30th NATIONAL RADIO SCIENCE CONFERENCE
(NRSC 2013)
April 16-18, 2013, National Telecommunication Institute, Egypt
B5. Perforated Nanoantenna Relectarray
S.H Zainud-Deen1, .A. Malhal*, .M Gabe?, and .. Awadalla1.
1Faculty of Electonic Eng., Menouia Universiy, Eypt. *echonidal@yahoo.com
2Egyptian Russian University, Egypt.
ABSTACT
This paper presents a design of perforated nanoantenna relectarray. The use of metallic nanostructures made of
Silver and/or Gold at appropriate wavelength cause fascinating unusual electromagnetic effects. Relectarray
consists of an array of unit cell made rom Silver is investigated. The effect of the number of perforated holes in
the unit cell conigurations is investigated for proper relection coeficient phase compensation. A linearly polarized
pyramidal nano-hon is used to feed the perforated nanoantenna relectarray. The radiation characteristics of 9 x 9
perforated nanoantenna relectarray are illustrated. A high gain of 20.5 dB is obtained at the designed requency of
735 THz. A comparison between solid Silver sheet with no perforation holes and the proposed perforated
relectarray is explained.
Kewords: Nanoantenna, Relectarray, Tera-Hertz.
I. INTRODUCTION
Relectarray antennas have received considerable attention over the years and are quickly inding many
applications. Relectarray antennas combine the same features of parabolic relectors and phased arrays providing
a directive beam in a desired scanned angle. The most important advantages of relectarrays over phased arrays are
the elimination of complexity and losses of the feeding network and the higher eiciency [1-4]. The relectarray
consists of an array of unit-cells illuminated by a primary feed. Each unit-cell is designed in such a way to correct
the phase of the incident wave as is done in the traditional parabolic relector. In relectarrays, the phase of the
relected ield rom each element is adjusted so that the main beam can be directed to a desired direction. Several
relectarray approaches are proposed to control the relection phase in the literature [5]. For example, one is to use
identical microstrip patches with different-length phase-delay lines attached so that they can compensate for the
phase delays over the different paths rom the illuminating feed. The other is to use variable-size patches, dipoles,
or rings so that elements can have different scattering impedances and, thus, diferent phases to compensate for the
different feed-path delays. To overcome the shortcoming of narrow bandwidth of the relectarray, dual-band multi
layer relectarrays using variable patch size, anular rings, and crossed dipoles are being developed [6-8].
Generally, conventional antennas act as a source and transformer of electromagnetic (EM) radiation at radio
requencies (RF) and microwaves, resulting in their sizes being comparable with the operational wavelength. Recent
success in the fabrication of nanoscale elements allows bringing the concept of the RF antennas to optics, leading
to the development of optical nanoantennas [9]. Nanoantennas are metal nanostructures used to enhance, conine,
receive, and transmit optical ields [10]. Nanoantenna is one of the most developing in plasmonics due to their
ability to overcome the size and impedance mismatch between subwavelength emitters and ree space radiation
[11]. The ability to redirect propagating radiation and transfer it into localized subwavelength modes at the
nanoscale makes the optical nanoantennas highly desirable for many applications. Several applications of
nanoantennas have been used such as spectroscopy and high resolution near-ield microscopy [9], subwavelength
light coninement and enhancement [12], photovoltaic [13], sensing [14], molecular response enhancement [15],
non-classical light emission [16], and communication [17]. Different nanoantennas have been investigated in the
literature. By far, research in this nanoantennas ield has utilized the coinage metals, Gold, Copper, and Silver, yet
many potential commercial applications would be optimally realized by inexpensive plasmonic materials
compatible with either high-technology or high-troughput manufacturing methods. Recently, a method of realizing
nanoantennas relectarray system at optical requencies by using nanoparticles of concentric structures with cores
made of ordinary dielectrics and shell of plasmonic materials has been suggested in [18]. The scattering resonance
of these concentric structures can be tailored at different wavelength range by adjusting the core and shell radii or
the material properties. Later, the perforated technique was used in the dielectric Fresnel lens design to obtain
proper phase compensation and gain enhancement [19]. The technique of perforating a dielectric sheet eliminates
the need to position and bond individual elements in an array. Perforations create different effective dielectric
permittivity and make the fabrication of the arrays feasible. The array is made rom one piece of material; and
978-1-4673-6222-11131$31.00 ©2013 IEEE
48
30th NATIONAL RADIO SCIENCE CONFERENCE
(NRSC 2013)
April 16-18, 2013, National Telecommunication Institute, Egypt
inserting air holes with deined diameters and spacing in the required position. The perforations result in lowering
the effective dielectric constant for the substrate at the region of air holes. The inite-integration technique, FIT [20]
is used to ca1culate the characteristics of the relectarray nanoantenna. In this paper, the radiation characteristics of
perforated nanoantennas relectarray are calculated. The paper is organized as folIows. Section 11 describes basic
material properties used in designing nanoantennas. Section III offers the complete description of the design of
nanoantenna relectarray unit cell and its designing parameters. Finally, in Section IV conclusions are drawn.
11. THEORY
Generally, the relectarray transforms the spherical wave emanating rom the feed into a plane wave at the
output, like conventional parabolic relector antenna. The array is located in the x-y plane and is illuminated by a
feed nano-horn. A required phase distribution, Ji(Xi,Yi), at each element of the array is needed to collimate a beam
in the (80, Jo) direction and is determined by
pi(Xi'yJ
di
=
=
ko(di -sinBo(xi cospo + Yi sinpJ)
�(Xi -X
[
)
2
+ (Yi
- Y[ )
2
+
( z[ )
(1)
2
(2)
where ko is the propagation constant in vacuum, diis the distance rom the feed nano-horn x. y. z} to the element
j of the array and xi,) are the coordinates of the cell element . The phase shit of each cell element in the
relectarray should between 0 and 360°.The use of metallic nanostructures made of Silver and/or Gold may become
promising. The light illumination of such metals at appropriate wavelength may cause fascinating unusual
electromagnetic effects [21] leading to promising applications for optical telecommunication, integrated optics and
optical sensors. Most of these effects appears due to an excitation of Surface Plasmon Polariton (SPP) resonances.
Due to high losses at optical requencies, the assumption of perfect electrical conductor is no longer valid. The
dielectric unction of material substrate, used in nanoantennas, have been derived by itting a Drude model given
by [18]
E
=
Eo
[1 -
_ �_
( - jvp)
-::
w
w
W...
7
]
(3)
(4)
where o is the angular resonance requency of the antenna, vp is the angular scattering requency, and op is the
electron plasma angular requency for bulk material at the operating wavelength A. It can be seen that as the metal
conductivity decreases, the losses will increase. In this paper, the desired phase shit compensation required at each
unit cell element is obtained by changing the effective permitivity of the Gold substrate trough the variation of
the diameters of the holes according to the theory of perforated dielectric material [22].
111. NUMEICAL RESULTS
a. The Unit-Cell
The detailed construction of the proposed perforated nanoantenna relectarray unit cell element is shown in
Fig.l. The antenna was designed to operate at 735 THz. The proposed square unit cell has length L 600 nm,
height = 70 nm, with four identical circular holes of equal radii, and perforation depth d= 35 nm. The unit cell is
constructed rom Silver substrate with its material properties are introduced using the Drude model. The Silver
angular scattering requency vp is (4.35x 1012 rad/sec) and the electron plasma angular requency op is (1.3665x 1016
rad/sec) at the operating wavelength of A = 408 nm. To ca1culate the required relection coeficient phase
compensation in each unit cell, the unit cell is placed in a waveguide simulator [5]. The perfect electric and magnetic
wall boundary conditions are applied to the sides of the surrounding waveguide, and result in image planes on all
sides of the unit cell to represent an ininite array approximation. A linearly polarized plane wave was used as the
excitation of the unit cells inside the waveguide simulator and only normal incidence angle was considered. There
are several limitations to the ininite array approach. Fist, all elements of the relectarray are identical; this is
clearly not the case in the real relectarray in which the diameters of the holes in each cell element must vary
according to the required phase compensation. Secondly, the relectarray itself is not ininite in extent.
=
49
30th NATIONAL RADIO SCIENCE CONFERENCE
(NRSC 2013)
April 16-18, 2013, National Telecommunication Institute, Egypt
x
x
z
Fig.
1.
a. 3-D view
b. Side view
The detailed construction of the perforated relectarray nanoantenna unit cell.
-.- •• one hole
.......... two holes
L
----- three holes
-
lour holes
Two holes
L
L
75
80
85
90
95
1 0
Hole radius (nm)
Three holes
FOf holes
a. The unit cell with different number of holes
c
0
)
�
t
::
b. The relection coeicient phase variation
-3
-.- .• one hole
..•••.••.•
4
-
-5
70
two holes
----- three holes
75
80
90
85
Hole radius (mm)
our holes
95
100
c. The relection coeficient magnitude variation
Fig. 2. The variation of the relection coeficient phase variation of the unit cell with different number of
perforated holes.
The separation between the holes and the number of holes are optimized to maximize the relection rom the
structfe as shown in Fig.2a. The variation of the relection coeicient phase versus the hole radius for one, two,
three and fOf holes at 735 THz is determined using the FIT technique and is shown in Fig.2b. The unit cell having
only one hole produce a phase shit ranging rom 0° to about 200°, by increasing the number of the holes the
relection coeicient phase is increased. The unit cell having two holes achieves phase of 250°, unit cell with three
holes achieves 300°, and inally the unit cell having fOUf holes achieves 360° relection coeficient phase variation.
The noticed variation of the relection coeficient phase is due to the variation of the effective dielectric constant of
the Silver material. Fig.2c shows the variation of the magnitude of the relection coeicient in dBs for unit cell with
50
30th NATIONAL RADIO SCIENCE CONFERENCE
(NRSC 2013)
April 16-18, 2013, National Telecommunication Institute, Egypt
different number of perforated holes. The magnitude of the relection coeicient of the unit cell having only one
hole changes rom -1.9 dB to 0 dB. By increasing the number of holes in the unit cell, the magnitude of the relection
coeicient is increased. The unit cell having two holes achieves relection coeicient magnitude variation rom 1.3 dB to 0 dB, unit cell with three holes achieves relection coeficient magnitude variation rom -1 dB to 0 dB,
and inally the unit cell having four holes achieves relection coeficient magnitude variation rom -0.2 dB to 0 B.
b. Nano-Horn Antenna
Different nano-horn antennas made rom nano-materials operating at THz range are investigated in [23, 24]. A
linearly polarized pyramidal nano-hon antenna is used to feed the perforated nanoantenna relectarray. The nano
hon was positioned such that the array was prime-focus fed. The nano-horn antenna is constructed rom Gold
substrate with its material properties introduced using the Drude model. The Gold angular scattering requency vp
is (1.2566xlQ13 rad/sec) and the electron plasma angular requency Op is (1.2566xlQ1 6 rad/sec) at the operating
wavelength of A= 408 m. The feed nano-horn was Lh=1080 nm long, with an aperture size bxa of 1700 m x 850
nm as shown in Fig. 3a. The 3-D radiation patten rom the nano-horn antenna at 735 THz is shown in Fig. 3b. The
nano-horn antenna has a gain of 11.7dB at 735 THz. The radiation pattens for the pyramidal nano-hon antenna in
E-plane and H-plane are shown in FigA at 735 THz. The irst sidelobe levels ( SLL) in the E- and H-planes are
approximately -18.95dB and -18.86 dB below the main peaks respectively.
dU
11 .7
9.55
7.43
5.31
3.18
a
1.06
-2.57
-7.72
-12.9
-18
-23.2
-
a. 3-D view of horn antenna
28 . 3
b. 3-D gain pattern
Fig.3. The detailed construction ofthe feeding hon and the 3-D gain radiation patten at =735 THz.
15 --�-
15 -�-
"
D
ii
s
::
"j
l
s
::
"j
l
-25 ---_�__�_�__�_�__l
-60
-180
120
-120
180
60
o
Eleation (degrees)
-120
a. E-plane
-60
60
o
Eleation (degrees)
120
180
b. H-plane
FigA. The normalized gain radiation patens of the feeding hon antenna in different planes at 735 THz.
c. 9x9 Nanoantenna Relectarray
A schematic of the perforated nanoantenna relectarray showing the feed is shown in Fig.5. 9x 9 unit cell
elements are used to construct the array. The element spacing between the unit cells is 600 m. The relectarray has
total dimensions of 5400 x 5400 m2 located in the x-y plane. The perforated nanoantenna relectarray is
symmetrical about the x-axis and y-axis. The feeding horn is located at a distance F=5400 m (FID=l) in the normal
direction of the array plane. Table 1 describes the phase shit and the corresponding hole radii relevant to the unit
51
30th NATIONAL RADIO SCIENCE CONFERENCE
(NRSC 2013)
April 16-18, 2013, National Telecommunication Institute, Egypt
cells of the irst quadrant of the array. The E and H-plane radiation patterns at 735 THz of the perforated
nanoantenna relectarray are shown in Fig. 6. The irst SLL is 10.34 dB and 8.9 dB in E-plane and H-plane
respectively. The HPBW of the relectarray is 4 degrees in E-plane and 5.7 degrees in H-plane. The 3-D radiation
pattern of the relectarray at f=735 THz is shown in Fig.7a. The perforated nanoantenna relectarray has a gain of
20.5 dB at 735 THz. Most of the radiated ield is relected through the perforated nanoantenna relectarray and the
back lobe is due to the variation of the conductivity of the relectarray material. The perforated nanoantenna
relectarray gain variation as a unction ofrequency is shown in Fig. 7b.
z
Fig. 5. The detailed 3-D construction of the 9x9 perforated nanoantenna relectarray.
Table 1 The phase shit and the corresponding hole radii relevant to the unit cells of the irst
quadrant of the array.
ll
�
:
"j
l
50.4°
88.03nm
79.5 1°
87.lnm
165.79°
79.36 nm
306.27°
72.33 nm
136.55°
80.87 nm
79.5°
87.1 nm
108.44°
82.99 nm
194.21°
78.21 nm
333.9°
70.97nm
163.17°
79.45 nm
165.79°
79.36 nm
194.21°
78.21 m
278.50°
73.86nm
55.88°
87.93nm
242.21°
76.11nm
306.27°
72.33 nm
333.9°
70.97nm
55.88°
87.93nm
189.65°
78.39nm
11.38°
91.14nm
136.55°
80.87 nm
163.17°
79.45 m
242.21°
76.11nm
11.38°
91.14nm
187.2°
78.47nm
25
25
20
20
15
15
10
10
ll
5
�
:
0
"j
l
-5
-10
5
0
-5
-10
-15
-15
-20
-20
-25
-180
-120
-60
0
60
120
180
x
-25
-180
-120
-60
0
60
120
180
Elevation (degrees)
b. H-plane
Elevation (degrees)
a. E-plane
Fig. 6. The gain pattens plot far a 9
Perforated
----'Horn
-
9 center-feed center-beam perforated nanoantenna relectarray at 735 THz.
52
30th NATIONAL RADIO SCIENCE CONFERENCE
(NRSC 2013)
April 16-18, 2013, National Telecommunication Institute, Egypt
25
dB
20
20.8
17
13.2
9.46
15
m
�
5.67
c
1.89
10
�
-1.75
-5.24
5
-8.73
-12.2
-15.7
0
725
-19.2
730
735
Frequency liz)
740
745
a. 3-D gain pattern at =735 THz
b. The gain variation versus requency
Fig .7. The 3-D gain radiation patteren at =735 THz and the gain variation versus requency and for a 9
perforated nanoantenna relectarray.
25
25
----- 728.5 Iz
20
�
c
B
----- 741.5 Iz
15
10
-7351Hz
10
5
5
l
.
0
0
c
·5
�
·10
·15
·
5
·10
·
·20
9
----- 728.5 Iz
20
15
x
15
·20
·120
Eleation (degrees)
a.
E-plane
Elewtion (degrees)
b. H-plane
Fig. 8. The gain radiation patterns plot for a 9 x 9 center-feed center-beam perforated nanoantenna relectarray
at different requencies.
x
z
Fig. 9. The detailed 3-D construction of the solid Silver sheet.
The gain is varied rom 14.5 dB to 20.5 dB over the requency range with 1 dB gain variations bandwidth (6.5
THz). Theoretical radiation pattens are shown in Fig.8 at different requencies to check the bandwidth of the array.
At the extreme requencies, the radiation pattens are similar, with some increase in sidelobe levels. A solid Silver
sheet with the same dimensions of the proposed 9 x 9 perforated nanoantenna relectarray (5400 x 5400 nm2) but
with no holes is shown in Fig. 9. The solid array is fed with the same nano-hon antenna placed at a distance F=D
as that in Fig. 5. The radiation patterns rom the solid sheet rom Silver material (with no holes) with the same size
of the nanoantenna relectarray are shown in Fig.l0. The gain of the solid Silver sheet is almost a copy of the gain
53
30th NATIONAL RADIO SCIENCE CONFERENCE
(NRSC 2013)
April 16-18, 2013, National Telecommunication Institute, Egypt
of the horn except that the side lobes are a little stronger. This is because the solid Silver sheet has a inite size. It is
quite Iikely that the side lobes are Airy features (difraction limit) since the caIculated area of the sheet is only about
12 wavelengths in size. The nano-horn antenna in ront of the solid sheet blocks a part of the relected beam and
reduce the main beam. The gain variation versus requency of the solid sheet is illustrated in Fig.ll. Due to the lack
of phase compensation in the solid surface, the gain is much less than the ree space hon antenna gain. In the solid
surface, there is no transformation of the spherical wave into a plane wave.
25 --,
25 --,
20 - Silver solid sheet
20
15
15
----'Horn
10
10
5
5
c
o
'i
)
Silver solid sheet
----'Horn
-
o
-5
-15
-20
-25 ---�
120
-180
-120
180
-60
o
60
Elevation (degrees)
a. E-plane
-25-��-180
-120
120
-60
o
60
180
Elevation (degrees)
b. H-plane
Fig. 10. The gain radiation patterns plot for a solid Silver sheet at =735 THz.
25
20
n 15
�
c
"i
) 10
5
0
725
730
735
Frequency (THz)
740
745
Fig .11. The gain variation versus requency for a solid Silver sheet.
IV. CONCLUSION
In this paper, a design of perforated nanoantenna relectarray is investigated. A perforated Silver material is used
to construct the array unit cell. The number of the perforation holes is optimized in order to obtain relection
coeicient phase variation rom 0° to 360° with maximum relection magnitude (�1). The radiation characteristics
of linearly polarized pyramidal nano-hon antenna are investigated. The nano-horn antenna has a gain of 11.7 dB
at 735 THz. The irst side lobe levels (SLL) in the E- and H-planes are approximately -18.95 dB and -18.86 B
below the main peaks respectively. Using the proposed unit cell a 9 x 9 perforated nanoantenna relectarray is
designed. The irst SLL is 10.34 B and 8.9 dB in E-plane and H-plane respectively. The HPBW of the relectarray
is 4 degrees in E-plane and 5.7 degrees in H-plane. The gain is varied rom 14.5 dB to 20.5 dB over the requency
range with 1 dB gain variations bandwidth (6.5 THz). A comparison between solid Silver sheet and a perforated
nanoantenna relectarray shows that the variation of the perforation hole radius better compensate the phase of the
spherical wave to be transformed into a plane wave and thus improves the relectarray gain.
54
30th NATIONAL RADIO SCIENCE CONFERENCE
(NRSC 2013)
April 16-18, 2013, National Telecommunication Institute, Egypt
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