18,463 research outputs found
The Effect of Random Surface Inhomogeneities on Microresonator Spectral Properties: Theory and Modeling at Millimeter Wave Range
The influence of random surface inhomogeneities on spectral properties of
open microresonators is studied both theoretically and experimentally. To solve
the equations governing the dynamics of electromagnetic fields the method of
eigen-mode separation is applied previously developed with reference to
inhomogeneous systems subject to arbitrary external static potential. We prove
theoretically that it is the gradient mechanism of wave-surface scattering
which is the highly responsible for non-dissipative loss in the resonator. The
influence of side-boundary inhomogeneities on the resonator spectrum is shown
to be described in terms of effective renormalization of mode wave numbers
jointly with azimuth indices in the characteristic equation. To study
experimentally the effect of inhomogeneities on the resonator spectrum, the
method of modeling in the millimeter wave range is applied. As a model object
we use dielectric disc resonator (DDR) fitted with external inhomogeneities
randomly arranged at its side boundary. Experimental results show good
agreement with theoretical predictions as regards the predominance of the
gradient scattering mechanism. It is shown theoretically and confirmed in the
experiment that TM oscillations in the DDR are less affected by surface
inhomogeneities than TE oscillations with the same azimuth indices. The DDR
model chosen for our study as well as characteristic equations obtained
thereupon enable one to calculate both the eigen-frequencies and the Q-factors
of resonance spectral lines to fairly good accuracy. The results of
calculations agree well with obtained experimental data.Comment: 17+ pages, 5 figure
Computation of Electromagnetic Fields Scattered From Objects With Uncertain Shapes Using Multilevel Monte Carlo Method
Computational tools for characterizing electromagnetic scattering from
objects with uncertain shapes are needed in various applications ranging from
remote sensing at microwave frequencies to Raman spectroscopy at optical
frequencies. Often, such computational tools use the Monte Carlo (MC) method to
sample a parametric space describing geometric uncertainties. For each sample,
which corresponds to a realization of the geometry, a deterministic
electromagnetic solver computes the scattered fields. However, for an accurate
statistical characterization the number of MC samples has to be large. In this
work, to address this challenge, the continuation multilevel Monte Carlo
(CMLMC) method is used together with a surface integral equation solver. The
CMLMC method optimally balances statistical errors due to sampling of the
parametric space, and numerical errors due to the discretization of the
geometry using a hierarchy of discretizations, from coarse to fine. The number
of realizations of finer discretizations can be kept low, with most samples
computed on coarser discretizations to minimize computational cost.
Consequently, the total execution time is significantly reduced, in comparison
to the standard MC scheme.Comment: 25 pages, 10 Figure
Quasi-specular reflection from particulate media
Specular reflection is known to play an important role in many fields of
scattering applications, e.g., in remote sensing, computer graphics,
optimization of visual appearance of industrial products. Usually it can be
assumed that the object has a solid surface and that the properties of the
surface will dictate the behavior of the specular component. In this study I
will show that media consisting of wavelength-sized particles can also have a
quasi-specular reflection in cases where there is ordered structure in the
media. I will also show that the quasi-specular reflection in particulate media
is more than just a surface effect, and planar particle arrangement below the
very surface can give arise to quasi-specular reflection. This study shows that
the quasi-specular reflection may contribute in some cases in the
backscattering direction, together with coherent backscattering and
shadow-hiding effects
Radiative Transfer in a Discrete Random Medium Adjacent to a Half-Space with a Rough Interface
For a macroscopically plane-parallel discrete random medium, the boundary conditions for the specific coherency dyadic at a rough interface are derived. The derivation is based on a modification of the Twersky approximation for a scattering system consisting of a group of particles and the rough surface, and reduces to the solution of the scattering problem for a rough surface illuminated by a plane electromagnetic wave propagating in a discrete random medium with non-scattering boundaries. In a matrix-form setting, the boundary conditions for the specific coherency dyadic imply the boundary conditions for specific intensity column vectors which in turn, yield the expressions for the reflection and transmission matrices. The derived expressions are shown to be identical to those obtained by applying a phenomenological approach based on a facet model to the solution of the scattering problem for a rough surface illuminated by a plane electromagnetic wave
A surface-scattering model satisfying energy conservation and reciprocity
In order for surface scattering models to be accurate they must necessarily
satisfy energy conservation and reciprocity principles. Roughness scattering
models based on Kirchoff's approximation or perturbation theory do not satisfy
these criteria in all frequency ranges. Here we present a surface scattering
model based on analysis of scattering from a layer of particles on top of a
substrate in the dipole approximation which satisfies both energy conservation
and reciprocity and is thus accurate in all frequency ranges. The model takes
into account the absorption in the substrate induced by the particles but does
not take into account the near-field interactions between the particles.Comment: 15 pages, 10 figure
Fundamental remote sensing science research program. Part 1: Scene radiation and atmospheric effects characterization project
Brief articles summarizing the status of research in the scene radiation and atmospheric effect characterization (SRAEC) project are presented. Research conducted within the SRAEC program is focused on the development of empirical characterizations and mathematical process models which relate the electromagnetic energy reflected or emitted from a scene to the biophysical parameters of interest
Analytical modeling and 3D finite element simulation of line edge roughness in scatterometry
The influence of edge roughness in angle resolved scatterometry at
periodically structured surfaces is investigated. A good description of the
radiation interaction with structured surfaces is crucial for the understanding
of optical imaging processes like, e.g. in photolithography. We compared an
analytical 2D model and a numerical 3D simulation with respect to the
characterization of 2D diffraction of a line grating involving structure
roughness. The results show a remarkably high agreement. The diffraction
intensities of a rough structure can therefore be estimated using the numerical
simulation result of an undisturbed structure and an analytically derived
correction function. This work allows to improve scatterometric results for the
case of practically relevant 2D structures
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