S. S. ISLAM et al.: A NEW WIDEBAND NEGATIVE-REFRACTIVE-INDEX METAMATERIAL
873–877
A NEW WIDEBAND NEGATIVE-REFRACTIVE-INDEX
METAMATERIAL
NOVI [IROKOPASOVNI METAMATERIAL Z NEGATIVNIM
LOMNIM KOLI^NIKOM
Sikder Sunbeam Islam1, Mohammad Rashed Iqbal Faruque1, Mohammad Jakir Hossain1,
Mohammad Tariqul Islam2
1Space Science Centre (ANGKASA)
2Universiti Kebangsaan Malaysia, Faculty of Engineering and Built Environment,
Department of Electrical, Electronic and Systems Engineering, 43600 UKM, Bangi, Selangor, Malaysia
sikder_islam@yahoo.co.uk
Prejem rokopisa – received: 2015-06-30; sprejem za objavo – accepted for publication: 2015-12-15
doi:10.17222/mit.2015.144
This paper reveals the design and analysis of a new wideband negative-refractive-index (NRI) metamaterial unit cell. The
proposed metamaterial unit-cell exhibits resonance in the C-band and displays negative permittivity and permeability there with
a wideband NRI property. It also shows a wider negative peak of the refractive index in the major area of the C- and X-band and
minor area of the S- and Ku-band, and a maximum 3-GHz negative bandwidth was achieved compared to the reference
metamaterial. In the basic design, a square-shaped copper resonator was constructed with a metal strip on the FR-4 substrate
material. The measured result was presented and it shows good conformity with the simulated result. Moreover, an analysis was
performed with the same design by replacing the substrate material with the popular Rogers RT 6010 instead of the FR-4
material and then it shows NRI properties in the C-, X- and Ku-band.
Keywords: metamaterials, negative refractive index, wideband
^lanek obravnava zgradbo in analizo osnovne celice novega, {irokopasovnega metamateriala z negativnim lomnim koli~nikom.
Predlagana osnovna celica iz metamateriala ka`e resonanco v C-pasu in ka`e negativno permitivnost ter permeabilnost z
lastnostmi NRI {irokega pasu. Ka`e tudi {ir{i negativni vrh lomnega koli~nika v ve~ini podro~ja C-pasu in X-pasu in v manj{em
podro~ju S-pasu in Ku-pasu. Najve~ja {irina negativnega pasu (3 GHz) je bila dose`ena v primerjavi z referen~nim
metamaterialom. Osnovna zgradba je bila konstruirana kot bakren rezonator {tirikotne oblike, s kovinskim trakom na podlagi iz
FR-4 materiala. Predstavljeni so izmerjeni rezultati, ki ka`ejo dobro ujemanje z rezultati simulacije. Poleg tega je bila izvedena
analiza z enako zgradbo in z nadome{~anjem podlage s popularnim Rogers RT 6010 namesto FR-4 materialom in prikazane so
NRI lastnosti v C-, X- in Ku-pasu.
Klju~ne besede: metamateriali, negativni lomni koli~nik, {irokopasovnost
1 INTRODUCTION
Metamaterials are engineered (at the atomic level)
materials that have unique and extraordinary properties
not found in nature. A metamaterial as a composite
material, usually gains these properties due to the
arrangement of its constituents (in a unit cell) rather than
individual properties. There are some exotic properties
that are not possible with naturally available materials,
but can be achieved with metamaterials, like negative
permittivity (<0) or negative permeability (μ<0), nega-
tive refractive index, inverted Snell’s law, etc. In 1967
the Russian physicist Victor Veselago1 predicted that it is
possible to develop a material of such reverse characte-
ristics that it will behave opposite to the natural material.
It was also stated by him that a material could exhibit a
negative refractive index if it gains negative permittivity
and permeability. Around 30 years later in 2000 D. R.
Smith et al.2 successfully demonstrated a composite
material with such negative properties. Due to these
uncommon characteristics it can used in many important
applications, like antenna design, EM absorption reduc-
tion, electromagnetic cloaking operation, filter deign,
sensor design, etc.3–7 A metamaterial with both negative
permittivity () and negative permeability (μ) is called a
double-negative (DNG) metamaterial or a negative
refractive index (NRI) or negative index material (NIM)
or a metamaterial with either permittivity or permeability
negative is called a single negative (SNG) metamaterial.
However, a metamaterial with the DNG property can
only exhibit the negative refractive index property
properly. There are many metamaterials found in the
literature, but not enough metamaterials with a double
negative property are found. However, very few of them
were designed to exhibit DNG characteristics in the
C-band of the microwave region. H. Benosman et al.8
presented a double-negative metamaterial, but their
metamaterial was applicable for the Ku-band only. O.
Turkmen et al.9, showed a metamaterial for X-band
operation, but their metamaterial was not double
negative. A. Dhouibi et al.10, proposed a metamaterial for
C-band applications, but they claimed these property for
an epsilon negative (ENG) metamaterial. S.S. Islam et
al.11 designed an S-band metamaterial, but their
Materiali in tehnologije / Materials and technology 50 (2016) 6, 873–877 873
UDK 67.017:537.87:620.1 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(6)873(2016)
metamaterial was showing ENG properties as well.
Moreover, recently in 12, a DNG material was introduced
where a maximum 1.05-GHz bandwidth of the negative
refractive index region was claimed.
In this study, a new double-negative metamaterial is
revealed that exhibits negative refractive index property
in the major region of C- and X-band of microwave
spectra with a wider bandwidth. Commercially available
finite-difference time-domain (FDTD) based CST-micro-
wave studio software was used to retrieve the S-para-
meters for the unit cell. For further investigation, the
structure was then designed on a Rogers RT 6010
substrate material instead of FR-4 material and an
analysis was performed.
2 DESIGN AND METHODOLOGY
Figure 1a shows the geometry of the proposed
square-shaped metamaterial unit cell. The proposed
metamaterial unit cell consists of a simple square-shaped
copper structure with a vertical copper stripe in the mid-
dle of the material, all having a thickness of 0.035 mm.
The copper strip in the middle was placed in such a way
that it maintains an equal gap for the two opposite sides
of the metal strip. The outer length and width of the unit
cell were denoted by a = b = 10 mm. The width of the
square-shaped structure was expressed by w = 1 mm.
The tiny gap at the two ends of the metal strip was
symbolized as s = 0.5 mm. The length and width (e) of
the metal strip were 7 mm and 1 mm. The distance from
the central metal strip to the square border was denoted
by, c = d = 3.5 mm.
The structure was designed on a 20 mm × 20 mm
square-shaped FR-4 substrate material having a dielec-
tric constant of 4.3 and a loss tangent of 0.025. The
thickness of the substrate was 1.6 mm. In this study,
commercially available finite-difference time-domain
(FDTD) based computer simulation technology (CST)
microwave studio software was adopted for the design
and calculation of the reflection (S11) and transmission
(S21) coefficients of the unit cell. These parameters were
used to compute the effective parameters (permittivity,
permeability and refractive index) for the unit cell.
Figure 1 displays the simulation arrangements. For the
simulation, the designed unit cell was placed between
two waveguide ports of positive, negative of z-axis, and
exited by transverse electromagnetic (TEM) waves. The
rest of the axes were defined as perfect electric conduc-
tor (PEC) and perfect magnetic conductor (PMC) boun-
dary conditions. The frequency-domain solver was used
for the whole simulation. The simulation was executed
for the frequency range of 1–15 GHz. For the compu-
tation of the effective parameters, the Nicolson-Ross-
Weir method was utilized to avoid the inverse cosine-
branch-index problem.13 However, as part of further
investigation, the unit cell was rotated by 90° and the S
parameters were estimated for gaining the effective
parameters using the same method.
In this study, the open-space measurement techno-
logy was adopted. The open-space measurement techno-
logy was chosen to observe the realistic effect. For
measurement purpose, a prototype of 160 mm × 200 mm
was fabricated that contains 8×10 unit cell. The fabri-
cated prototype is seen in Figure 2a. The fabricated
prototype was placed between two horn antennas. The
antennas were acting as the transmitting and receiving
end and they were connected to an Agilent E8363D
vector network analyzer to calculate the S parameters.
However, the distance between the prototype and the
horn antenna was kept at 35 cm to avoid the near-field
effect. As a part of calibration process, measurements
with and without the prototype were performed as well.
3 RESULTS AND DISCUSSION
Figure 2b displays the simulated magnitude of the
transmission coefficient (S21) for the z-axis wave pro-
pagation through the unit cell. It is evident from Figure
2b that for the z-axis wave propagation a clear resonance
is seen in the range of the C-band at the frequency of
7.48 GHz of the microwave spectra. Figure 2c shows the
measured magnitude of the transmission coefficient
where it has been compared with the simulated one. It is
apparent from Figure 2c that the measured result shows
almost good conformity with the simulated result.
However, a slight distortion is found in the measured
magnitude of S21 than the simulated result that might
have occurred due to the noise effect in the open-space
measurement process and fabrication error.
Figures 3a and 3b show the real magnitude of the
effective permittivity and the permeability against the
frequency for the z-axis wave propagation through the
unit cell. It is clear from Figure 3a that the permittivity
curve has two clear resonances at the frequencies of
S. S. ISLAM et al.: A NEW WIDEBAND NEGATIVE-REFRACTIVE-INDEX METAMATERIAL
874 Materiali in tehnologije / Materials and technology 50 (2016) 6, 873–877
Figure 1: a) Design of the unit cell, b) simulation geometry in CST
software
Slika 1: a) Zgradba osnovne celice, b) simulacija geometrije s CST
programsko opremo
(a)
(b)
2.94 GHz and 7.45 GHz. Moreover, the negative portion
of this curve is found from 2.91 GHz to 5.57 GHz, which
covers almost 2.66 GHz of bandwidth, more than 1 GHz
bandwidth from the frequency of 7.42 GHz to 8.71 GHz,
and nearly 3 GHz bandwidth from the frequency of
10.75 GHz to 13.65 GHz. Similarly, the permeability
curve of Figure 3b displays a negative region from the
frequency of 3.74 GHz to 7.51 GHz that covers almost
3.77 GHz bandwidth. Another negative portion is visible
there from the frequency of 11.15 GHz to 14.77GHz,
which also covers nearly 3.62 GHz bandwidth.
For this design, for the varying magnetic field, a
charge builds up in the gaps between the metal strip and
the ring. At low frequency the current remains in phase
with the applied field, but at higher frequency it starts
lagging and produces a negative permeability at that
frequency.
Figure 4 reveals the real magnitude of the refractive
index () against frequency for the unit cell. In this
paper, it was mentioned earlier that for a material the
refractive index curve would be negative if its per-
mittivity and permeability curve appears negative simul-
taneously. Therefore, from Figure 4 it is apparent that
for the proposed material the refractive index curve exhi-
bits a negative magnitude from the frequency of
3.74 GHz to 5.57GHz; 7.42 GHz to 7.51 GHz and
11.15 GHz to 13.65 GHz. It is notable that two wide
bandwidths of 1.83 GHz (3.74 GHz to 5.57GHz) and
3.73 GHz (11.15 GHz to 13.65 GHz) are seen as the
negative region in the refractive index curve. These
bandwidths have fallen in the few portion of S- and
Ku-band and major portion of C- and X-band of the
microwave region.
Moreover, in these negative regions of the refractive
index curve, the permittivity and permeability curve are
also found to be displaying a negative peak. As a result,
the material can be characterized as a double-negative
(DNG) metamaterial in these regions of microwave spec-
tra. Another important feature is in the frequency range
between 7.42 GHz and 7.51 GHz where the refractive
index curve was found to be negative, both the simulated
and measured transmittance (S21) are also found to be
exhibiting sharp resonance clearly at the frequency of
7.48 GHz with a refractive index = -4.40. Thus, it
S. S. ISLAM et al.: A NEW WIDEBAND NEGATIVE-REFRACTIVE-INDEX METAMATERIAL
Materiali in tehnologije / Materials and technology 50 (2016) 6, 873–877 875
Figure 2: a) Prototypes for measurement, b) simulated transmission
coefficient (S21) for z-axis wave propagation, c) comparision of
measured and simulated result for S21
Slika 2: a) Prototipi za merjenje; b) simuliran koeficient prenosa (S21)
pri napredovanju vala po z-osi, c) primerjava izmerjenega in simuli-
ranega rezultata za S21
Figure 4: Real magnitude of refractive index () versus frequency
Slika 4: Realni obseg lomnega koli~nika () v odvisnosti od frekvence
Figure 3: a) Real magnitude of permittivity () against frequency,
b) real magnitude of permeability (μ) versus frequency
Slika 3: a) Realna magnituda permitivnosti () proti frekvenci,
b) realna magnituda permeabilnosti (μ) v odvisnosti od frekvence
reveals that for the z-axis wave propagation the material
is practically applicable for C-band applications in the
the microwave spectra.
As a part of a further investigation, the Rogers 6010
substrate material was used instead of the FR-4 substrate
material for the unit cell. Figures 5a and 5b depicts the
transmission coefficient as well as the real magnitude of
permittivity () against frequency for the unit cell on the
Rogers 6010 substrate material consecutively.
According to Figure 5a, the transmission coefficient
shows two resonances at the frequencies of 5.14 GHz
and 10.06 GHz. These frequencies are in the range of
C-band and X-band. The permittivity curve in Figure 5b
reveals a negative magnitude from the frequency of 3.29
GHz to 6.55 GHz, which covers more than 3 GHz band-
width in the C-band. Similarly, it also shows negative
peak from the frequency of 8.80 GHz to 10.30 GHz in
the X-band and 12.84 GH to 15 GHz in the Ku-band. In
Figures 6a and 6b the real magnitude of the per-
meability and refractive index is depicted. It is apparent
from Figure 6a that the permeability curve shows a
negative peak from the frequency of 4.33 GHz to 8.90
GHz and 13.25 GHz to 15 GHz. Therefore, from the
permittivity and permeability curve of Figures 5b and 6a
it is evident that the material exhibits a double-negative
property from the frequency of 4.33 GHz to 6.55 GHz,
8.80 GHz to 8.89 GHz and 13.25 GHz to 15 GHz.
Usually, the properties of permittivity and per-
meability are most likely affected by the polarization due
to the internal architecture of the material. When electro-
magnetic waves enter anisotropic materials, which have
unequal lattice axes, it is affected by the polarization
inside the material. As a result, the value of the per-
mittivity and permeability changes due to changes in the
design. In the same way, the refractive index curve is
also affected by the polarization.
Similarly, in the refractive index curve in Figure 6b,
it is clear that the curve shows negative magnitude from
the frequency of 4.33 GHz to 6.55 GHz, 8.80 GHz to
8.89 GHz and 13.32 GHz to 15 GHz and these frequency
ranges cover 2.22 GHz, 9 MHz, 2.32 GHz bandwidth in
the microwave ranges. These regions of negative refrac-
tive index curve fall in the range of C, X and Ku-band of
microwave spectra. Moreover, these frequencies obey the
permittivity and permeability curves in Figures 5b and
6a as well.
4 CONCLUSIONS
In this paper, a new square-shaped negative refractive
index metamaterial was demonstrated that exhibits a
wider negative peak in the major area of the C-and
X-band and the minor area of S- and Ku-band. A more
than 3-GHz wider bandwidth of the negative peak was
achieved for the proposed metamaterial than the latest
reference metamaterial. The measured result also agrees
well with the simulated result. Moreover, the material
shows a negative refractive index zone in the C-, X and
Ku-band of the microwave spectra as well as when it is
designed on the Rogers 6010 substrate material. How-
ever, C- and X-, Ku-band are widely used for long-dist-
ance and satellite communications. So, this metamaterial
can be practically applied in these frequency bands,
S. S. ISLAM et al.: A NEW WIDEBAND NEGATIVE-REFRACTIVE-INDEX METAMATERIAL
876 Materiali in tehnologije / Materials and technology 50 (2016) 6, 873–877
Figure 6: a) Real magnitude of permeability (μ) against frequency, b)
real magnitude of refractive index () versus frequency for the unit
cell on the Rogers 6010 substrate material
Slika 6: a) Realen obseg magnitude permeabilnosti (μ) v odvisnosti od
frekvence, b) realen obseg lomnega koli~nika () v odvisnosti od frek-
Figure 5: a) Transmission coefficient (S21) for the unit cell on the
Rogers 6010 substrate material, b) real magnitude of permittivity ()
against frequency for the unit cell on the Rogers 6010 substrate
Slika 5: a) koeficient prenosa (S21) osnovne celice na podlagi iz mate-
riala Rogers 6010, b) realna magnituda dielektri~ne konstante () v
odvisnosti od frekvence osnovne celice na podlagi iz Rogers 6010
especially for wider bandwidth application besides the
other metamaterials in the microwave range.
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S. S. ISLAM et al.: A NEW WIDEBAND NEGATIVE-REFRACTIVE-INDEX METAMATERIAL
Materiali in tehnologije / Materials and technology 50 (2016) 6, 873–877 877