2,479 research outputs found
100 GHz resonant cavity enhanced Schottky photodiodes
Cataloged from PDF version of article.Resonant cavity enhanced (RCE) photodiodes are promising candidates for applications in optical communications and interconnects where ultrafast high-efficiency detection is desirable. We have designed and fabricated RCE Schottky photodiodes in the (Al, In) GaAs material system for 900-nm wavelength. The observed temporal response with 10-ps pulsewidth was limited
by the measurement setup and a conservative estimation of the bandwidth corresponds to more than 100 GHz. A direct comparison of RCE versus conventional detector performance was performed by high speed measurements under optical excitation at resonant wavelength (895 nm) and at 840 nm where the device functions as a single-pass conventional photodiode. A more than two-fold bandwidth enhancement with the RCE detection scheme was demonstrated
Design and Optimization of High-Speed Resonant Cavity Enhanced Schottky Photodiodes
Cataloged from PDF version of article.Resonant cavity enhanced (RCE) photodiodes (PDâs)
are promising candidates for applications in optical communications
and interconnects where high-speed high-efficiency photodetection
is desirable. In RCE structures, the electrical properties
of the photodetector remain mostly unchanged; however, the
presence of the microcavity causes wavelength selectivity accompanied
by a drastic increase of the optical field at the resonant
wavelengths. The enhanced optical field allows to maintain a high
efficiency for faster transit-time limited PDâs with thinner absorption
regions. The combination of an RCE detection scheme with
Schottky PDâs allows for the fabrication of high-performance
photodetectors with relatively simple material structures and
fabrication processes. In top-illuminated RCE Schottky PDâs,
a semitransparent Schottky contact can also serve as the top
reflector of the resonant cavity. We present theoretical and
experimental results on spectral and high-speed properties of
GaAsâAlAsâInGaAs RCE Schottky PDâs designed for 900-nm
wavelength
A New Giant Branch Clump Structure In the Large Magellanic Cloud
We present Washington C, T1 CCD photometry of 21 fields located in the
northern part of the Large Magellanic Cloud (LMC), and spread over a region of
more than 2.52 degrees approximately 6 degrees from the bar. The surveyed areas
were chosen on the basis of their proximity to SL 388 and SL 509, whose fields
showed the presence of a secondary giant clump, observationally detected by
Bica et al. (1998, AJ, 116, 723). From the collected data we found that most of
the observed field CMDs do not show a separate secondary clump, but rather a
continuous vertical structure (VS), which is clearly seen for the first time.
Its position and size are nearly the same throughout the surveyed regions: it
lies below the Red Giant Clump (RGC) and extends from the bottom of the RGC to
approximately 0.45 mag fainter, spanning the bluest color range of the RGC. The
more numerous the VS stars in a field, the larger the number of LMC giants in
the same zone. Our analysis demonstrate that VS stars belong to the LMC and are
most likely the consequence of some kind of evolutionary process in the LMC,
particularly in those LMC regions with a noticeable large giant population. Our
results suggest that in order to trigger the formation of VS stars, there
should be other conditions besides the appropriate age, metallicity, and the
necessary red giant star density. Indeed, stars satisfying the requisites
mentioned above are commonly found throughout the LMC, but the VS phenomenon is
only clearly seen in some isolated regions. Finally, the fact that clump stars
have an intrinsic luminosity dispersion further constrains the use of the clump
magnitude as a reliable distance indicator.Comment: 25 pages, 9 figures, 3 tables; to be published in AJ, Dec. issu
WIYN Open Cluster Study 1: Deep Photometry of NGC 188
We have employed precise V and I photometry of NGC 188 at WIYN to explore the
cluster luminosity function (LF) and study the cluster white dwarfs (WDs). Our
photometry is offset by V = 0.052 (fainter) from Sandage (1962) and Eggen &
Sandage (1969). All published photometry for the past three decades have been
based on these two calibrations, which are in error by 0.05 +- 0.01. We employ
the Pinsonneault etal (1998) fiducial main sequence to derive a cluster
distance modulus of 11.43 +- 0.08. We report observations that are >= 50%
complete to V = 24.6 and find that the cluster central-field LF peaks at M_I ~
3 to 4. This is unlike the solar neighborhood LF and unlike the LFs of
dynamically unevolved portions of open and globular clusters, which rise
continuously until M_I ~ 9.5. Although we find that >= 50% of the unresolved
cluster objects are multiple systems, their presence cannot account for the
shape of the NGC 188 LF. For theoretical reasons (Terlevich 1987; Vesperini &
Heggie 1997) having to do with the survivability of NGC 188 we believe the
cluster is highly dynamically evolved and that the missing low luminosity stars
are either in the cluster outskirts or have left the cluster altogether. We
identify nine candidate WDs, of which we expect three to six are bona fide
cluster WDs. The luminosities of the faintest likely WD indicates an age
(Bergeron, Wesemael, & Beauchamp 1995) of 1.14 +- 0.09 Gyrs. This is a lower
limit to the cluster age and observations probing to V = 27 or 28 will be
necessary to find the faintest cluster WDs and independently determine the
cluster age. While our age limit is not surprising for this ~6 Gyr old cluster,
our result demonstrates the value of the WD age technique with its very low
internal errors. (abridged)Comment: 26 pages, uuencoded gunzip'ed latex + 16 postscrip figures, to be
published in A
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The ACS Survey Of Globular Clusters. V. Generating A Comprehensive Star Catalog For Each Cluster
The ACS Survey of Globular Clusters has used Hubble Space Telescope's Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous data set for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available point-spread function models. We describe here in detail the program's rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we ran a standard set of similar to 10(5) tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the World Wide Web.Astronom
Wide-Field Survey of Globular Clusters in M31. I. A Catalog of New Clusters
We present the result of a wide-field survey of globular clusters (GCs) in
M31 covering a 3deg x 3deg field c. We have searched for GCs on CCD images
taken with Washington CMT1 filters at the KPNO 0.9 m telescope using steps: (1)
inspection of morphological parameters given by the SExtractor package such as
stellarity, full maximum, and ellipticity; (2) consulting the spectral types
and radial velocities obtained from spectra takena spectrograph at the WIYN 3.5
m telescope; and (3) visual inspection of the images of each object. We have
and GC candidates, of which 605 are newly found GCs and GC candidates and 559
are previously known GCs. Amoects there are 113 genuine GCs, 258 probable GCs,
and 234 possible GCs, according to our classification critee known objects
there are 383 genuine GCs, 109 probable GCs, and 67 possible GCs. In total
there are 496 genprobable GCs and 301 possible GCs. Most of these newly found
GCs have T1 magnitudes of 17.5 - 19.5 mag, [17.9 < V < 19.9 mag assuming (C-T1)
~ 1.5], and (C-T1) colors in the range 1 - 2.Comment: accepted by AJ, using emulateapj.cl
High bandwidth-efficiency resonant cavity enhanced Schottky photodiodes for 800-850 nm wavelength operation
Cataloged from PDF version of article.High-speed resonant cavity enhanced Schottky photodiodes operating in 800-850 nm wavelength region are demonstrated. The devices are fabricated in the AlGaAs/GaAs material system. The Schottky contact is a semitransparent Au film which also serves as the top reflector of the Fabry-Perot cavity. The detectors exhibit a peak quantum efficiency of eta = 0.5 at lambda = 827 nm wavelength and a 3 dB bandwidth of more than 50 GHz resulting in a bandwidth-efficiency product of more than 25 GHz. (C) 1998 American Institute of Physic
Wide-Field Survey of Globular Clusters in M31. II. Kinematics of the Globular Cluster System
We present a kinematic analysis of the globular cluster(GC) system in M31.
Using the photometric and spectroscopic database of 504 GCs, we have
investigated the kinematics of the M31 GC system. We find that the all GC
system shows strong rotation, with rotation amplitude of v_rot~190km/s, and
that a weak rotation persists even for the outermost samples at |Y|>5kpc. The
rotation-corrected velocity dispersion for the GC system is estimated to be
sigma_{p,r}~130km/s, and it increases from sigma_{p,r}~120km/s at |Y|<1kpc to
sigma_{p,r}~150km/s at |Y|>5kpc. These results are very similar to those for
the metal-poor GCs. This shows that there is a dynamically hot halo in M31 that
is rotating but primarily pressure-supported. We have identified 50
"friendless" GCs, and they appear to rotate around the major axis of M31. For
the subsamples of metal-poor and metal-rich GCs, we have found that the
metal-rich GCs are more centrally concentrated than the metal-poor GCs, and
both subsamples show strong rotation. For the subsamples of bright and faint
GCs, it is found that the rotation for the faint GCs is stronger than that for
the bright GCs. We have identified 56 GCs and GC candidates with X-ray
detection. It is found that the majority of X-ray emitting GCs follow the disk
rotation, and that the redder, more metal-rich, and brighter GCs are more
likely to be detected as X-ray emitting GCs. We have derived a rotation curve
of M31 using the GCs at |Y|<0.6kpc. We have estimated the dynamical mass of M31
using `Projected Mass Estimator(PME)' and `Tracer Mass Estimator(TME)'. We
finally discuss the implication of these results and compare the kinematics of
GCs with that of planetary nebulae in M31.Comment: 62 pages, 26 figues, Accepted by Ap
Giving Miss Marple a makeover : graduate recruitment, systems failure and the Scottish voluntary sector
The voluntary sector in Scotland, as across the globe, is becoming increasingly business like. Resultantly, there is an increasing demand for graduates to work in business and support functions. In Scotland, however, despite an oversupply of graduates in the labor market, the voluntary sector reports skills shortages for graduate-level positions; a leadership deficit was also reported in countries such as the United States. Through exploratory, mainly qualitative, case study and stakeholder research, this article proposes that one reason for this mismatch between the supply of and demand for graduates is a systems failure within the sector. Many graduates and university students remain unaware of potentially suitable paid job opportunities, in part because of the sector's voluntary label. To rectify this systems failure, thought needs to be given to the sector's nomenclature and the manner in which voluntary sector organizations attract graduate recruits, for example, through levering value congruence in potential recruits
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