2,857 research outputs found
Project-based assessment for graduate coursework in physics
Project-based assessment, in the form of take-home exams, was trialed in an
honours/masters level electromagnetic theory course. This assessment formed an
integral part of the learning experience of the students, and students felt
that this was effective method of learning.Comment: 5 pages, no figures, presented at 17th AIP Congress, Brisbane, 200
Double-resonant extremely asymmetrical scattering of electromagnetic waves in periodic arrays separated by a gap
Two strong simultaneous resonances of scattering--double-resonant extremely
asymmetrical scattering (DEAS)--are predicted in two parallel, oblique,
periodic Bragg arrays separated by a gap, when the scattered wave propagates
parallel to the arrays. One of these resonances is with respect to frequency
(which is common to all types of Bragg scattering), and another is with respect
to phase variation between the arrays. The diffractional divergence of the
scattered wave is shown to be the main physical reason for DEAS in the
considered structure. Although the arrays are separated, they are shown to
interact by means of the diffractional divergence of the scattered wave across
the gap from one array into the other. It is also shown that increasing
separation between the two arrays results in a broader and weaker resonance
with respect to phase shift. The analysis is based on a recently developed new
approach allowing for the diffractional divergence of the scattered wave inside
and outside the arrays. Physical interpretations of the predicted features of
DEAS in separated arrays are also presented. Applicability conditions for the
developed theory are derived.Comment: 8 pages, 5 figure
Extremely asymmetrical scattering of electromagnetic waves in gradually varying periodic arrays
This paper analyses theoretically and numerically the effect of varying
grating amplitude on the extremely asymmetrical scattering (EAS) of bulk and
guided optical modes in non-uniform strip-like periodic Bragg arrays with
stepwise and gradual variations in the grating amplitude across the array. A
recently developed new approach based on allowance for the diffractional
divergence of the scattered wave is used for this analysis. It is demonstrated
that gradual variations in magnitude of the grating amplitude may change the
pattern of EAS noticeably but not radically. On the other hand, phase
variations in the grating may result in a radically new type of Bragg
scattering - double-resonant EAS (DEAS). In this case, a combination of two
strong simultaneous resonances (one with respect to frequency, and another with
respect to the phase variation) is predicted to take place in non-uniform
arrays with a step-like phase and gradual magnitude variations of the grating
amplitude. The tolerances of EAS and DEAS to small gradual variations in the
grating amplitude are determined. The main features of these types of
scattering in non-uniform arrays are explained by the diffractional divergence
of the scattered wave inside and outside the array.Comment: 13 pages, 10 figure
Grazing-angle scattering of electromagnetic waves in gratings with varying mean parameters: grating eigenmodes
A highly unusual pattern of strong multiple resonances for bulk
electromagnetic waves is predicted and analysed numerically in thick periodic
holographic gratings in a slab with the mean permittivity that is larger than
that of the surrounding media. This pattern is shown to exist in the geometry
of grazing-angle scattering (GAS), that is when the scattered wave (+1
diffracted order) in the slab propagates almost parallel to the slab (grating)
boundaries. The predicted resonances are demonstrated to be unrelated to
resonant generation of the conventional guided modes of the slab. Their
physical explanation is associated with resonant generation of a completely new
type of eigenmodes in a thick slab with a periodic grating. These new slab
eigenmodes are generically related to the grating; they do not exist if the
grating amplitude is zero. The field structure of these eigenmodes and their
dependence on structural and wave parameters is analysed. The results are
extended to the case of GAS of guided modes in a slab with a periodic groove
array of small corrugation amplitude and small variations in the mean thickness
of the slab at the array boundaries.Comment: 16 pages, 6 figure
Adaptive Segmentation of Knee Radiographs for Selecting the Optimal ROI in Texture Analysis
The purposes of this study were to investigate: 1) the effect of placement of
region-of-interest (ROI) for texture analysis of subchondral bone in knee
radiographs, and 2) the ability of several texture descriptors to distinguish
between the knees with and without radiographic osteoarthritis (OA). Bilateral
posterior-anterior knee radiographs were analyzed from the baseline of OAI and
MOST datasets. A fully automatic method to locate the most informative region
from subchondral bone using adaptive segmentation was developed. We used an
oversegmentation strategy for partitioning knee images into the compact regions
that follow natural texture boundaries. LBP, Fractal Dimension (FD), Haralick
features, Shannon entropy, and HOG methods were computed within the standard
ROI and within the proposed adaptive ROIs. Subsequently, we built logistic
regression models to identify and compare the performances of each texture
descriptor and each ROI placement method using 5-fold cross validation setting.
Importantly, we also investigated the generalizability of our approach by
training the models on OAI and testing them on MOST dataset.We used area under
the receiver operating characteristic (ROC) curve (AUC) and average precision
(AP) obtained from the precision-recall (PR) curve to compare the results. We
found that the adaptive ROI improves the classification performance (OA vs.
non-OA) over the commonly used standard ROI (up to 9% percent increase in AUC).
We also observed that, from all texture parameters, LBP yielded the best
performance in all settings with the best AUC of 0.840 [0.825, 0.852] and
associated AP of 0.804 [0.786, 0.820]. Compared to the current state-of-the-art
approaches, our results suggest that the proposed adaptive ROI approach in
texture analysis of subchondral bone can increase the diagnostic performance
for detecting the presence of radiographic OA
Orientation of biological cells using plane-polarized Gaussian beam optical tweezers
Optical tweezers are widely used for the manipulation of cells and their
internal structures. However, the degree of manipulation possible is limited by
poor control over the orientation of trapped cells. We show that it is possible
to controllably align or rotate disc shaped cells - chloroplasts of Spinacia
oleracea - in a plane polarised Gaussian beam trap, using optical torques
resulting predominantly from circular polarisation induced in the transmitted
beam by the non-spherical shape of the cells.Comment: 9 pages, 6 figure
Nanotrapping and the thermodynamics of optical tweezers
Particles that can be trapped in optical tweezers range from tens of microns
down to tens of nanometres in size. Interestingly, this size range includes
large macromolecules. We show experimentally, in agreement with theoretical
expectations, that optical tweezers can be used to manipulate single molecules
of polyethylene oxide suspended in water. The trapped molecules accumulate
without aggregating, so this provides optical control of the concentration of
macromolecules in solution. Apart from possible applications such as the
micromanipulation of nanoparticles, nanoassembly, microchemistry, and the study
of biological macromolecules, our results also provide insight into the
thermodynamics of optical tweezers.Comment: 5 pages, 3 figures, presented at 17th AIP Congress, Brisbane, 200
Estimating seasonal variations in cloud droplet number concentration over the boreal forest from satellite observations
Seasonal variations in cloud droplet number concentration (NCD) in low-level stratiform clouds over the boreal forest are estimated from MODIS observations of cloud optical and microphysical properties, using a sub-adiabatic cloud model to interpret vertical profiles of cloud properties. An uncertainty analysis of the cloud model is included to reveal the main sensitivities of the cloud model. We compared the seasonal cycle in NCD, obtained using 9 yr of satellite data, to surface concentrations of potential cloud activating aerosols, measured at the SMEAR II station at Hyytiälä in Finland. The results show that NCD and cloud condensation nuclei (CCN) concentrations have no clear correlation at seasonal time scale. The fraction of aerosols that actually activate as cloud droplet decreases sharply with increasing aerosol concentrations. Furthermore, information on the stability of the atmosphere shows that low NCD is linked to stable atmospheric conditions. Combining these findings leads to the conclusion that cloud droplet activation for the studied clouds over the boreal forest is limited by convection. Our results suggest that it is important to take the strength of convection into account when studying the influence of aerosols from the boreal forest on cloud formation, although they do not rule out the possibility that aerosols from the boreal forest affect other types of clouds with a closer coupling to the surfac
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