561 research outputs found
A high efficiency input/output coupler for small silicon photonic devices
Coupling light from an optical fibre to small optical waveguides is particularly problematic in semiconductors, since the refractive index of the silica fibre is very different from that of a semiconductor waveguide. There have been several published methods of achieving such coupling, but none are sufficiently efficient whilst being robust enough for commercial applications. In this paper experimental results of our approach called a Dual-Grating Assisted Directional Coupler, are presented. The principle of coupling by this novel method has been successfully demonstrated, and a coupling efficiency of 55% measured
DNA electrophoresis studied with the cage model
The cage model for polymer reptation, proposed by Evans and Edwards, and its
recent extension to model DNA electrophoresis, are studied by numerically exact
computation of the drift velocities for polymers with a length L of up to 15
monomers. The computations show the Nernst-Einstein regime (v ~ E) followed by
a regime where the velocity decreases exponentially with the applied electric
field strength. In agreement with de Gennes' reptation arguments, we find that
asymptotically for large polymers the diffusion coefficient D decreases
quadratically with polymer length; for the cage model, the proportionality
coefficient is DL^2=0.175(2). Additionally we find that the leading correction
term for finite polymer lengths scales as N^{-1/2}, where N=L-1 is the number
of bonds.Comment: LaTeX (cjour.cls), 15 pages, 6 figures, added correctness proof of
kink representation approac
Stability of the non-extremal enhancon solution I: perturbation equations
We consider the stability of the two branches of non-extremal enhancon
solutions. We argue that one would expect a transition between the two branches
at some value of the non-extremality, which should manifest itself in some
instability. We study small perturbations of these solutions, constructing a
sufficiently general ansatz for linearised perturbations of the non-extremal
solutions, and show that the linearised equations are consistent. We show that
the simplest kind of perturbation does not lead to any instability. We reduce
the problem of studying the more general spherically symmetric perturbation to
solving a set of three coupled second-order differential equations.Comment: 20 pages, 1 figure, references added, typos fixed, version to appear
in PR
Strings in Gravimagnetic Fields
We provide a complete solution of closed strings propagating in Nappi-Witten
space. Based on the analysis of geodesics we construct the coherent
wavefunctions which approximate as closely as possible the classical
trajectories. We then present a new free field realization of the current
algebra using the gamma, beta ghost system. Finally we construct the quantum
vertex operators, for the tachyon, by representing the wavefunctions in terms
of the free fields. This allows us to compute the three- and four-point
amplitudes, and propose the general result for N-point tachyon scattering
amplitude.Comment: final version, 29 pages + 4 app
Holographic mesons in various dimensions
We calculate the spectrum of fluctuations of a probe Dk-brane in the
background of N Dp-branes, for k=p,p+2,p+4 and p< 5. The result corresponds to
the mesonic spectrum of a (p+1)-dimensional super-Yang-Mills (SYM) theory
coupled to `dynamical quarks', i.e., fields in the fundamental representation
-- the latter are confined to a defect for k=p and p+2. We find a universal
behaviour where the spectrum is discrete and the mesons are deeply bound. The
mass gap and spectrum are set by the scale M ~ m_q/g_{eff}(m_q), where m_q is
the mass of the fundamental fields and g_{eff}(m_q) is the effective coupling
evaluated at the quark mass, i.e. g_{eff}^2(m_q)=g_{ym}^2 N m_q^{p-3}. We
consider the evolution of the meson spectra into the far infrared of
three-dimensional SYM, where the gravity dual lifts to M-theory. We also argue
that the mass scale appearing in the meson spectra is dictated by holography.Comment: 44 pages, 2 figures; v2: typos corrected, references adde
The calibration of the Sudbury Neutrino Observatory using uniformly distributed radioactive sources
The production and analysis of distributed sources of 24Na and 222Rn in the
Sudbury Neutrino Observatory (SNO) are described. These unique sources provided
accurate calibrations of the response to neutrons, produced through
photodisintegration of the deuterons in the heavy water target, and to low
energy betas and gammas. The application of these sources in determining the
neutron detection efficiency and response of the 3He proportional counter
array, and the characteristics of background Cherenkov light from trace amounts
of natural radioactivity is described.Comment: 24 pages, 13 figure
The clustering of ultra-high energy cosmic rays and their sources
The sky distribution of cosmic rays with energies above the 'GZK cutoff'
holds important clues to their origin. The AGASA data, although consistent with
isotropy, shows evidence for small-angle clustering, and it has been argued
that such clusters are aligned with BL Lacertae objects, implicating these as
sources. It has also been suggested that clusters can arise if the cosmic rays
come from the decays of very massive relic particles in the Galactic halo, due
to the expected clumping of cold dark matter. We examine these claims and show
that both are in fact not justified.Comment: 13 pages, 8 figures, version in press at Phys. Rev.
Strings in the Extended BTZ Spacetime
We study string theory on the extended spacetime of the BTZ black hole, as
described by an orbifold of the SL(2,R) WZW model. The full spacetime has an
infinite number of disconnected boundary components, each corresponding to a
dual CFT. We discuss the computation of bulk and boundary correlation functions
for operators inserted on different components. String theory correlation
functions are obtained by analytic continuation from an orbifold of the
SL(2,C)/SU(2) coset model. This yields two-point functions for general
operators, including those describing strings that wind around the horizon of
the black hole.Comment: 35 pages, harvmac, 5 eps figures, uses epsf.tex. (v2): Extended
discussion in section 3.1, typo corrections, references adde
A study of the centrally produced baryon-antibaryon systems in pp interactions at 450 GeV/c
A study of the centrally produced ppbar, ppbarpi, ppbarpipi and lambda lambda
channels has been performed in pp collisions using an incident beam momentum of
450 GeV/c. No significant new structures are observed in the mass spectra,
however, important new information on the production dynamics is obtained. A
systematic study of the production properties of these systems has been
performed and it is found that these systems are not produced dominantly by
double Pomeron exchange.Comment: 13 pages, Latex, 4 Figure
A study of pseudoscalar states produced centrally in pp interactions at 450 GeV/c
A study has been made of pseudoscalar mesons produced centrally in pp
interactions. The results show that the eta and etaprime appear to have a
similar production mechanism which differs from that of the pi0. The production
properties of the eta and etaprime are not consistent with what is expected
from double Pomeron exchange. In addition the production mechanism for the eta
and etaprime is such that the production cross section are greatest when the
azimuthal angle between the pT vectors of the two protons is 90 degrees.Comment: 11 pages, Latex, 3 Figure
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