Multi-layered bowtie nano-antennasMonir Morshed, Abdul Khaleque, and Haroldo T. Hattori
Citation: Journal of Applied Physics 121, 133106 (2017); doi: 10.1063/1.4979862View online: https://doi.org/10.1063/1.4979862View Table of Contents: http://aip.scitation.org/toc/jap/121/13Published by the American Institute of Physics
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Multi-layered bowtie nano-antennas
Monir Morshed,a) Abdul Khaleque, and Haroldo T. HattoriSchool of Engineering and Information Technology, University of New South Wales at Canberra, Canberra,Australian Capital Territory 2610, Australia
(Received 8 December 2016; accepted 26 March 2017; published online 6 April 2017)
This paper analyzes a multi-layered bowtie nano-antenna, consisting of alternate layers of silica
(SiO2) and gold (Au). We show that the multi-layered structure can produce six times higher elec-
tric field enhancement than a purely gold bowtie antenna. The antennas may find applications in
sensing (e.g., Surface Enhanced Raman Scattering) and imaging. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4979862]
I. INTRODUCTION
The emergence of plasmonic nano-antennas has attracted
significant interest because of their ability to convert freely
propagating optical radiation into localized optical waves and
vice versa.1 Localization and confinement of optical radiation
in the antenna gap are achieved by the coupling of localized
surface plasmon resonances (LSPRs), resulting in high elec-
tric field enhancement.1–5 Due to their electric field enhance-
ment capacity, they have been used as sensors or probes in
Surface Enhanced Raman Scattering (SERS), enabling the
detection of low concentrations of cancer cells,6 single algae
cell detection,7 and single-molecule detection.8 In addition,
they have been used in high-sensitive photodetection,9 near-
field optical trapping,10 nanoscale light sources,11,12 and solar
energy harvesting.13,14
In recent years, different nano-antennas have been devel-
oped such as dipole,15 bowtie,16 Yagi-Uda,17 spiral,18 phased-
array,19 staircase,20 and log-periodic antennas.18 Amongst the
different designs, bowtie nano-antennas (BNAs) provide
strong electric field confinement and enhancement because of
the near-field coupling across the gap.2,21 In addition, they are
suitable for broad-band operation18 and single molecule detec-
tion because of their design flexibility.22
Currently, many strategies are used to improve and engi-
neer the electric field enhancement and optical properties of a
bowtie nano-antenna. For example, Zhou and co-workers pro-
posed an aluminium (Al) based BNA with a fused silica sub-
strate for ultraviolet (UV) wavelengths, which could enhance
the electric field at the nano-antenna gap due to the localized
surface plasmon resonance.23 Ding et al.24 proposed and opti-
mized a silver (Ag) bowtie nano-antenna by varying bow
angles and achieved a high electric field. Wang et al.25
designed a plasmonic gold (Au) bowtie nano-ring array to pro-
duce a high electric field enhancement factor by optimizing
the antenna size and the period of the bowtie nano-ring array.
In addition, Sederberg and Elezzabi26 applied a triangular
contour on a bowtie antenna for the near to mid-infrared
region: they were able to improve the electric field enhance-
ment factor by 28% when compared to Wang’s geometry.
Moreover, Li et al.27 used nano-holes inside the triangular
arms of the antenna: they numerically computed electric field
enhancement and sensitivity by using the finite difference
time domain (FDTD) method. An Au-BNA based on a SiO2
pillar was introduced and analyzed experimentally and numer-
ically as a function of the BNA gap spacing, array periodicity,
and pillar height to enhance the electric field in the gap.28
However, the large optical spot size and less flexibility of
SERS probes are the main limitations of these antennas, and
therefore, it is still necessary to find a new geometry for bow-
tie nano-antennas that can overcome the above limitations as
well as improve the electric field enhancement factor.
On the other hand, composite structures are attracting
growing interest in nano-photonics because they can confine
more light by increasing the effective refractive index at the
interface between two media and reduce the ohmic loss.29,30
In addition, when the separation between two interfaces is
comparable to or less than the penetration depth of the inter-
face modes, interactions between surface plasmon polaritons
(SPPs) give rise to coupled modes, whereby the electric field
strength can be amplified due to coupling of symmetric
mode profiles.31–34 Dey et al.33 introduced a composite
structure for single nano-rod at the wavelength of 5.98 lm
that enhanced the electric field intensity. A similar approach
was applied to design silica filled slot waveguides based on
hyperbolic metamaterials,29,35 multi-layered waveguides,36
and multi-layered dipole nano-antennas.37 In this paper, we
propose a multi-layered bowtie nano-antenna with a combi-
nation of gold (Au) and silica (SiO2), which produces a
615% higher electric field enhancement factor than a pure
gold bowtie antenna.
II. GENERAL DESCRIPTION OF THE STRUCTURES
Figures 1(a) and 1(b) show the three dimensional (3D)
view and the 2D side view (x-y plane) of the proposed multi-
layered bowtie structure, respectively. Key geometric parame-
ters of the bowtie antenna include the length of each triangle
l, width of the triangle w, apex angle a, gap width between
two triangles g, and total height H. The device is optimized
for the wavelength of 1053 nm. In addition, the thicknesses of
the gold and silica layers are referred by hg and hs,
respectively.
The orange color in Fig. 1 indicates a metallic gold
region, while the green color indicates a quartz substrate
a)Author to whom correspondence should be addressed. Electronic mail:
0021-8979/2017/121(13)/133106/9/$30.00 Published by AIP Publishing.121, 133106-1
JOURNAL OF APPLIED PHYSICS 121, 133106 (2017)
(SiO2). The whole structure is surrounded by air, and the
incident wave is assumed to be a quasi-plane wave propagat-
ing along the þy direction. Commercial 3D FDTD software
is used38 with a perfectly matched layer (PML)39 boundary
condition for the numerical analysis of the nano-antennas.
According to Lesina et al.,40 the FDTD method provides
faster convergence and better accuracy for the bowtie anten-
nas when compared with other methods (e.g., finite-element
method). In addition, in different research articles,20,41 they
had used mesh sizes of 10 nm and 20 nm for the theoretical
analysis and found similar results to experiments. In our
case, we are using non-uniform grid sizes, being refined at
the boundaries between the metallic and dielectric regions,
with Dx¼ 10 nm and Dy¼Dz¼ 20 nm, while close to the
metallic regions, Dx¼Dy¼Dz¼ 3 nm. The time step is cho-
sen as 5.7� 10�18 s. The refractive index of SiO2 is set to a
value of 1.45, and the refractive index of gold (Au) is given
by the following dispersive model:38
e xFullwaveð Þ ¼ 1þX6
k¼1
Dek
�akx2Fullwave � bk ixFullwaveð Þ þ ck
;
(1)
where xFullwave is defined by xFullwave ¼ x0=cvac, with x0
being the angular frequency of incoming light and cvac being
the speed of the light in vacuum, and ak, bk, ck, and Dek are
the built in coefficients of Fullwave software as listed in
Table I.
In the simulation, the incident wave is considered to be
a Gaussian wave with a large spot size diameter of 20 lm.
Since the electric field in the gap of the nano-antenna is
enhanced, the relative electric field enhancement factor can
be calculated and formulated as
Frel ¼jEgap;peakjjEinc;peakj
; (2)
where jEgap; peakj is the magnitude of the electric field calcu-
lated in the subwavelength gap of the nano-antenna, and
jEinc; peakj is the magnitude of the electric field of incoming
light.
The magnitude of the electric field may be higher at the
center of the nano-antenna gap but might not be uniform
along the vertical direction (þy direction). Therefore, it is
also better to calculate the average electric field enhance-
ment factor Favg which can be defined as37
Favg ¼
ÐH0
Frel x ¼ 0; y; z ¼ 0ð Þdy
H: (3)
The filling factor, FF, is defined by37
FF ¼HAu totalð Þ
HAu totalð Þ þ HSiO2 totalð Þ� �
!; (4)
where HAu(total) and HSiO2(total) are the total thickness of the
gold and silica layer, respectively.
III. RESULTS AND DISCUSSION
A. Comparative discussion of multi-layered andsingle-layered bowtie structures
The relative and the average electric field enhancement
factors for the optimum design of the proposed structure and
a comparison with a single-layered bowtie antenna at the
wavelength of 1053 nm are shown in Figs. 2(a) and 2(b),
respectively. From the figures, it is clear that the proposed
antenna has 615% and 387% higher peak and average elec-
tric field enhancement factors, respectively, than a standard
gold bowtie antenna, as a result of the constructive interfer-
ence among the different electric modes in the gap along the
vertical direction. Note that the single-layered antenna is
optimized by considering a thickness of 500 nm.
In addition, the electric field profiles of the proposed
geometry are shown in Fig. 3. Although, in the lateral direc-
tion, the electric field in both single-layered (Fig. 3(a)) and
multi-layered structures (Fig. 3(b)) looks similar, there is a
significantly stronger vertical confinement in a multi-layered
FIG. 1. (a) 3D diagram of the multi-
layered bowtie nano-antenna and (b)
2D side view in the x-y plane.
TABLE I. Dispersion coefficients of gold from the Fullwave material
library.38
De A b c
1589.516 1 0.268419 0
50.19525 1 1.220548 4.417455
20.91469 1 1.747258 17.66982
148.4943 1 4.406129 226.0978
1256.973 1 12.63 475.1378
9169 0 11.21284 4550.765
133106-2 Morshed, Khaleque, and Hattori J. Appl. Phys. 121, 133106 (2017)
structure when compared with a single-layered structure, as
it is clearly observed in Fig. 3(c). An initial explanation for a
higher electric field enhancement in the multi-layered
antenna is that it has a higher refractive index, creating more
abrupt transitions between the antenna/air and antenna/sub-
strate interfaces—leading to a stronger confinement of
energy in the central region of the antenna. However, the
stronger concentration of light in the central region might be
FIG. 2. Comparative performance of
optimized (a) multi-layered (H¼ 500 nm,
FF¼ 50%, a¼ 90�, l¼ 211 nm,
w¼ 422 nm, and g¼ 50 nm) and (b)
single-layered (H¼ 500 nm, a¼ 90�,l¼ 128 nm, w¼ 256 nm, and g¼ 50 nm)
bowtie structures at the wavelength of
1053 nm.
FIG. 3. Electric field profile (x-z plane) of (a) single-layered and (b) multi-layered bowtie nanoantennas for optimum parameters; (c) electric field distribution
along the y plane at x¼ 0 and z¼ 0.
133106-3 Morshed, Khaleque, and Hattori J. Appl. Phys. 121, 133106 (2017)
due to the multiple scattering and interference of light in the
vertical direction,31–34 leading to a much stronger concentra-
tion of the electric field energy in the central region (x¼ 0,
y¼ 250 nm, and z¼ 0) and, consequently, a higher electric
field enhancement factor. Moreover, the multi-layered struc-
ture provides additional flexibility to concentrate the energy
of the electric field in the vertical direction.
B. Optimization of the geometric parameters of theproposed structure
The optimized electric field enhancement factor at the
resonant wavelength can be determined by tuning the geo-
metrical parameters of the bowtie multi-layered shape
because the optical properties such as the optical wave-
length, electric field intensity, and confinement of the electric
field depend on the geometrical parameters.23,25–28 In this
work, the resonant condition of the proposed antenna at the
wavelength of 1053 nm is obtained by using the FDTD
method. The optimization of the parameters at this wave-
length has been carried out by following the procedure dis-
cussed by Zhou et al.23 We have simulated different sets of
antennas by varying the length l, width w, and height of gold
hg, height of the silica layer hs, and total height of layers Hwith fixed gap width g and apex angle a.
The filling factor, FF, is optimized for the total thick-
ness H¼ 300 nm by keeping the other parameters such as the
gap width (g) and apex angle (a) constant at 50 nm and 90�,respectively. To optimize the filling factor for the fixed
parameters at the wavelength of 1053 nm, we have changed
the length of the antenna, which is shown in Fig. 4(a). The
width of this antenna is also changed because of the relation
w¼ 2l for a triangular shape since the apex angle is fixed at
90�. Figure 4(a) shows that the highest electric field enhance-
ment factor occurs for a 50% filling factor (hg¼ 50 nm and
hs¼ 75 nm) at the length of 206 nm, with a value of 21.92. In
the case of the 50% filling factor, the amount of gold and sil-
ica is approximately the same, leading to a reduction in the
overall loss of the nano-antenna37—however, we have noted
that in metals with higher losses (e.g., titanium), the fraction
of silica needs to be increased to overcome the higher losses
of the metal. In addition, the strength of the electric field
increases by the collective electron oscillation characteristic
of a strongly coupled plasma.34 Fig. 4(b) shows that the
resonance first blue shifts and then red shifts as the filling
factor increases. The resonance blue shifts while increasing
the filling factor from 40% because the thickness of the gold
layer is increased, which decreases the effective refractive
index. According to Ding et al.,42 the reduction in the effec-
tive refractive index of SPPs blue shifts the resonance. In
addition, the peak wavelength can be determined by34
k0 � 2Ð l=2
�l=2nef f xð Þdxþ d, where d is the offset introduced
by the gap width and the reflection of the two extremities.
Since the length and the gap are fixed in Fig. 4(b), the effect
of d is minor. Now, it is clear that the reduction of the effec-
tive index blue shifts the resonance wavelength. However,
this equation is no longer valid when FF exceeds 50%. It can
be observed in Fig. 4(b) that the resonance wavelength red
shifts when the thickness of gold is changed from a 50% fill-
ing factor. In this case, large filling factors increase the thick-
ness of the gold layer, which isolates the SPPs at the
interfaces resulting in the decoupling of the plasmonic
modes. Therefore, the electric field is reduced, but back and
forth propagation of SPPs in each of the interfaces and addi-
tive interferences gives rise to standing waves whose wave-
length scales with the length of the triangles, thus redshifting
the resonance as the filling factor increases.24
The total thickness of the antenna (H) is changed as
shown in Figs. 5(a)–5(c): it is noted that the optimum length
of the antenna changes as H changes as expected since the
total height affects the interference of the electromagnetic
fields that are scattered by the different gold/silica interfaces
and also affects the coupling of the plasmonic modes propa-
gating at different gold/silica interfaces.32,34 However, the
optimum filling factor remains FF¼ 50% for different
heights as can be observed in Figs. 5(a)–5(c). A maximum
electric field enhancement Frel¼ 32.3 is reached for
H¼ 500 nm, as can be observed in Fig. 5(b). The introduc-
tion of the silica layers has reduced the overall losses of the
antennas,37 but at the same time, it is necessary to have
enough metal to produce the surface plasmon polaritons at
different boundaries of gold/silica layers. It seems that a
50% filling factor produces a good compromise between
reducing the antenna ohmic losses and, at the same time, cre-
ating surface plasmon waves. Another factor that needs to be
taken into account is that FF¼ 50% leads to an electric field
which has a maximum at the center of the antenna, while
FIG. 4. (a) Optimization of the filling
factor (FF) for the height H¼ 300 nm
while other parameters are constant
(a¼ 90� and g¼ 50 nm), (b) Frel as a
function of wavelength for different
filling factors (FF), while other param-
eters are constant (H¼ 300 nm,
a¼ 90�, l¼ 206 nm, w¼ 412 nm, and
g¼ 50 nm).
133106-4 Morshed, Khaleque, and Hattori J. Appl. Phys. 121, 133106 (2017)
other filling factors produce more asymmetric electric field
profiles.
In Fig. 6, we examine the effect of changing the total
height H for the multi-layered nano-antenna, and for an opti-
mized antenna, FF¼ 50%, a¼ 90�, l¼ 211 nm, w¼ 422 nm,
and g¼ 50 nm. It can be observed that the electric field
enhancement factor reaches a maximum for H¼ 500 nm but
does not change significantly in the region between
H¼ 400 nm and 600 nm. In a multi-layered antenna, there is
multiple scattering of the incident electric field along the
boundaries of different layers, and for a height H¼ 500 nm,
the interference between the multiple scattered fields reaches
a maximum at the center of the antenna. In addition, differ-
ent plasmonic modes at the silica-gold interfaces may poten-
tially couple and interfere with each other,32,34 creating a
very complex scenario. In the case of a single-layered
antenna, the boundaries are limited to air/gold and gold/sub-
strate interfaces with a gap between the metallic regions,
somewhat simplifying the analysis since the plasmonic
modes do not strongly interfere with each other in the verti-
cal direction. However, there is still interference between the
multiple scattered fields along the vertical direction, leading
to a maximum at the mid-height as can be observed in Fig.
3(c), but the electric field energy is more uniformly distrib-
uted along the volume of the gap.
So far, the apex angle and the number of layers have
been kept constant to optimize the length, filling factor, and
total height of the antenna. Now, the effects of the apex
angle on the electric field enhancement factor are further
examined. Figure 7 shows the impact of the apex angle (a)
on the electric field in the sub-wavelength gap as a function
of the wavelength, while other parameters such as length (l),gap (g), and total height H are kept constant, but the width
(w) is changed because of the relationship w ¼ 2l sin a=2ð Þfor a bowtie shape. From Fig. 7, it can be inferred that as the
apex angle increases, the resonance wavelength is red shifted
because the back and forth propagation and interference of
surface plasmon polaritons (SPPs) at the interfaces of trian-
gles give rise to a standing wave in which its resonance
wavelength scales with the length of the triangle edges and
FIG. 5. Effect of the length (l) on Frel
at the wavelength of 1053 nm with dif-
ferent filling factors (FF), while other
parameters are constant (a¼ 90� and
g¼ 50 nm) for (a) H¼ 400 nm, (b)
H¼ 500 nm, and (c) H¼ 600 nm.
FIG. 6. Effect of the total height (H) on Frel at the wavelength of 1053 nm
by keeping other parameters constant as FF¼ 50%, a¼ 90�, l¼ 211 nm,
w¼ 422 nm, and g¼ 50 nm.
133106-5 Morshed, Khaleque, and Hattori J. Appl. Phys. 121, 133106 (2017)
thus redshifts as the apex angles increase.24 In addition, the
electric field enhancement factor increases with the increas-
ing apex angles, which is the maximum for the angle of 90�
at the wavelength of 1053 nm. However, the enhancement is
reduced due to the destructive interferences and increasing
losses in the wider part of the structure16 when the apex
angle exceeds 90�.A variation of the electric field with respect to the num-
ber of layers for different total heights (H) in the gap is
shown in Fig. 8, while all other parameters such as length (l),width (w), gap (g), filling factor (FF), and apex angle (a) are
kept constant as 211 nm, 422 nm, 50 nm, 50%, and 90�,respectively. It can be observed from Fig. 8 that the proposed
antenna has the highest electric field enhancement for a five
layer antenna. However, the electric field enhancement fac-
tor is reduced when the number of layers is increased since
at this stage the metal losses start to dominate the response
of the structure and the decoupling of the resonant electric
fields.31–34
The magnitude of Frel for all the above figures was taken
at the mid-point of the gap (x¼ 0, y¼ 250 nm, and z¼ 0), but
the electric field distribution inside the antenna might not be
uniform. We have placed several monitors along the þx and
þy direction to calculate the electric field along these direc-
tions. To distinguish the electric field distribution for the dif-
ferent position in the y-plane, we placed 8 electric field
monitors with a constant distance from top to bottom in the
plane at x¼ 0 and z¼ 0 for optimum parameters: H¼ 500 nm,
FF¼ 50%, a¼ 90�, l¼ 211 nm, w¼ 422 nm, and g¼ 50 nm.
The relative electric field distribution along the þy direction
is shown in Fig. 9. The electric field enhancement factor at the
mid-point (x¼ 0, y¼ 250 nm, and z¼ 0) of the middle metal-
lic layer is 32.30, which reduces for the other metallic layer
away from the middle layer and is comparatively lower in the
silica substrate.
The magnitude of the electric field, along the x plane, is
measured at y¼ 250 nm and z¼ 0, which is shown in Fig.
10, and the electric field distribution is almost uniform along
the gap. However, it is higher at the edges of the nano-
antenna, for example, Frel with a gap width of 50 nm is 32.30
at the middle position of the gap (x¼ 0 nm, y¼ 250 nm,
z¼ 0 nm) while it reaches the value of 47.06 (x¼625 nm,y¼ 250 nm, and z¼ 0 nm) at the edges, i.e., at the interface
between metal and air in the gap.
From Fig. 9, it is clear that the electric field is not uni-
form along the vertical direction and it can significantly
change from top to bottom. For this reason, the average elec-
tric field enhancement factor is calculated for the optimum
parameters from Equation (3), which is shown in Fig. 11.
The maximum Favg at the wavelength of 1053 nm is 17.2.
Finally, Fig. 12 shows the effect of the gap width (g) on
the average electric field enhancement factor. Any variation
of the gap between the two arms of the antenna can change
the coupling and thereby the resonance wavelength.37 To
make a precise comparison and obtain the highest electric
field enhancement factor at the resonant wavelength of
1053 nm for each gap width, the length has been retuned and
optimized with the fixed total height H¼ 500 nm, filling ratio
FF¼ 50%, and apex angle a¼ 90�. The optimized length
sizes for the gap width of 10 nm, 20 nm, 40 nm, 60 nm,
80 nm, and 100 nm are 189 nm, 190 nm, 210 nm, 211 nm, and
212 nm, respectively, and the corresponding average electric
FIG. 7. Frel as a function of wavelength for different apex angles (a) of the
nano-antenna while other parameters (H¼ 500 nm, FF¼ 50%, l¼ 211 nm,
and g¼ 50 nm) are in the optimized condition.
FIG. 8. Effect of the number of layers on Frel at the wavelength of 1053 nm,
while other parameters (FF¼ 50%, a¼ 90�, l¼ 211 nm, w¼ 422 nm, and
g¼ 50 nm) are constant.
FIG. 9. Distribution of Frel along the þy direction in the gap of the nano-
antenna at x¼ 0 and z¼ 0 for the optimum parameters: H¼ 500 nm,
FF¼ 50%, a¼ 90�, l¼ 211 nm, w¼ 422 nm, and g¼ 50 nm.
133106-6 Morshed, Khaleque, and Hattori J. Appl. Phys. 121, 133106 (2017)
field enhancement factors are 60.45, 36.39, 18.43, 13.7, 8.4,
and 6.14, respectively. In this paper, we have optimized the
structure and compared with the pure gold single-layered
bowtie antenna for the gap width of 50 nm. As expected, the
average electric field enhancement changes exponentially
with the gap width.
C. Impedance analysis of the proposed structures
Impedance is an important property of antennas: in this
section, we calculate the impedance of multi-layered and
single-layered nano-antennas. Since nano-antennas do not
support TEM modes, the impedance of the devices for TE
modes can be calculated as43 Z ¼Ð1�1 Exdx=
Ð1�1 Hzdz,
where Ex and Hz are the in-plane electric field and magnetic
field components. The integral paths are chosen along the
central cuts of the antenna’s cross-section (paths along the
mid-height and the center of the antenna gap, as shown in
Fig. 1(a)).43 The impedance is calculated by using the Finite
Difference Time Domain (FDTD) method, which employs a
Fast Fourier Transform to calculate the wavelength depen-
dent real and imaginary parts of the electric and magnetic
fields along the integration paths.
Figures 13(a) and 13(b) show the real (solid curves) and
imaginary (dotted curves) parts of the impedance for the opti-
mized single and multi-layered antennas, respectively. It is
also clear from the figures that the impedance of the optimized
single and multi-layered antennas is 245.19þ j94.16 X and
1929.05þ j1811.33 X, respectively. Choi and Sarabandi44
have found an impedance of 2400 � j1200 X for their bow-tie
antenna, meaning that our results are comparable to those
reported in the literature. While calculating the impedance, we
noted that the values of the magnetic field had not changed
significantly, although the values of the electric field were sig-
nificantly higher in the multi-layered antenna, which seems to
indicate that the higher electric fields in the multi-layered
antenna partially explain the higher impedance of the multi-
layered antenna. If we consider that the intrinsic impedance of
free-space is about Z0¼ 377 X and that the reflectance can be
estimated by R ¼ j ZL�Z0
ZLþZ0j2 (ZL is the impedance of the nano-
antenna), then the reflectances for single and multi-layered
antennas are 6.63% and 66.17%, respectively. The reflectance
formula R ¼ j ZL�Z0
ZLþZ0j2 assumes that the source is solely excit-
ing the antenna, and for a small source placed close to the
antenna, we got reflectance values close to the calculated
ones. Given this initial estimate, it seems that impedance
matching does not provide a good explanation for the larger
electric field in the multi-layered structure.
However, in many practical situations, an array of nano-
antennas will be fabricated and illuminated by a source with
a large spot-size diameter (e.g., coming from a laser source),
meaning that the source will also illuminate regions with no
nano-antennas. We have calculated the reflectance for an
array where the antennas are separated by 10 lm in a square
array (this distance is chosen so that the antennas do not cou-
ple with each other), and the calculated reflectances for the
single and multi-layered antennas are 3.01% and 4.08%,
respectively (it is true that this reflectance will strongly
depend upon how you configure the array, but we have just
used the array as an example). The significant lower reflec-
tance for the multi-layered antennas can be explained by the
fact that the source with a large spot diameter is illuminating
mostly the silica substrate and the antennas are small struc-
tures on top of the substrate.
FIG. 10. Distribution of Frel along the lateral direction in the gap of the
nano-antenna at y¼ 250 nm and z¼ 0 for the optimum parameters:
H¼ 500 nm, FF¼ 50%, a¼ 90�, l¼ 211 nm, w¼ 422 nm, and g¼ 50 nm.
FIG. 11. Favg as a function of wavelength for the optimum parameters:
H¼ 500 nm, FF¼ 50%, a¼ 90�, l¼ 211 nm, w¼ 422 nm, and g¼ 50 nm.
FIG. 12. Effect of the gap width (g) on Favg at the wavelength of 1053 nm
by keeping other parameters constant as H¼ 500 nm, FF¼ 50%, and
a¼ 90�.
133106-7 Morshed, Khaleque, and Hattori J. Appl. Phys. 121, 133106 (2017)
It is true that the impedance matching could be
improved by using tapers45,46 or other impedance matching
techniques, for example, a proper choice of parallel load
(i.e., different permittivity materials in the gap) can be used
for a good matching between the antenna and the feeder
without changing the dimension.47 However, it would add
further complexity to our devices which are generally
excited through the vertical direction.41 It might also be chal-
lenging to fabricate the impedance matching structures on
top of the existing antennas, leading to complicated multi-
step fabrication processes. In addition, given the small area
of the antennas, most of the reflected fields will likely be
scattered into different directions rather than returning to the
laser source.
IV. CONCLUSION
In conclusion, we have designed a bowtie-composite
structure by using gold and silica materials and examined how
to optimize antenna parameters to get the highest electric field
enhancement factor. For the optimum parameters, the proposed
antenna reached a relative and average electric field enhance-
ment factor of 32.3 and 17.2, respectively, which are six and
about four times higher compared to a pure gold bowtie nano-
antenna for the gap width of 50 nm at the wavelength of
1053 nm. Due to the improved performance of the nano-
antenna, it might be useful for different applications such as
surface enhanced Raman spectroscopy and nano light emitting
diode (nano-LED) sources. Moreover, its smaller spot size
than the subwavelength gap can help single molecule detec-
tion. Although the antenna has been optimized with a fixed air
gap between the two arms, the smaller the gap is, the larger the
electric field enhancement factor is (although very small gaps,
i.e., g< 30 nm, are hard to be fabricated in practice).
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial
support of the University of New South Wales (Canberra
Australia). We also acknowledge the financial support from
the Asian Office of Aerospace Research and Development
(AOARD)-USAF (FA2386-15-1-4064) and the Australian
Research Council (ARC) (LP160100253).
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