2,892 research outputs found
InGaAsP/InP laser development for single-mode, high-data-rate communications
Materials studies as well as general and specific device development were carried out in the InGaAsP system. A comparison was made of three standard methods of evaluating substrate quality by means of dislocation studies. A cause of reduced yield of good wafers, the pullover of melt from one bin to the next, has been analyzed. Difficulties with reproducible zinc acceptor doping have been traced to segregation of zinc in the In/Zn alloy used for the doping source. Using EBIC measurments, the pn junction was shown to drift in location depending on factors not always under control. An analysis of contact structures by SIMS showed that the depth to which the sintered Au/Zn contact penetrates into the structure is typically 0.13 microns, or well within the cap layer and out of the p-type cladding and thus not deleterious to laser prformance. The problem of single-mode laser development was investigated and it was shown to be related to the growth habit over four different possible substrate configurations. The fabrication of constricted double heterojunctions, mesa stripe buried heterostructures, and buried heterostructures was discussed, and measurements were presented on the device properties of single-mode buried heterostructure lasers. Results include single spectral line emission at 3 mW and a threshold current of 60 mA
Aerosol physical properties in the stratosphere (APPS) radiometer design
The measurement concepts and radiometer design developed to obtain earth-limb spectral radiance measurements for the Aerosol Physical Properties in the Stratosphere (APPS) measurement program are presented. The measurements made by a radiometer of this design can be inverted to yield vertical profiles of Rayleigh scatterers, ozone, nitrogen dioxide, aerosol extinction, and aerosol physical properties, including a Junge size-distribution parameter, and a real and imaginary index of refraction. The radiometer design provides the capacity for remote sensing of stratospheric constituents from space on platforms such as the space shuttle and satellites, and therefore provides for global measurements on a daily basis
The Lennard-Jones-Devonshire cell model revisited
We reanalyse the cell theory of Lennard-Jones and Devonshire and find that in
addition to the critical point originally reported for the 12-6 potential (and
widely quoted in standard textbooks), the model exhibits a further critical
point. We show that the latter is actually a more appropriate candidate for
liquid-gas criticality than the original critical point.Comment: 5 pages, 3 figures, submitted to Mol. Phy
Modelling the early time behaviour of type Ia supernovae: effects of the 56Ni distribution
Recent studies have demonstrated the diversity in type Ia supernovae (SNe Ia)
at early times and highlighted a need for a better understanding of the
explosion physics as manifested by observations soon after explosion. To this
end, we present a Monte Carlo code designed to model the light curves of
radioactively driven, hydrogen-free transients from explosion to approximately
maximum light. In this initial study, we have used a parametrised description
of the ejecta in SNe Ia, and performed a parameter study of the effects of the
Ni distribution on the observed colours and light curves for a fixed
Ni mass of 0.6 . For a given density profile, we find that
models with Ni extending throughout the entirety of the ejecta are
typically brighter and bluer shortly after explosion. Additionally, the shape
of the density profile itself also plays an important role in determining the
shape, rise time, and colours of observed light curves. We find that the
multi-band light curves of at least one SNe Ia (SN 2009ig) are inconsistent
with less extended Ni distributions, but show good agreement with models
that incorporate Ni throughout the entire ejecta. We further demonstrate
that comparisons with full colour light curves are powerful tools in
discriminating various Ni distributions, and hence explosion models.Comment: 14 pages, 8 figures, 2 tables. Minor changes in notation to match
published version in Astronomy & Astrophysic
Identifying gravitationally lensed supernovae within the Zwicky Transient Facility public survey
Strong gravitational lensing of supernovae is exceedingly rare. To date, only
a handful of lensed supernovae are known. Despite their rarity, lensed
supernovae have emerged as one of the most promising methods for measuring the
current expansion rate of the Universe and breaking the Hubble tension. We
present an extensive search for gravitationally lensed supernovae within the
Zwicky Transient Facility (ZTF) public survey, covering 12,524 transients with
good light curves discovered during four years of observations. We crossmatch a
catalogue of known and candidate lens galaxies with our transient sample and
find only one coincident source, which was due to chance alignment. To search
for supernovae magnified by unknown lens galaxies, we test multiple methods
that have been suggested in the literature, for the first time on real data.
This includes selecting objects with extremely red colours and those that
appear inconsistent with the host galaxy redshift. In both cases, we find a few
hundred candidates, most of which are due to contamination from activate
galactic nuclei, bogus detections, or unlensed supernovae. The false positive
rate from these methods presents significant challenges for future surveys. In
total, 65 unique transients were identified across all of our selection methods
that required detailed manual rejection, which would be infeasible for larger
samples. Overall, we do not find any compelling candidates for lensed
supernovae, which is broadly consistent with previous estimates for the rate of
lensed supernovae in the ZTF public survey and the number expected to pass the
selection cuts we apply.Comment: Submitte
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