1,930 research outputs found
Nd-doped polymer waveguide amplifiers at 850-930 nm
Nd-complex-doped, polymer channel waveguides were realized on thermally oxidized silicon wafers by a simple fabrication procedure. Broadband optical gain was demonstrated at 850-930 nm. Internal net gain up to 5.3 dB/cm was obtained at 850 nm, which is very promising for optical amplification in optical backplanes. With this result a route toward low-cost integrated waveguide amplifiers for optical interconnects has been opened
Nd-doped polymer waveguide amplifiers
Nd3+-complex-doped polymer channel waveguide amplifiers with various lengths and Nd3+ concentrations are fabricated by a simple procedure. Internal net gain at 840–950 nm and 1064 nm is experimentally and theoretically investigated under continuous-wave excitation at 800 nm. Internal net gain in the range 865–930 nm is observed and a peak gain of 2.8 dB at 873 nm is obtained in a 1.9-cm-long waveguide with a Nd3+ concentration of 0.6x10e20 cm-3 at a launched pump power of 25 mW. The small-signal gain measured in a 1-cm-long sample with a Nd3+ concentration of 1.03x10e20 cm-3 is 2.0 dB/cm and 5.7 dB/cm at 873 nm and 1064 nm, respectively. By use of a rate-equation model, the internal net gain at these two wavelengths is calculated and the macroscopic parameter of energy-transfer upconversion as a function of Nd3+ concentration is derived. Ease of fabrication, compatibility with other materials, and low cost make such rare-earth-ion-doped polymer waveguide amplifiers suitable for providing gain in many integrated optical devices
A Coupled Equations Model for Epitaxial Growth on Textured Surfaces
We have developed a continuum model that explains the complex surface shapes
observed in epitaxial regrowth on micron scale gratings. This model describes
the dependence of the surface morphology on film thickness and growth
temperature in terms of a few simple atomic scale processes including adatom
diffusion, step-edge attachment and detachment, and a net downhill migration of
surface adatoms. The continuum model reduces to the linear part of the
Kardar-Parisi-Zhang equation with a flux dependent smoothing coefficient in the
long wavelength limit.Comment: 11 pages, 4 figures. Submitted to the Journal of Crystal Growt
The Spectrum of Bogomol'nyi Solitons in Gauged Linear Sigma Models
Gauged linear sigma models with C^m-valued scalar fields and gauge group
U(1)^d, d \leq m, have soliton solutions of Bogomol'nyi type if a suitably
chosen potential for the scalar fields is also included in the Lagrangian. Here
such models are studied on (2+1)-dimensional Minkowski space. If the dynamics
of the gauge fields is governed by a Maxwell term the appropriate potential is
a sum of generalised Higgs potentials known as Fayet-Iliopoulos D-terms. Many
interesting topological solitons of Bogomol'nyi type arise in models of this
kind, including various types of vortices (e.g. Nielsen-Olesen, semilocal and
superconducting vortices) as well as, in certain limits, textures (e.g.
CP^(m-1) textures and gauged CP^(m-1) textures). This is explained and general
results about the spectrum of topological defects both for broken and partially
broken gauge symmetry are proven. When the dynamics of the gauge fields is
governed by a Chern-Simons term instead of a Maxwell term a different scalar
potential is required for the theory to be of Bogomol'nyi type. The general
form of that potential is given and a particular example is discussed.Comment: 32 pages, harvmac, no figure
The Abundance of New Kind of Dark Matter Structures
A new kind of dark matter structures, ultracompact minihalos (UCMHs) was
proposed recently. They would be formed during the radiation dominated epoch if
the large density perturbations are existent. Moreover, if the dark matter is
made up of weakly interacting massive particles, the UCMHs can have effect on
cosmological evolution because of the high density and dark matter annihilation
within them. In this paper, one new parameter is introduced to consider the
contributions of UCMHs due to the dark matter annihilation to the evolution of
cosmology, and we use the current and future CMB observations to obtain the
constraint on the new parameter and then the abundance of UCMHs. The final
results are applicable for a wider range of dark matter parametersComment: 4 pages, 1 tabl
The dependence of the EIT wave velocity on the magnetic field strength
"EIT waves" are a wavelike phenomenon propagating in the corona, which were
initially observed in the extreme ultraviolet (EUV) wavelength by the EUV
Imaging Telescope (EIT). Their nature is still elusive, with the debate between
fast-mode wave model and non-wave model. In order to distinguish between these
models, we investigate the relation between the EIT wave velocity and the local
magnetic field in the corona. It is found that the two parameters show
significant negative correlation in most of the EIT wave fronts, {\it i.e.},
EIT wave propagates more slowly in the regions of stronger magnetic field. Such
a result poses a big challenge to the fast-mode wave model, which would predict
a strong positive correlation between the two parameters. However, it is
demonstrated that such a result can be explained by the fieldline stretching
model, \emph{i.e.,} that "EIT waves" are apparently-propagating brightenings,
which are generated by successive stretching of closed magnetic field lines
pushed by the erupting flux rope during coronal mass ejections (CMEs).Comment: 11 pages, 8 figures, accepted for publication in Solar Phy
The effects of knee arthroplaste on walking speed. A meta-analysis.
Background: Patients with knee osteoarthritis patients have problems with walking, and tend to walk slower. An important aim of knee arthroplasty is functional recovery, which should include a post-operative increase in walking speed. Still, there are several problems with measuring walking speed in groups of knee osteoarthritis patients. Nevertheless, test-retest reliability of walking speed measurements is high, and when the same investigators monitor the same subjects, it should be possible to assess the walking speed effects of knee arthroplasty. The present study reports a meta-analysis of these effects. Methods. A total of 16 independent pre-post arthroplasty comparisons of walking speed were identified through MEDLINE, Web of Science, and PEDro, in 12 papers, involving 419 patients. Results: For 0.5-5 months post-operatively, heterogeneity was too large to obtain a valid estimate of the overall effect-size. For 6-12 and 13-60 months post-operatively, heterogeneity was absent, low, or moderate (depending on estimated pre-post correlations). During these periods, subjects walked on average 0.8 standard-deviations faster than pre-operatively, which is a large effect. Meta-regression analysis revealed significant effects of time and time squared, suggesting initial improvement followed by decline. Conclusion: This meta-analysis revealed a large effect of arthroplasty on walking speed 6-60 months post-operatively. For the first 0.5-5 months, heterogeneity of effect-sizes precluded a valid estimate of short-term effects. Hence, patients may expect a considerable improvement of their walking speed, which, however, may take several months to occur. Meta-regression analysis suggested a small decline from 13 months post-operatively onwards. © 2012 Abbasi-Bafghi et al; BioMed Central Ltd
Edge states and topological orders in the spin liquid phases of star lattice
A group of novel materials can be mapped to the star lattice, which exhibits
some novel physical properties. We give the bulk-edge correspondence theory of
the star lattice and study the edge states and their topological orders in
different spin liquid phases. The bulk and edge-state energy structures and
Chern number depend on the spin liquid phases and hopping parameters because
the local spontaneous magnetic flux in the spin liquid phase breaks the time
reversal and space inversion symmetries. We give the characteristics of bulk
and edge energy structures and their corresponding Chern numbers in the
uniform, nematic and chiral spin liquids. In particular, we obtain analytically
the phase diagram of the topological orders for the chiral spin liquid states
SL[\phi,\phi,-2\phi], where \phi is the magnetic flux in two triangles and a
dodecagon in the unit cell. Moreover, we find the topological invariance for
the spin liquid phases, SL[\phi_{1},\phi_{2},-(\phi_{1}+\phi_{2})] and
SL[\phi_{2},\phi_{1},-(\phi_{1}+\phi_{2})]. The results reveal the relationship
between the energy-band and edge-state structures and their topological orders
of the star lattice.Comment: 7 pages, 8 figures, 1 tabl
The ultraviolet Behaviour of Integrable Quantum Field Theories, Affine Toda Field Theory
We investigate the thermodynamic Bethe ansatz (TBA) equations for a system of
particles which dynamically interacts via the scattering matrix of affine Toda
field theory and whose statistical interaction is of a general Haldane type. Up
to the first leading order, we provide general approximated analytical
expressions for the solutions of these equations from which we derive general
formulae for the ultraviolet scaling functions for theories in which the
underlying Lie algebra is simply laced. For several explicit models we compare
the quality of the approximated analytical solutions against the numerical
solutions. We address the question of existence and uniqueness of the solutions
of the TBA-equations, derive precise error estimates and determine the rate of
convergence for the applied numerical procedure. A general expression for the
Fourier transformed kernels of the TBA-equations allows to derive the related
Y-systems and a reformulation of the equations into a universal form.Comment: 37 pp Latex, 5 figure
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