medium that filled the hidden region. The cloak reduced the
size of the concealed object making use of the form invariance
of Maxwell’s equation instead of hiding it. Optimization in
the design of cloak could be done by considering different
mappings and finding the most appropriate one. Scattering
and radiation from the imperfect cylindrical cloak was solved
analytically to obtain the electric field intensity in terms of
Bessel’s and Hankel’s functions, and the results were
confirmed by full-wave finite element simulation (COMSOL
Multiphysics).
An effort to investigate the challenges of realizing a cloak in
practice using full wave simulation of cylindrical cloaking
using ideal and non-ideal EM parameters was reported by
Cummer
et al. (2008a). The electric field distribution and
electromagnetic power flow were plotted (using COMSOL
Multiphysics S/w) for various configurations such as, cloak
with simplified parameters, with ideal parameters, with and
without loss, and cloak with 8-layer approximation. Cloaking
effect was clear in ideal case. However, the performance of
cloak deteriorated with increase in loss.
A scheme for cloaking objects on a dielectric half-space was
proposed by Zhang
et al. (2008b), in which two vertical
matching strips at the bottom surfaces of a semi-cylindrical
cloaking cover were introduced to achieve the cloaking effect.
The cloaking effect was tested by placing a PEC scatterer on
the axis of the cloaking cover. The matching strips eliminated
the non-uniformity of intensity distribution making the
reflected cylindrical wave uniform resulting in cloaking.
Co-ordinate transformation approach was further used by W.
X. Jiang (2008
a) for design of cloaks of arbitrary shape. The
anisotropic, inhomogeneous permittivity and permeability
tensors were derived for an elliptical cloak, which could be
extended to spherical cloak. The cloaking phenomenon was
illustrated using an elliptical cylindrical cloak operating at a
frequency of 2 GHz. The object to be hidden was a perfect
electric conductor (PEC). The electric field distributions and
power flow lines were plotted for lossy and lossless cloaking
material, which showed that fields in the interior region were
zero, and the power flow lines propagated smoothly around
the PEC. Outside the cloak, the waves however were
unchanged thus achieving cloaking. Loss degraded the
performance of the cloak in the forward scattering direction
but the invisibility effect was good for the backward and other
direction. The computational domain used for full wave
simulation of elliptical cylindrical cloaks is as shown in
Figure 3.
The finite element analysis of a diffraction problem involving
a coated cylinder for cloaking a lossy object with sharp
wedges was carried out by Zolla
et al. (2007). Nicolet et al.
(2008) then extended this analysis to a finite conducting
object with sharp wedges. The EM wave picture in the case of
a 2D cylindrical object with a cloak of circular shape was
given and then the results were extended to an elliptical cloak.
From the computations it was observed that invisibility should
be restricted to some approximations of the material
properties.
Figure 3 Full-wave simulations domain of elliptical-cylindrical cloaks.
The cloaking object is an elliptical cylindrical PEC, and the cloak was
made up of anisotropic material.
Design of different types of invisibility cloaks were presented
by Ribeiro and Paiva (2008) using the equivalence principle
of electromagnetics. A new metamaterial transformation
called the left translation in a group structure was used. A
metric perturbation
g, which corresponds to the cloaking
topology, was determined. The geometric interpretation was
converted to material interpretation by using this metric
perturbation. Then the equivalence principle was used to
determine the metamaterial that creates the new geometry.
Illustration was done for spherical and elliptical cylindrical
co-ordinates.
A general transformation method was presented by Li
et al.
(2008c) to design cylindrical cloak with arbitrary regular or
irregular cross-sections. The strategy was to introduce a new
co-ordinate system conformal to the surface of the scatterer to
be concealed. The EM properties of a 2D cloak with irregular
geometry was analyzed and designed. The frequency of the
harmonic wave considered was 1 GHz. The design was
validated using full wave simulations based on finite element
method (FEM) and the performance was evaluated based on
Huygens’s principle.
Cloaks of conical shape with increasing cross-section along
the axial direction were introduced by Luo
et al. (2008b). Full
wave scattering model was used to analyze the EM
characteristics of this cloak, which showed that perfect
conical cloak can guide any kind of EM wave. It was shown
that for azimuthally invariant incoming field, simplified
conical cloak of wide working frequency with permittivity
and permeability tensors greater than one could be
constructed. The design was also used to achieve a
polarization rotator.
A general transformation method was used by Li and Li
(2008
b) to design cylindrical cloaks with arbitrary regular or
irregular cross-section. This method was extended to design
2D cloaks with non-conformal inner and outer boundaries (Li
et al., 2008c). The frequency of operation was 1GHz. The EM
waves were guided around the inner region by the cloak with