2,815 research outputs found
Lorenz-Mie theory for 2D scattering and resonance calculations
This PhD tutorial is concerned with a description of the two-dimensional
generalized Lorenz-Mie theory (2D-GLMT), a well-established numerical method
used to compute the interaction of light with arrays of cylindrical scatterers.
This theory is based on the method of separation of variables and the
application of an addition theorem for cylindrical functions. The purpose of
this tutorial is to assemble the practical tools necessary to implement the
2D-GLMT method for the computation of scattering by passive scatterers or of
resonances in optically active media. The first part contains a derivation of
the vector and scalar Helmholtz equations for 2D geometries, starting from
Maxwell's equations. Optically active media are included in 2D-GLMT using a
recent stationary formulation of the Maxwell-Bloch equations called
steady-state ab initio laser theory (SALT), which introduces new classes of
solutions useful for resonance computations. Following these preliminaries, a
detailed description of 2D-GLMT is presented. The emphasis is placed on the
derivation of beam-shape coefficients for scattering computations, as well as
the computation of resonant modes using a combination of 2D-GLMT and SALT. The
final section contains several numerical examples illustrating the full
potential of 2D-GLMT for scattering and resonance computations. These examples,
drawn from the literature, include the design of integrated polarization
filters and the computation of optical modes of photonic crystal cavities and
random lasers.Comment: This is an author-created, un-copyedited version of an article
published in Journal of Optics. IOP Publishing Ltd is not responsible for any
errors or omissions in this version of the manuscript or any version derived
from i
Adding SALT to Coupled Microcavities: the making of active photonic molecule lasers
A large body of work has accumulated over the years in the study of the
optical properties of single and coupled microcavities for a variety of
applications, ranging from filters to sensors and lasers. The focus has been
mostly on the geometry of individual resonators and/or on their combination in
arrangements often referred to as photonic molecules (PMs).
Our primary concern will be the lasing properties of PMs as ideal candidates
for the fabrication of integrated microlasers, photonic molecule lasers.
Whereas most calculations on PM lasers have been based on cold-cavity (passive)
modes, i.e. quasi-bound states, a recently formulated steady-state ab initio
laser theory (SALT) offers the possibility to take into account the spectral
properties of the underlying gain transition, its position and linewidth, as
well as incorporating an arbitrary pump profile. We will combine two
theoretical approaches to characterize the lasing properties of PM lasers: for
two-dimensional systems, the generalized Lorenz-Mie theory will obtain the
resonant modes of the coupled molecules in an active medium described by SALT.
Not only is then the theoretical description more complete, the use of an
active medium provides additional parameters to control, engineer and harness
the lasing properties of PM lasers for ultra-low threshold and directional
single-mode emission.Comment: 16th International Conference on Transparent Optical Networks (2014
Phase Space Engineering in Optical Microcavities I: Preserving near-field uniformity while inducing far-field directionality
Optical microcavities have received much attention over the last decade from
different research fields ranging from fundamental issues of cavity QED to
specific applications such as microlasers and bio-sensors. A major issue in the
latter applications is the difficulty to obtain directional emission of light
in the far-field while keeping high energy densities inside the cavity (i.e.
high quality factor). To improve our understanding of these systems, we have
studied the annular cavity (a dielectric disk with a circular hole), where the
distance cavity-hole centers, d, is used as a parameter to alter the properties
of cavity resonances. We present results showing how one can affect the
directionality of the far-field while preserving the uniformity (hence the
quality factor) of the near-field simply by increasing the value of d.
Interestingly, the transition between a uniform near- and far-field to a
uniform near- and directional far-field is rather abrupt. We can explain this
behavior quite nicely with a simple model, supported by full numerical
calculations, and we predict that the effect will also be found in a large
class of eigenmodes of the cavity.Comment: 12th International Conference on Transparent Optical Network
Optimization of integrated polarization filters
This study reports on the design of small footprint, integrated polarization
filters based on engineered photonic lattices. Using a rods-in-air lattice as a
basis for a TE filter and a holes-in-slab lattice for the analogous TM filter,
we are able to maximize the degree of polarization of the output beams up to 98
% with a transmission efficiency greater than 75 %. The proposed designs allow
not only for logical polarization filtering, but can also be tailored to output
an arbitrary transverse beam profile. The lattice configurations are found
using a recently proposed parallel tabu search algorithm for combinatorial
optimization problems in integrated photonics
Ab initio investigation of lasing thresholds in photonic molecules
We investigate lasing thresholds in a representative photonic molecule
composed of two coupled active cylinders of slightly different radii.
Specifically, we use the recently formulated steady-state ab initio laser
theory (SALT) to assess the effect of the underlying gain transition on lasing
frequencies and thresholds. We find that the order in which modes lase can be
modified by choosing suitable combinations of the gain center frequency and
linewidth, a result that cannot be obtained using the conventional approach of
quasi-bound modes. The impact of the gain transition center on the lasing
frequencies, the frequency pulling effect, is also quantified
S and Q Matrices Reloaded: applications to open, inhomogeneous, and complex cavities
We present a versatile numerical algorithm for computing resonances of open
dielectric cavities. The emphasis is on the generality of the system's
configuration, i.e. the geometry of the (main) cavity (and possible inclusions)
and the internal and external dielectric media (homogeneous and inhomogeneous).
The method is based on a scattering formalism to obtain the position and width
of the (quasi)-eigenmodes. The core of the method lies in the scattering
S-matrix and its associated delay Q-matrix which contain all the relevant
information of the corresponding scattering experiment. For instance, the
electromagnetic near- and far-fields are readily extracted. The flexibility of
the propagation method is displayed for a selected system.Comment: 15th International Conference on Transparent Optical Networks (2013
Matrix metalloproteinase inhibitors: Present achievements and future prospects
Matrix metalloproteinases (MMPs) are a class of structurally related enzymes that function in the degradation of extracellular matrix proteins that constitute the pericellular connective tissue and play an important role in both normal and pathological tissue remodelling. Increased MMP activity is detected in a wide range of cancers and seems correlated to their invasive and metastatic potential. MMPs thus seem an attractive target for both diagnostic and therapeutic purposes. Several synthetic matrix metalloproteinase inhibitors (MMPIs) are currently being developed. Preclinical studies are promising as they suggest inhibition of several steps in the metastatic process. Marimastat is the first MMPI to enter comparative phase III trials after early clinical trials established the safety profile. Clinical trials will need to be specifically designed to optimally evaluate the therapeutic potential of this novel class of cytostatic drugs. Safety studies should consider the markedly different toxicity profile and determine the range of biologically active dosage, while efficacy studies should be performed in selected clinical settings with appropriate end-points. We review the present achievements in preclinical and clinical studies with MMPIs, discuss specific considerations for appropriate study design and reflect on the future prospects of this novel class of agents
Coherent beam shaping using two-dimensional photonic crystals
Optical devices based on photonic crystals such as waveguides, lenses and
beam-shapers, have received considerable theoretical and experimental attention
in recent years. The production of these devices has been facilitated by the
wide availability of silicon-on-insulator fabrication techniques. In this
theoretical work, we show the possibility to design a coherent PhC-based
beam-shaper. The basic photonic geometry used is a 2D square lattice of air
holes in a high-index dielectric core. We formulate the beam shaping problem in
terms of objective functions related to the amplitude and phase profile of the
generated beam. We then use a parallel tabu search algorithm to minimize the
two objectives simultaneously. Our results show that optimization of several
attributes in integrated photonics design is well within reach of current
algorithms.Comment: 15th International Conference on Transparent Optical Networks (2013
New insights into the pathogenesis of glucocorticoid-induced avascular necrosis: microarray analysis of gene expression in a rat model
The role and challenges of cluster randomised trials for global health
Evaluating whether an intervention works when trialled in groups of individuals can pose complex challenges for clinical research. Cluster randomised controlled trials involve the random allocation of groups or clusters of individuals to receive an intervention, and they are commonly used in global health research. In this paper, we describe the potential reasons for the increasing popularity of cluster trials in low-income and middle-income countries. We also draw on key areas of global health research for an assessment of common trial planning practices, and we address their methodological shortcomings and pitfalls. Lastly, we discuss alternative approaches for population-level intervention trials that could be useful for research undertaken in low-income and middle-income countries for situations in which the use of cluster randomisation might not be appropriate
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