13,172 research outputs found
Light scattering from self-affine fractal silver surfaces with nanoscale cutoff: Far-field and near-field calculations
We study the light scattered from randomly rough, one-dimensional self-affine
fractal silver surfaces with nanoscale lower cutoff, illuminated by s- or
p-polarized Gaussian beams a few microns wide. By means of rigorous numerical
calculations based on the Green theorem integral equation formulation, we
obtain both the far- and near-field scattered intensities. The influence of
diminishing the fractal lower scale cutoff (from below a hundred, down to a few
nanometers) is analyzed in the case of both single realizations and ensemble
average magnitudes. For s polarization, variations are small in the far field,
being only significant in the higher spatial frequency components of evanescent
character in the near field. In the case of p polarization, however, the
nanoscale cutoff has remarkable effects stemming from the roughness-induced
excitation of surface-plasmon polaritons. In the far field, the effect is
noticed both in the speckle pattern variation and in the decrease of the total
reflected energy upon ensemble averaging, due to increased absorption. In the
near field, more efficient excitation of localized optical modes is achieved
with smaller cutoff, which in turn leads to huge surface electric field
enhancements.Comment: REVTeX 4, 10 page
Quantum calculations of the carrier mobility in thin films: Methodology, Matthiessen's rule and comparison with semi-classical approaches
We discuss the calculation of the carrier mobility in silicon films within
the quantum Non-Equilibrium Green's Functions (NEGF) framework. We introduce a
new method for the extraction of the carrier mobility that is free from contact
resistance contamination, and provides accurate mobilities at a reasonable
cost, with minimal needs for ensemble averages. We then introduce a new
paradigm for the definition of the partial mobility associated with a
given elastic scattering mechanism "M", taking phonons (PH) as a reference
(). We argue that this definition
makes better sense in a quantum transport framework as it is free from long
range interference effects that can appear in purely ballistic calculations. As
a matter of fact, these mobilities satisfy Matthiessen's rule for three
mechanisms [surface roughness (SR), remote Coulomb scattering (RCS) and
phonons] much better than the usual, single mechanism calculations. We also
discuss the problems raised by the long range spatial correlations in the RCS
disorder. Finally, we compare semi-classical Kubo-Greenwood (KG) and quantum
NEGF calculations. We show that KG and NEGF are in reasonable agreement for
phonon and RCS, yet not for SR. We point to possible deficiencies in the
treatment of SR scattering in KG, opening the way for further improvements.Comment: Submitted to Journal of Applied Physic
Numerical studies of the scattering of light from a two-dimensional randomly rough interface between two dielectric media
The scattering of polarized light incident from one dielectric medium on its
two-dimensional randomly rough interface with a second dielectric medium is
studied. A reduced Rayleigh equation for the scattering amplitudes is derived
for the case where p- or s-polarized light is incident on this interface, with
no assumptions being made regarding the dielectric functions of the media.
Rigorous, purely numerical, nonperturbative solutions of this equation are
obtained. They are used to calculate the reflectivity and reflectance of the
interface, the mean differential reflection coefficient, and the full angular
distribution of the intensity of the scattered light. These results are
obtained for both the case where the medium of incidence is the optically less
dense medium, and in the case where it is the optically more dense medium.
Optical analogues of the Yoneda peaks observed in the scattering of x-rays from
metal surfaces are present in the results obtained in the latter case. Brewster
scattering angles for diffuse scattering are investigated, reminiscent of the
Brewster angle for flat-interface reflection, but strongly dependent on the
angle of incidence. When the contribution from the transmitted field is added
to that from the scattered field it is found that the results of these
calculations satisfy unitarity with an error smaller than .Comment: 25 pages, 14 figure
Optomagnetic composite medium with conducting nanoelements
A new type of metal-dielectric composites has been proposed that is
characterised by a resonance-like behaviour of the effective permeability in
the infrared and visible spectral ranges. This material can be referred to as
optomagnetic medium. The analytical formalism developed is based on solving the
scattering problem for considered inclusions with impedance boundary condition,
which yields the current and charge distributions within the inclusions. The
presence of the effective magnetic permeability and its resonant properties
lead to novel optical effects and open new possible applications.Comment: 48 pages, 13 figures. accepted to Phys. Rev. B; to appear vol. 66,
200
A full wave method for rough surface scattering using fictitious current distributions
Rough surface scattering is a current topic of interest in many diverse fields. But, despite its importance, the two most widely used solution methods, the Kirchhoff and first order perturbation methods, are valid only for a restricted range of surface types. There is a large range of surface statistics for which neither of these theories is valid. There are purely numerical solutions to the problem, i.e., the integral equation technique and FDTD method, but these methods require a prohibitively large amount of computer time and storage space for use in practical applications. A full wave method has been introduced by E. Bahar which agrees with the Kirchhoff method in its range of validity, but does not bridge the gap between the later two standard theories and does not provide understanding of the physical processes involved in rough surface scattering. Consequently, it has been a center of controversy since modifications made to improve the method seem arbitrary and are without mathematical or physical justification.
The method presented here is a new full wave method which uses equivalent currents to provide insight into the physical scattering processes. This full wave method analytically reduces to the two standard theories in their respective regions of validity and bridges the gap between the two, which was shown by comparison to the integral equation method. The results presented here are for statistically rough surfaces with Gaussian distributed heights and slopes. A Monte Carlo procedure is used to generate the radar cross section data for this new full wave method
Trefftz Difference Schemes on Irregular Stencils
The recently developed Flexible Local Approximation MEthod (FLAME) produces
accurate difference schemes by replacing the usual Taylor expansion with
Trefftz functions -- local solutions of the underlying differential equation.
This paper advances and casts in a general form a significant modification of
FLAME proposed recently by Pinheiro & Webb: a least-squares fit instead of the
exact match of the approximate solution at the stencil nodes. As a consequence
of that, FLAME schemes can now be generated on irregular stencils with the
number of nodes substantially greater than the number of approximating
functions. The accuracy of the method is preserved but its robustness is
improved. For demonstration, the paper presents a number of numerical examples
in 2D and 3D: electrostatic (magnetostatic) particle interactions, scattering
of electromagnetic (acoustic) waves, and wave propagation in a photonic
crystal. The examples explore the role of the grid and stencil size, of the
number of approximating functions, and of the irregularity of the stencils.Comment: 28 pages, 12 figures; to be published in J Comp Phy
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