure before it happens. We describe the design and implementation of SafeWeb, a Ruby-based middleware that associates data with security labels and transparently tracks their propagation at different granularities across a multi-tier web architecture with storage and complex event processing. For efficiency, maintainability and ease-of-use, SafeWeb exploits the dynamic features of the Ruby programming language to achieve label propagation and data flow enforcement. We evaluate SafeWeb by reporting our experience of implementing a web-based cancer treatment application and deploying it as part of the UK National Health Service (NHS)
The Subaru Coronagraphic Extreme Adaptive Optics system: enabling high-contrast imaging on solar-system scales
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a
multipurpose high-contrast imaging platform designed for the discovery and
detailed characterization of exoplanetary systems and serves as a testbed for
high-contrast imaging technologies for ELTs. It is a multi-band instrument
which makes use of light from 600 to 2500nm allowing for coronagraphic direct
exoplanet imaging of the inner 3 lambda/D from the stellar host. Wavefront
sensing and control are key to the operation of SCExAO. A partial correction of
low-order modes is provided by Subaru's facility adaptive optics system with
the final correction, including high-order modes, implemented downstream by a
combination of a visible pyramid wavefront sensor and a 2000-element deformable
mirror. The well corrected NIR (y-K bands) wavefronts can then be injected into
any of the available coronagraphs, including but not limited to the phase
induced amplitude apodization and the vector vortex coronagraphs, both of which
offer an inner working angle as low as 1 lambda/D. Non-common path, low-order
aberrations are sensed with a coronagraphic low-order wavefront sensor in the
infrared (IR). Low noise, high frame rate, NIR detectors allow for active
speckle nulling and coherent differential imaging, while the HAWAII 2RG
detector in the HiCIAO imager and/or the CHARIS integral field spectrograph
(from mid 2016) can take deeper exposures and/or perform angular, spectral and
polarimetric differential imaging. Science in the visible is provided by two
interferometric modules: VAMPIRES and FIRST, which enable sub-diffraction
limited imaging in the visible region with polarimetric and spectroscopic
capabilities respectively. We describe the instrument in detail and present
preliminary results both on-sky and in the laboratory.Comment: Accepted for publication, 20 pages, 10 figure
Occupational choice, number of entrepreneurs and output: theory and empirical evidence with Spanish data
This paper extends the (Lucas, Bell J Econ 9:508–523,1978) model of occupational choices by individuals with different skills, beyond the simple options of self-employment or wage-employment, by including a second choice for the self-employed. That is, an option to hire employees and so become self-employed with employees (SEWEs), or to be self-employed without employees (SEWNEs). We solve for the market equilibrium and examine the sensitivity of relative sizes of occupational groups, and of the level of productivity, to changes in the exogenous parameters. The results show that the positive (negative) association between number of SEWEs (SEWNEs) and productivity, observed in the Spanish data, can be explained, under certain conditions, as the result of cross-region and time differences in average skills. These findings point to the importance of distinguishing between SEWEs and SEWNEs in drawing valid conclusions concerning any link between entrepreneurship and economic development
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