Thin absorbing screens using metamaterial surfaces

TLDR: In this paper, a resistive sheet is placed on top of a surface to achieve a thin structure that can be an efficient absorber for incident electromagnetic energy, which can be regarded as frequency-selective surfaces.

Abstract: Metamaterial surfaces can be conceptualized by a 2-dimensional periodic arrangement of many small flat inclusions on an otherwise homogeneous host surface. The electromagnetic properties of such metamaterial plates, which can indeed be regarded as frequency-selective surfaces, are influenced by the shape and geometry of these inclusions. When a metamaterial surface is closely placed above a perfectly conducting plate, at a certain frequency band, this structure may possess a high surface impedance at its top surface, thus providing a high-impedance ground plane (HIGP). The center frequency and bandwidth over which such a high-impedance electromagnetic surface is achieved depend on inclusion shapes and compositions, among other parameters. By placing a resistive sheet on top of this surface, we can achieve a thin structure that can be an efficient absorber for incident electromagnetic energy.

Full text

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Thin Absorbing Screens
Using
Metamaterial Surfaces
Nader Engheta
University of Pennsylvania
Department of Electrical Engineering
Philadelphia. Pennsylvania
19104,
U.S.A.
Tel:
11-215-898-9777,
Fax:
tl-215-573-2068
E-mail: endictac~uc.upei,nedo, URL
httv:/i\~.\~~w.ee.~~pIrnn.cdili-cnehetai
Abstracl
Metamaterial surfaces can be conceptualized by 2-dimensional periodic arrangement
of
many small flat inclusions
on
an
otherwise homogeneous host surface. Electromagnetic
properties of such metamaterial plates, which can indeed be regarded
as
frcquency-
selective surfaces,
are
influenced by the shape and geometry of these inclusions.
When
a
metamaterial
surface
is
closely placed abovc
a
perfectly conducting plate, at certain band
of frequency, this stmcture may possess high surface impedance at its top surface, and
thus providing
a
high-impedance ground plane
(HIGP).
The center frequency and
bandwidth
over
which such high-impedance electromagnetic surface
is
achieved depend
on
inclusion shapes
and
compositions, among other parameters.
By
placing
a
resistive
sheet
on
top of this surface, we can achieve
a
thin stmcture that
can
be
an
efficient
absorber for incident electromagnetic energy.
Inlroduetion
Study of electromagnetic properties of complex media and complex surfaces has
received considerable attention
in
the past
several
decades
(see,
e.g.,
[I
]-[S]).
Researchers in several groups all over the world have been investigating
vannus
features
of electromagnetic composite media,
also
known
as
metamateiials, which
can
possess
novel electromagnetic properties. not readily
available
in
nature, but physically
realizable.
Owing
to
their exciting features, these "artificial" materials
can
offer
interesting potential applications in antennas, devices and components.
Here, we
describe
one
of
our
ideas, namely the possibility
of
having thin absorbing screens far
electromagnetic energy,
as
a
potential application of metamaterial surfaces.
Thin
Absorbing
Screens
As
is
well
known,
a highly conductive flat
surface
has
a
very
low
surface impedance,
Z,
=0,
that would result
in
a reflectivity of
R
=-I
for incident ware on the
conducting surface. However, if
a
surface
is
designed to have
a
very
high surface
impedance, the reflectivity for the incident wave
an
it
would be
R
=
+1~ which
effectively suggests that such a surfare may act as a "magnetic wall"
in
contrast
to
the
convcntianal electric wall for which
R
=
-1,
These surfaces
can
obviously
have
interesting applications.
For
example, for
a
surface with
R
=
+1
I
it
has
been shown that
a
Small dipole antennas
can
be laid horizontally near the surface, and the image current
will be
in
phasc with the antenna
current,
resulting
in
good radiation efficiency
(see
e.g.
~91).
392
o7803-73308m?$$l7.a)02002aEE

Metamaterial
sulfates,
when they
are
placed
near,
and parallel with, a conducting plate
may
offer
an
interesting possibility for achieving high-impedance surfaces.
For
example,
let
us
take a metamaterial surface that can conceptually
be
made by having many small
flat inclusions distributed,
in
a periodic fashion,
on
a
flat surface. Such a surface
can
indeed behave as
a
frequency-selective surface when an incident electromagnetic
wave
interacts with it. Such
a
thin metamaterial plate by itself can be characterized
electromagnetically by a shunt impedance matrix
z,,,,
.
This implies that when
an
incident plane wave
is
illuminating this metamaterial plate by itself, the reflected and
transmitted waves can
be
obtained by treating the plate
as
the
shunt impedance
g,*"",
.
Such shunt impedance does in general depend
on
frequency and angle
of
incidence of
incoming wave, and the specific form of this function depends
on
several parameters
including the shape, density, and size
of
the inclusions.
Now
we place
this
metamaterial
surface
in
front ofa highly conductive metallic plate at
a
distance
d,
which is assumed
to be much smaller than the operating wavelength
1,
with
a
dielectric layer sandwiched
behveen the metamalerial
surface
and
the metallic
plate.
(Fig.
la).
The
surface
impedance at the lop
surface
of
this
combined
layer
of metamaterial surface, dielectric
spacer, and the
ground
plane can
be
obtained using the transmission-line theory
as
given
below:
where
Z,vflecc
represents the surface impedance at the top surface
of
the combined layer,
q,d
is the transverse intrinsic impedance of the dielectric
layer,
and k,,
is
the
normal
component of wave vector in the dielectric layer.
(Here
far the
sake
of mathematical
simplicity,
we
assume the metamaterial plate is "isotropic", i.e..
z,,,.,
can be witten
in
a
scalar
form
Z,*"",
.
The
general
anisotropic surface
can
also
be easily treated using the
matrix manipulation
of
transmission-line theory.)
As
can be seen From the above
equation, if the metamaterial surface
is
designed such that for
a
given frequency range
its
equivalent shunt impedance
Z,h,,,
satisfies the fallowing relation
Z,,,,,
=
-&
tan(kndd)
=
-jn,k,d
then the surface impedance
Zvv,nte(~)
can
in principle attain a
very
high
value
in this
frequency
range.
Therefore, the top surface
of
the combined layer would have
a
reflectivity
of
R
=
+I,
effectively acting
as
a magnetic wall. Variation
of
the equivalent
shunt impedance
Z,,,,
as
a function of frequency relies
on
various parameters among
which
one
should mention the shape
of
the inclusions.
For
instance,
in
our
work
on
wire
media, we have obtained interesting variation for
Z,*"",
for a plane made
of
many long,
psrallel,
thin wire inclusions
[6],
which may allow the possibility
of
achieving the
R=+l
[IO].
393

For
such
a
combined layer, the reflectivity for the incident
wave
will then become
R
=
+l.
Now
we assume
a
normally incident plane wave impinging on this structure.
(Here,
only far the sake of simplicity of the argument,
we
assume
the normally incident
wave. We can easily extend this argument to the oblique incidence.) Since
R
=
+I,
the
total tangential component of the electric field and
of
the magnetic field
on
the top
surface
of
this shucture
are,
respectively,
EY'
:
2ETU"'
and
HY'
=
0,
so
as
we said
earlier
the surface may effectively act
as
a
"magnetic
wall".
Thus unlike the
case
of
a
simple metallic ground plane (i.e., electric wall), here the tangential component of the
total electric field
is
significant
on
the top surface. If
we
can
now lay a thin layer of
resistive sheet on top of this surface
(Fig.
Ib),
since the electric field
is
high
an
this
surface, part of the incident
energy
can
be dissipated
on
this resistive sheer,
and
thus the
reflection coefficient
can
be reduced appreciably. This can be quantitatively shown by
using the transmission-line theoly. If the shunt resistance of the resistive sheet
is
taken
to be
r,,,%,,,,
,
and if
we
assume that the metamaterial surface
can
be designed
so
that the
reflectivity for the combined structure
(before we
put the resistive sheet
on)
is
R
=+I,
then the normal-incidence reflection coefficient
=$er
the resistive sheet is
a
placed
on
this
structure
can
be
obtained
as
R
=
(rm,,,,w
-qo)/(<aa,jw
+%).
With the proper
choice
of
the value
of
resistive sheet impedance,
one
may then have the possibility of
reducing the reflection coefficient and thus reducing the scattering
cross
section.
This potential idea resembles the idea
of
single-screen Salisbury shields
[I
I].
However,
in
the
case
of Salisbury shields the electric resistive sheet has to be placed at the
114
distance away
From
the metallic plate in order to be at the location with the maximum
electric field. But the distance
L/4
makes the combined shucture thick, and thus makes
it unsuitable for many applications.
However,
in
our
idea described above if the
metamaterial surface can be designed to achieve the high surface impedance at the top
surface of the sttactwe, then the resistive sheet
can
be laid right
on
top of the structure,
providing the potential far
thin.
lighhveight absorbing
screens.
394

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Thin absorbing screens using metamaterial surfaces" ?

However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions @ ieee. This conference paper is available at ScholarlyCommons: https: //repository. 

Q2. How does the electric resistive sheet work?

in the case of Salisbury shields the electric resistive sheet has to be placed at the 114 distance away From the metallic plate in order to be at the location with the maximum electric field. 

Q3. What is the description of the metamaterial surface?

if a surface is designed to have a very high surface impedance, the reflectivity for the incident wave an it would be R = + 1 ~ which effectively suggests that such a surfare may act as a "magnetic wall" in contrast to the convcntianal electric wall for which R = -1, These surfaces can obviously have interesting applications. 

Q4. What is the nature of the metamaterial plates?

Electromagnetic properties of such metamaterial plates, which can indeed be regarded as frcquencyselective surfaces, are influenced by the shape and geometry of these inclusions. 

Q5. What is the normal-incidence reflection coefficient of the resistive sheet?

If the shunt resistance of the resistive sheet is taken to be r,,,%,,,, and if the authors assume that the metamaterial surface can be designed so that the reflectivity for the combined structure (before the authors put the resistive sheet on) is R =+I, then the normal-incidence reflection coefficient =$er the resistive sheet is a placed on this structure can be obtained as R = (rm,,,,w -qo)/(<aa,jw +%). 

Q6. What is the center frequency and bandwidth of a high-impedance electromagnetic surface?

The center frequency and bandwidth over which such high-impedance electromagnetic surface is achieved depend on inclusion shapes and compositions, among other parameters. 

Q7. What is the shunt impedance of a thin absorbing screen?

Such shunt impedance does in general depend on frequency and angle of incidence of incoming wave, and the specific form of this function depends on several parameters including the shape, density, and size of the inclusions. 

Q8. What is the simplest description of the metamaterial surface?

The surface impedance at the lop surface of this combined layer of metamaterial surface, dielectric spacer, and the ground plane can be obtained using the transmission-line theory as given below:where Z,vflecc represents the surface impedance at the top surface of the combined layer, q,d is the transverse intrinsic impedance of the dielectric layer, and k,, is the normal component of wave vector in the dielectric layer. 

Q9. What is the simplest explanation for the oblique incidence of the shunt?

If the authors can now lay a thin layer of resistive sheet on top of this surface (Fig. Ib), since the electric field is high an this surface, part of the incident energy can be dissipated on this resistive sheer, and thus the reflection coefficient can be reduced appreciably. 

Q10. What is the name of the paper?

Tel: 11-215-898-9777, Fax: tl-215-573-2068 E-mail: endictac~uc.upei,nedo, URL httv:/i\\~.\\~~w.ee.~~pIrnn.cdili-cnehetaiMetamaterial surfaces can be conceptualized by 2-dimensional periodic arrangement of many small flat inclusions on an otherwise homogeneous host surface. 

Q11. What is the effect of the resistive sheet on the structure?

in their idea described above if the metamaterial surface can be designed to achieve the high surface impedance at the top surface of the sttactwe, then the resistive sheet can be laid right on top of the structure, providing the potential far thin. 

Q12. What is the purpose of this article?

the authors describe one of their ideas, namely the possibility of having thin absorbing screens far electromagnetic energy, as a potential application of metamaterial surfaces. 

Q13. What is the shunt resistance of the resistive sheet?

In this talk, the authors will present some of the results of their theoretical analysis on thin absorbing screens, with the metamaterial surfaces being taken IO be a wiremedium surface, which can be synthesized by having many long, parallel, thin wire inclusions placed on a flat surface. 

Q14. What is the simplest explanation for the shunt impedance?

Variation of the equivalent shunt impedance Z,,,, as a function of frequency relies on various parameters among which one should mention the shape of the inclusions. 

Q15. What is the tangential component of the electric field and of the magnetic field on the top?

the total tangential component of the electric field and of the magnetic field on the top surface of this shucture are, respectively, EY' : 2ETU"' and HY' = 0 , so as the authors said earlier the surface may effectively act as a "magnetic wall".