27,962 research outputs found
Theory of disorder-induced multiple coherent scattering in photonic crystal waveguides
We introduce a theoretical formalism to describe disorder-induced extrinsic
scattering in slow-light photonic crystal waveguides. This work details and
extends the optical scattering theory used in a recent \emph{Physical Review
Letter} [M. Patterson \emph{et al.}, \emph{Phys. Rev. Lett.} \textbf{102},
103901 (2009)] to describe coherent scattering phenomena and successfully
explain complex experimental measurements. Our presented theory, that combines
Green function and coupled mode methods, allows one to self-consistently
account for arbitrary multiple scattering for the propagating electric field
and recover experimental features such as resonances near the band edge. The
technique is fully three-dimensional and can calculate the effects of disorder
on the propagating field over thousands of unit cells. As an application of
this theory, we explore various sample lengths and disordered instances, and
demonstrate the profound effect of multiple scattering in the waveguide
transmission. The spectra yield rich features associated with disorder-induced
localization and multiple scattering, which are shown to be exasperated in the
slow light propagation regime
Interplay between disorder and local field effects in photonic crystal waveguides
We introduce a theory to describe disorder-induced scattering in photonic
crystal waveguides, specifically addressing the influence of local field
effects and scattering within high-index-contrast perturbations. Local field
effects are shown to increase the predicted disorder-induced scattering loss
and result in significant resonance shifts of the waveguide mode. We
demonstrate that two types of frequency shifts can be expected, a mean
frequency shift and a RMS frequency shift, both acting in concert to blueshift
and broaden the nominal band structure. For a representative waveguide, we
predict substantial meV frequency shifts and band structure broadening for a
telecommunications operating frequency, even for state of the art fabrication.
The disorder-induced broadening is found to increase as the propagation
frequency approaches the slow light regime (mode edge) due to restructuring of
the electric field distribution. These findings have a dramatic impact on
high-index-contrast nanoscale waveguides, and, for photonic crystal waveguides,
suggest that the nominal slow-light mode edge may not even exist. Furthermore,
our results shed new light on why it has hitherto been impossible to observe
the very slow light regime for photonic crystal waveguides.Comment: 4 page lette
Streaks to Rings to Vortex Grids: Generic Patterns in Transient Convective Spin-Up
We observe the transient formation of a ringed pattern state during spin-up
of an evaporating fluid on a time scale of order a few Ekman spin-up times. The
ringed state is probed using infrared thermometry and particle image
velocimetry and it is demonstrated to be a consequence of the transient balance
between Coriolis and viscous forces which dominate inertia, each of which are
extracted from the measured velocity field. The breakdown of the ringed state
is quantified in terms of the antiphasing of these force components which
drives a Kelvin-Helmholtz instability and we show that the resulting vortex
grid spacing scales with the ring wavelength. This is the fundamental route to
quasi-two dimensional turbulent vortex flow and thus may have implications in
astrophysics and geophysics wherein rotating convection is ubiquitous. sics.Comment: 4 pages, 5 figure
Measurements of aerosol properties needed to infer backscatter characteristics in support of the NASA Doppler Lidar program
During the first year two areas of work were emphasized, analysis of aerosol data to provide improved estimates of backscatter over the Pacific ocean and development of a global model for backscatter at different wavelengths and preparation and planning for the aircraft flights of the GLOVE program. Significant progress was made in each of these areas. The analytical work is a continuation of the GAMETAG analysis, and was directed toward the development of a backscatter model for the Pacific Oceanic free troposphere. This has included a further evaluation of relationships between the optical effects at different wavelengths, a comparison of modeled optical effects at differing temporal resolutions, and an investigation of the effects of sampling on the modeled results. An initial investigation of the predictability of 9 to 10 micrometers Beta values from other data sets as well as some preliminary comparisons of the modeling results with experimental data. The comparison of the optical effects was extended to make the comparisons for different sets of time bases between 1 and 20 minutes
A review of residual stress analysis using thermoelastic techniques
Thermoelastic Stress Analysis (TSA) is a full-field technique for experimental stress analysis
that is based on infra-red thermography. The technique has proved to be extremely effective for
studying elastic stress fields and is now well established. It is based on the measurement of the
temperature change that occurs as a result of a stress change. As residual stress is essentially a
mean stress it is accepted that the linear form of the TSA relationship cannot be used to
evaluate residual stresses. However, there are situations where this linear relationship is not
valid or departures in material properties due to manufacturing procedures have enabled
evaluations of residual stresses. The purpose of this paper is to review the current status of
using a TSA based approach for the evaluation of residual stresses and to provide some
examples of where promising results have been obtained
Oxidized basalts on the surface of Venus: Compositional implications of measured spectral properties
Venera Lander reflectance data are compared with high temperature spectra of the same basaltic materials. The dark, flat unoxidized basalts are still inconsistent with the Venera data in the near-infrared. Basaltic material with a ferric component, however, would satisfy both the increase in reflectance beyond 0.7 microns as well as the dark, relatively colorless character in the visible. Therefore, it is concluded that besaltic surfaces of Venus represented by these measurements either contain minerals with uncommon characteristics, or, more likely, are relatively oxidized
Distortion and regulation characterization of a Mapham inverter
Output voltage Total Harmonic Distortion (THD) of a 20kHz, 6kVA Mapham resonant inverter is characterized as a function of its switching-to-resonant frequency ratio, f sub s/f sub r, using the EASY5 engineering analysis system. EASY5 circuit simulation results are compared with hardware test results to verify the accuracy of the simulations. The effects of load on the THD versus f sub s/f sub r ratio is investigated for resistive, leading, and lagging power factor load impedances. The effect of the series output capacitor on the Mapham inverter output voltage distortion and inherent load regulation is characterized under loads of various power factors and magnitudes. An optimum series capacitor value which improves the inherent load regulation to better than 3 percent is identified. The optimum series capacitor value is different than the value predicted from a modeled frequency domain analysis. An explanation is proposed which takes into account the conduction overlap in the inductor pairs during steady-state inverter operation, which decreases the effective inductance of a Mapham inverter. A fault protection and current limit method is discussed which allows the Mapham inverter to operate into a short circuit, even when the inverter resonant circuit becomes overdamped
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