176,058 research outputs found
Microphotonic parabolic light directors fabricated by two-photon lithography
We have fabricated microphotonic parabolic light directors using two-photon lithography, thin-film processing, and aperture formation by focused ion beam lithography. Optical transmission measurements through upright parabolic directors 22 μm high and 10 μm in diameter exhibit strong beam directivity with a beam divergence of 5.6°, in reasonable agreement with ray-tracing and full-field electromagnetic simulations. The results indicate the suitability of microphotonic parabolic light directors for producing collimated beams for applications in advanced solar cell and light-emitting diode designs
WDVV Equations, Darboux-Egoroff Metric and the Dressing Method
Dressing technique is used to construct commuting Lax operators which provide
an integrable (canonical) structure behind
Witten--Dijkgraaf--Verlinde--Verlinde equations. The commuting flows are
related to the isomonodromic flows. Examples of the canonical integrable
structure are given in two- and three-dimensional cases. The three-dimensional
example is associated with the rational Landau-Ginzburg potentials.Comment: Contribution to the conference "Workshop on Integrable Theories,
Solitons and Duality", Unesp2002, LaTeX file w. JHEP style fil
Many-body wave scattering by small bodies
Scattering problem by several bodies, small in comparison with the
wavelength, is reduced to linear algebraic systems of equations, in contrast to
the usual reduction to some integral equations
An Interacting Galaxy System Along a Filament in a Void
Cosmological voids provide a unique environment for the study of galaxy
formation and evolution. The galaxy population in their interior have
significantly different properties than average field galaxies. As part of our
Void Galaxy Survey (VGS), we have found a system of three interacting galaxies
(VGS_31) inside a large void. VGS_31 is a small elongated group whose members
are embedded in a common HI envelope. The HI picture suggests a filamentary
structure with accretion of intergalactic cold gas from the filament onto the
galaxies. We present deep optical and narrow band H_alpha data, optical
spectroscopy, near-UV and far-UV GALEX and CO(1-0) data. We find that one of
the galaxies, a Markarian object, has a ring-like structure and a tail evident
both in optical and HI. While all three galaxies form stars in their central
parts, the tail and the ring of the Markarian object are devoid of star
formation. We discuss these findings in terms of a gravitational interaction
and ongoing growth of galaxies out of a filament. VGS_31 is one of the first
observed examples of a filamentary structure in a void. It is an important
prototype for understanding the formation of substructure in a void. This
system also shows that the galaxy evolution in voids can be as dynamic as in
high density environments.Comment: 17 pages, 8 figures, accepted for publication in A
KK246, a dwarf galaxy with extended H I disk in the Local Void
We have found that KK 246, the only confirmed galaxy located within the
nearby Tully Void, is a dwarf galaxy with an extremely extended H I disk and
signs of an H I cloud with anomalous velocity. It also exhibits clear
misalignment between the kinematical major and minor axes, indicative of an
oval distortion, and a general misalignment between the H I and optical major
axes. We measure a H I mass of 1.05 +- 0.08 x 10^8 M_sun, and a H I extent 5
times that of the stellar disk, one of the most extended H I disks known. We
estimate a dynamical mass of 4.1 x 10^9 M_sun, making this also one of the
darkest galaxies known, with a mass-to-light ratio of 89. The relative
isolation and extreme underdense environment make this an interesting case for
examining the role of gas accretion in galaxy evolution.Comment: 15 pages, 8 figures, 2 tables, accepted for publication in A
Optimal Tradeoff Between Exposed and Hidden Nodes in Large Wireless Networks
Wireless networks equipped with the CSMA protocol are subject to collisions
due to interference. For a given interference range we investigate the tradeoff
between collisions (hidden nodes) and unused capacity (exposed nodes). We show
that the sensing range that maximizes throughput critically depends on the
activation rate of nodes. For infinite line networks, we prove the existence of
a threshold: When the activation rate is below this threshold the optimal
sensing range is small (to maximize spatial reuse). When the activation rate is
above the threshold the optimal sensing range is just large enough to preclude
all collisions. Simulations suggest that this threshold policy extends to more
complex linear and non-linear topologies
Large magnetoresistance using hybrid spin filter devices
A magnetic "spin filter" tunnel barrier, sandwiched between a non-magnetic
metal and a magnetic metal, is used to create a new magnetoresistive tunnel
device, somewhat analogous to an optical polarizer-analyzer configuration. The
resistance of these trilayer structures depends on the relative magnetization
orientation of the spin filter and the ferromagnetic electrode. The spin
filtering in this configuration yields a previously unobserved
magnetoresistance effect, exceeding 100%.Comment: 3.5 pages, 3 figures, submitted to Appl. Phys. Let
A near-infrared study of the star forming region RCW 34
We report the results of a near-infrared imaging study of a
arcmin region centered on the 6.7 GHz methanol maser associated with the
RCW 34 star forming region using the 1.4m IRSF telescope at Sutherland. A total
of 1283 objects were detected simultaneously in J, H, and K for an exposure
time of 10800 seconds. The J-H, H-K two-colour diagram revealed a strong
concentration of more than 700 objects with colours similar to what is expected
of reddened classical T Tauri stars. The distribution of the objects on the K
{\it vs} J-K colour-magnitude diagram is also suggestive that a significant
fraction of the 1283 objects is lower mass pre-main sequence stars. We also
present the luminosity function for the subset of about 700 pre-main sequence
stars and show that it suggests ongoing star formation activity for about
years. An examination of the spatial distribution of the pre-main
sequence stars shows that the fainter (older) part of the population is more
dispersed over the observed region and the brighter (younger) subset is more
concentrated around the position of the O8.5V star. This suggests that the
physical effects of the O8.5V star and the two early B-type stars on the
remainder of the cloud out of which they formed, could have played a role in
the onset of the more recent episode of star formation in RCW 34.Comment: Accepted for publication in The Astronomical Journa
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