647 research outputs found
Quasi-TEM modes in rectangular waveguides: a study based on the properties of PMC and hard surfaces
Hard surfaces or magnetic surfaces can be used to propagate quasi-TEM modes
inside closed waveguides. The interesting feature of these modes is an almost
uniform field distribution inside the waveguide. But the mechanisms governing
how these surfaces act, how they can be characterized, and further how the
modes propagate are not detailed in the literature. In this paper, we try to
answer these questions. We give some basic rules that govern the propagation of
the quasi-TEM modes, and show that many of their characteristics (i.e. their
dispersion curves) can be deduced from the simple analysis of the reflection
properties of the involved surfaces
Design of metallic nanoparticles gratings for filtering properties in the visible spectrum
Plasmonic resonances in metallic nanoparticles are exploited to create
efficient optical filtering functions. A Finite Element Method is used to model
metallic nanoparticles gratings. The accuracy of this method is shown by
comparing numerical results with measurements on a two-dimensional grating of
gold nanocylinders with elliptic cross section. Then a parametric analysis is
performed in order to design efficient filters with polarization dependent
properties together with high transparency over the visible range. The behavior
of nanoparticle gratings is also modelled using the Maxwell-Garnett
homogenization theory and analyzed by comparison with the diffraction by a
single nanoparticle. The proposed structures are intended to be included in
optical systems which could find innovative applications.Comment: submitted to Applied Optic
Surface coupling effects on the capacitance of thin insulating films
A general form for the surface roughness effects on the capacitance of a
capacitor is proposed. We state that a capacitor with two uncoupled rough
surfaces could be treated as two capacitors in series which have been divided
from the mother capacitor by a slit. This is in contrast to the case where the
two rough surfaces are coupled. When the rough surfaces are coupled, the type
of coupling decides the modification of the capacitance in comparison to the
uncoupled case. It is shown that if the coupling between the two surfaces of
the capacitor is positive (negative), the capacitance is less (higher) than the
case of two uncoupled rough plates. Also, we state that when the correlation
length and the roughness exponent are small, the coupling effect is not
negligible
A modal model for diffraction gratings
A description of an algorithm for a rather general modal grating calculation
is presented. Arbitrary profiles, depth, and permittivity are allowed. Gratings
built up from sub-gratings are allowed, as are coatings on the sidewalls of
lines, and arbitrary complex structure. Conical angles and good conductors are
supported
Dynamic effective anisotropy: Asymptotics, simulations, and microwave experiments with dielectric fibers
International audienceWe investigate dynamic effective anisotropy in photonic crystals (PCs) through a combination of an effective medium theory, which is a high-frequency homogenization (HFH) method explicitly developed to operate for short waves, as well as through numerical simulations and microwave experiments. The HFH yields accurate predictions of the effective anisotropic properties of periodic structures when the wavelength is of comparable order to the pitch of the array; specifically, we investigate a square array of pitch 2 cm consisting of dielectric rods of radius 0.5 cm and refractive index n=6√ within an air matrix. This behaves as an effective medium, with strong artificial anisotropy, at a frequency corresponding to a flat band emerging from a Dirac-like point in transverse magnetic (TM) polarization. At this frequency, highly directive emission is predicted for an electric source placed inside this PC, and this artificial anisotropy can be shown to coincide with a change of character of the underlying effective equation from isotropic to unidirective, with coefficients of markedly different magnitudes appearing in the effective equation tensor. In transverse electric (TE) polarization, we note a second radical change of character of the underlying effective equation, this time from elliptic to hyperbolic, near a frequency at which a saddle point occurs in the corresponding dispersion curves. Delicate microwave experiments are performed in both polarizations for such a PC consisting of 80 rods, and we demonstrate that a directive emission in the form of a + (respectively, an X) is indeed seen experimentally at the predicted frequency 9.5 GHz in TM polarization (respectively, 5.9 GHz in TE polarization). These are clearly dynamic effects since in the quasistatic regime the PC just behaves as an isotropic medium
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