83,641 research outputs found
Theory of Transmission of Light by Sub-wavelength Cylindrical Holes in Metallic Films
This paper presents theory and finite-difference time-domain (FDTD)
calculations for a single and arrays of sub-wavelength cylindrical holes in
metallic films presenting large transmission. These calculations are in
excellent agreement with experimental measurements. This effect has to be
understood in terms of the properties exhibited by the dielectric constant of
metals which cannot be treated as ideal metals for the purpose of transmission
and diffraction of light. We discuss the cases of well-differentiated metals
silver and tungsten. It is found that the effect of surface plasmons or other
surface wave excitations due to a periodical set of holes or other roughness at
the surface is marginal. The effect can enhance but also can depress the
transmission of the arrays as shown by theory and experiments. The peak
structure observed in experiments is a consequence of the interference of the
wavefronts transmitted by each hole and is determined by the surface array
period independently of the material. Without large transmission through a
single hole there is no large transmission through the array. We found that in
the case of Ag which at the discussed frequencies is a metal there are
cylindrical plasmons at the wall of the hole, as reported by Economu et al 30
years ago, that enhanced the transmission. But it turns out, as will be
explained, that for the case of W which behaves as a dielectric, there is also
a large transmission when compared with that of an ideal metal waveguide. To
deal with this problem one has to use the measured dielectric function of the
metals. We discuss thoroughly all these cases and compare with the data.Comment: 13 pages and 9 figure
Low Voltage I-V Characteristics in Magnetic Tunnel Junctions
We show that elastic currents that take into account variations of the tunnel
transmitivity with voltage and a large ratio of majority to minority spin
densities of states of the band, can account for the low voltage current
anomalies observed in magnet-oxide-magnet junctions. The anomalies can be
positive, negative or have a mixed form, depending of the position of the Fermi
level in the band, in agreement with observations. Magnon contribution is
negligible small to account for the sharp drop of the magnetoresistance with
the voltage bias.Comment: 8 pages, 3 postscript figure
Towards High-order Methods for Rotorcraft Applications
This work presents CFD results obtained with an efficient, high-order, finite-volume scheme. The formulation is
based on the variable extrapolation MUSCL-scheme, and high-order spatial accuracy is achieved using correction
terms obtained through successive differentiation. The scheme is modified to cope with physical and multiblock
mesh interfaces, so stability, conservativeness, and high-order accuracy are guaranteed. Results with the proposed
scheme for steady flows, showed better wake and higher resolution of vortical structures compared with
the standard MUSCL, even when coarser meshes were employed. The method was also demonstrated for unsteady
flows using overset and moving grids for the UH-60A rotor in forward flight and the ERICA tiltrotor in aeroplane
mode. The present method adds CPU and memory overheads of 47% and 23%, respectively, in performing
multi-dimensional problems for routine computations
Light Collimation and Focussing by a Thin Flat Metallic Slab
We present experimental and theoretical work showing that a flat metallic
slab can collimate and focus light impinging on the slab from a punctual
source. The effect is optimised when the radiation is around the bulk, not at
the surface, plasma frequency. And the smaller the imaginary part of the
permittivity is, the better the collimation. Experiments for Ag in the visible
as well as calculations are presented. We also discuss the interesting case of
the Aluminium whose imaginary part of the permittivity is very small at the
plasma frequency in UV radiation. Generalization to other materials and
radiations are also discussed.Comment: 6 pages, 3 figures. To be published on Optics Lette
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