1,413 research outputs found
Near Field Lenses in Two Dimensions
It has been shown that a slab of materials with refractive index = -1 behaves
like a perfect lens focussing all light to an exact electromagnetic copy of an
object. The original lens is limited to producing images the same size as the
object, but here we generalise the concept so that images can be magnified. For
two dimensional systems, over distances much shorter than the free space
wavelength, we apply conformal transformations to the original parallel sided
slab generating a variety of new lenses. Although the new lenses are not
`perfect' they are able to magnify two dimensional objects. The results apply
equally to imaging of electric or magnetic sub wavelength objects in two
dimensions. The concepts have potential applications ranging from microwave
frequencies to the visible.Comment: PDF fil
The Theory of SNOM: A Novel Approach
In this paper we consider the application of electromagnetic theory to the
analysis of the Scanning Near-field Optical Microscope (SNOM) in order to
predict experimentally observable quantities such as the transmission or
reflection coefficients for a particular tip-surface configuration. In
particular we present the first application of a transfer matrix based
calculation to this challenging problem by using an adaptive co-ordinate
transformation to accurately model the shape of the SNOM tip.
We also investigate the possibility of increasing the transmitted light
through the SNOM tip by introducing a metal wire into the centre of the tip.
This converts the tip into a co-axial cable. We show that, in principle, this
can dramatically improve the transmission characteristics without having a
detrimental effect on the resolution.Comment: 19 pages, 11 figures. To be published in the Journal of Modern Optic
Focussing Light Using Negative Refraction
A slab of negatively refracting material, thickness d, can focus an image at
a distance 2d from the object. The negative slab cancels an equal thickness of
positive space. This result is a special case of a much wider class of
focussing: any medium can be optically cancelled by an equal thickness of
material constructed to be an inverted mirror image of the medium, with,
and reversed in sign. We introduce the powerful technique of
coordinate transformation, mapping a known system into an equivalent system, to
extend the result to a much wider class of structures including cylinders,
spheres, and intersecting planes and hence show how to produce magnified
images. All the images are perfect in the sense that both the near and far
fields are brought to a focus and hence reveal sub wavelength details.Comment: pdf file onl
Imaging the Near Field
In an earlier paper we introduced the concept of the perfect lens which
focuses both near and far electromagnetic fields, hence attaining perfect
resolution. Here we consider refinements of the original prescription designed
to overcome the limitations of imperfect materials. In particular we show that
a multi-layer stack of positive and negative refractive media is less sensitive
to imperfections. It has the novel property of behaving like a fibre-optic
bundle but one that acts on the near field, not just the radiative component.
The effects of retardation are included and minimized by making the slabs
thinner. Absorption then dominates image resolution in the near-field. The
deleterious effects of absorption in the metal are reduced for thinner layers.Comment: RevTeX, (9 pages, 8 figures
Calculating photonic Green's functions using a non-orthogonal finite difference time domain method
In this paper we shall propose a simple scheme for calculating Green's
functions for photons propagating in complex structured dielectrics or other
photonic systems. The method is based on an extension of the finite difference
time domain (FDTD) method, originally proposed by Yee, also known as the
Order-N method, which has recently become a popular way of calculating photonic
band structures. We give a new, transparent derivation of the Order-N method
which, in turn, enables us to give a simple yet rigorous derivation of the
criterion for numerical stability as well as statements of charge and energy
conservation which are exact even on the discrete lattice. We implement this
using a general, non-orthogonal co-ordinate system without incurring the
computational overheads normally associated with non-orthogonal FDTD.
We present results for local densities of states calculated using this method
for a number of systems. Firstly, we consider a simple one dimensional
dielectric multilayer, identifying the suppression in the state density caused
by the photonic band gap and then observing the effect of introducing a defect
layer into the periodic structure. Secondly, we tackle a more realistic example
by treating a defect in a crystal of dielectric spheres on a diamond lattice.
This could have application to the design of super-efficient laser devices
utilising defects in photonic crystals as laser cavities.Comment: RevTex file. 10 pages with 8 postscript figures. Submitted to Phys
Rev
Order N photonic band structures for metals and other dispersive materials
We show, for the first time, how to calculate photonic band structures for
metals and other dispersive systems using an efficient Order N scheme. The
method is applied to two simple periodic metallic systems where it gives
results in close agreement with calculations made with other techniques.
Further, the approach demonstrates excellent numerical stablity within the
limits we give. Our new method opens the way for efficient calculations on
complex structures containing a whole new class of material.Comment: Four pages, plus seven postscript figures. Submitted to Physical
Review Letter
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