236 research outputs found
Time-reversal in dynamically-tuned zero-gap periodic systems
We show that short pulses propagating in zero-gap periodic systems can be
reversed with 100% efficiency by using weak non-adiabatic tuning of the wave
velocity at time-scales that can be much slower than the period. Unlike
previous schemes, we demonstrate reversal of {\em broadband} (few cycle) pulses
with simple structures. Our scheme may thus open the way to time-reversal in a
variety of systems for which it was not accessible before.Comment: Accepted for publication in Phys. Rev. Letter
Full-wave simulations of electromagnetic cloaking structures
Based on a coordinate transformation approach, Pendry {\it et al.} have
reported electromagnetically anisotropic and inhomogeneous shells that, in
theory, completely shield an interior structure of arbitrary size from
electromagnetic fields without perturbing the external fields. We report
full-wave simulations of the cylindrical version of this cloaking structure
using ideal and nonideal (but physically realizable) electromagnetic parameters
in an effort to understand the challenges of realizing such a structure in
practice. The simulations indicate that the performance of the electromagnetic
cloaking structure is not especially sensitive to modest permittivity and
permeability variations. This is in contrast to other applications of
engineered electromagnetic materials, such as subwavelength focusing using
negative refractive index materials. The cloaking performance degrades smoothly
with increasing loss, and effective low-reflection shielding can be achieved
with a cylindrical shell composed of an eight (homogeneous) layer approximation
of the ideal continuous medium
Ray-optical negative refraction and pseudoscopic imaging with Dove-prism arrays
A sheet consisting of an array of small, aligned Dove prisms can locally (on the scale of the width of the prisms) invert one component of the ray direction. A sandwich of two such Dove-prism sheets that inverts both transverse components of the ray direction is a ray-optical approximation to the interface between two media with refractive indices +n and –n. We demonstrate the simulated imaging properties of such a Dove-prism-sheet sandwich, including a demonstration of pseudoscopic imaging
Time varying gratings model Hawking radiation
Diffraction gratings synthetically moving at trans-luminal velocities contain
points where wave and grating velocities are equal. We show these points can be
understood as a series of optical event horizons where wave energy can be
trapped and amplified, leading to radiation from the quantum vacuum state. We
calculate the spectrum of this emitted radiation, finding a quasi-thermal
spectrum with features that depend on the grating profile, and an effective
temperature that scales exponentially with the length of the grating, emitting
a measurable flux even for very small grating contrast.Comment: 13 pages, 4 figure
Customised broadband metamaterial absorbers for arbitrary polarisation
This paper shows that customised broadband absorption of electromagnetic
waves having arbitrary polarisation is possible by use of lossy cut-wire (CW)
metamaterials. These useful features are confirmed by numerical simulations in
which different lengths of CW pairs are combined as one periodic metamaterial
unit and placed near to a perfect electric conductor (PEC). So far metamaterial
absorbers have exhibited some interesting features, which are not available
from conventional absorbers, e.g. straightforward adjustment of electromagnetic
properties and size reduction. The paper shows how with proper design a broad
range of absorber characteristics may be obtained.Comment: 12 pages, 5 figures, submitted to Optics Expres
Design of Electromagnetic Cloaks and Concentrators Using Form-Invariant Coordinate Transformations of Maxwell's Equations
The technique of applying form-invariant, spatial coordinate transformations
of Maxwell's equations can facilitate the design of structures with unique
electromagnetic or optical functionality. Here, we illustrate the
transformation-optical approach in the designs of a square electromagnetic
cloak and an omni-directional electromagnetic field concentrator. The
transformation equations are described and the functionality of the devices is
numerically confirmed by two-dimensional finite element simulations. The two
devices presented demonstrate that the transformation optic approach leads to
the specification of complex, anisotropic and inhomogeneous materials with well
directed and distinct electromagnetic behavior.Comment: submitted to "Photonics and Nanostructures", Special Issue "PECS
VII", Elsevie
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
Optical design of reflectionless complex media by finite embedded coordinate transformations
Transformation optics offers an unconventional approach to the control of
electromagnetic fields. A transformation optical structure is designed by first
applying a form-invariant coordinate transform to Maxwell's equations, in which
part of free space is distorted in some desired manner. The coordinate
transformation is then applied to the permittivity and permeability tensors to
yield the specification for a complex medium with desired functionality. The
transformation optical structures proposed to date, such as electromagnetic
"invisibility" cloaks and concentrators, are inherently reflectionless and
leave the transmitted wave undisturbed. Here we expand the class of
transformation optical structures by introducing finite, embedded coordinate
transformations, which allow the electromagnetic waves to be steered or
focused. We apply the method to the design of several devices, including a
parallel beam shifter and a beam splitter, both of which exhibit unusual
electromagnetic behavior as confirmed by 2D full-wave simulations. The devices
are designed to be reflectionless, in accordance with a straightforward
topological criterion.Comment: submitted to the journal on Sep 10 2007, abstract changed to make it
more accessible, keywords adde
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