Split-Ring-Resonator-Coupled Enhanced Transmission
through a Single Subwavelength Aperture
Koray Aydin,
1,2,
*
A. Ozgur Cakmak,
1
Levent Sahin,
1
Z. Li,
1
Filiberto Bilotti,
3
Lucio Vegni,
3
and Ekmel Ozbay
1
1
Department of Physics, Nanotechnology Research Center, and Department of Electrical and Electronics Engineering,
Bilkent University, 06800, Ankara, Turkey
2
Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
3
Department of Applied Electronics, University of Roma Tre, Rome 00146, Italy
(Received 26 May 2008; revised manuscript received 20 October 2008; published 7 January 2009)
We report the enhanced transmission of electromagnetic waves through a single subwavelength
aperture by using a split-ring resonator (SRR) at microwave frequencies. By placing a single SRR at
the near field of the aperture, strongly localized electromagnetic fields are effectively coupled to the
aperture with a radius that is 20 times smaller than the resonance wavelength (r= ¼ 0:05). We obtained
740-fold transmission enhancement by exciting the electric resonance of SRR. A different coupling
mechanism, through the magnetic resonance of SRR, is also verified to lead to enhanced transmission.
DOI: 10.1103/PhysRevLett.102.013904 PACS numbers: 41.20.Jb, 42.25.Bs, 42.79.Ag
Enhanced transmission of light through apertures that
are much smaller than the wavelength has received a
burgeoning amount of interest [1,2], after the seminal
work by Ebbesen et al., realizing the extraordinary trans-
mission from subwavelength hole arrays that were milled
in optically thick metallic films [3]. Extraordinary light
transmission has been extensively studied by using sub-
wavelength periodic hole arrays [3–7] or metallic struc-
tures with a single aperture [7–12]. Based on Bethe’s
theoretical description, transmission through a single sub-
wavelength aperture of a radius r scales with ðr=Þ
4
[13]. However, one can increase the amount of light pass-
ing through a single hole by corrugating the metal surface
with periodic grooves [7], filling the hole with a material of
high dielectric permittivity [8,9], using alternative aperture
geometries [10,11], or placing artificially designed meta-
material covers in front of the aperture [12]. In all of the
approaches, transmission is enhanced by a resonant pro-
cess that leads to the effective coupling of light to a small
aperture. Although much of the work on enhanced trans-
mission has been carried out at optical frequencies, similar
results are obtained at microwave [14] and THz [15]
frequency regimes.
In this Letter, we propose and demonstrate an alternative
approach that utilizes the resonance of a split-ring resona-
tor (SRR) [16] in order to enhance the transmission
through a single subwavelength aperture at microwave
frequencies. We successfully demonstrated the extraordi-
nary transmission of microwave radiation through an ap-
erture of radius r, which is 20 times smaller than the
incident wavelength (r= ¼ 0:05). We measured a 740-
fold enhancement by using electrically coupled SRR in the
proximity of a single aperture. Moreover, we showed en-
hanced transmission by exciting the magnetic resonance of
SRRs. Enhanced transmission is attributed to the highly
localized electric fields at the resonance frequency of split-
ring resonators that couple incident electromagnetic (EM)
radiation to the aperture.
A commercial 1.6 mm thick FR4 printed circuit board
deposited with a thin (30 m) copper plate of the size L
L (L ¼ 200 mm) was used in the experiments. A circular
aperture with a radius of r ¼ 4mm at the center of a
metallic plate was created by mechanical etching. In the
experiments, single SRR was used to couple electromag-
netic waves to the aperture at a resonance frequency of
SRR [Fig. 1(a)]. SRR was made up of two concentric rings
FIG. 1 (color online). Schematic drawings of a subwavelength
aperture in a metallic plate and SRR plane (a) parallel and
(b) perpendicular to the aperture plane. (c) Measured trans-
mission spectra from a single SRR with four different orienta-
tions. (d) Schematic drawing of SRR orientations with respect to
the incident EM field.
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with the parameters as provided in [17]. The radius of the
SRR is 3.6 mm, which is comparable with the aperture size.
Figure 1(b) shows an alternative configuration of placing
SRR in front of the aperture, wherein a different resonance
mechanism (magnetic resonance) plays a role in the en-
hanced transmission.
The resonance of a single SRR for a different configu-
ration and the incident polarization of an electromagnetic
wave were studied simply by measuring the transmission
spectrum. Transmission measurements were performed by
using two waveguides as a transmitter and receiver, which
were connected to the Agilent N5230A network analyzer.
Four different orientations [Fig. 1(d)] of SRR with respect
to the incident EM wave were investigated, the results of
which are shown in Fig. 1(c). For the configurations SRR
(A) and SRR (B), the wave propagation was perpendicular
to the plane of SRRs [see Fig. 1(d)]. Although the magnetic
resonance could not be excited, it is possible to excite an
electrical resonance for SRR (A), due to the asymmetry of
the SRR structure with respect to the electric field [18]. The
resonance frequency for SRR (A) is at 3.55 GHz. For the
SRR (B) case, the transmission resonance was not ob-
served. The magnetic resonance in SRRs could only be
excited when H was perpendicular to the plane of SRR as
in SRR (C) and (D). Both electric and magnetic fields
contribute to the resonance mechanism of SRRs for con-
figuration (C) [18]. The resonance of SRR (D) is purely of
magnetic origin since the E field is symmetric with the
orientation of the splits. The resonance frequencies were
3.82 GHz for SRR (C) and 3.85 GHz for SRR (D).
In the measurements, we employed two waveguide an-
tennas to transmit and receive electromagnetic waves. A
transmitter antenna was placed 0.2 mm away from the
metallic plate with the aperture, and the transmitted power
was collected 5 cm away from the structure. Wave propa-
gation was along the z axis, where E k y and H k x
[Fig. 1(a)]. First, we measured transmission through the
metallic plate with a single subwavelength aperture [short-
dashed red line in Fig. 2(a)]. As expected from the diffrac-
tion theory, the transmission of electromagnetic waves
through a small aperture was very weak [13]. We then
placed a single SRR 0.1 mm away from the aperture with
the outer split region facing the center of the aperture as
shown in Fig. 1(a). The solid blue line in Fig. 2(a) plots the
measured intensity of the EM wave, which propagated
through the SRR (A) structure and aperture. The trans-
mission was significantly increased when a single SRR was
placed at the near field of the aperture. We observed a 740-
fold enhancement in the transmission at 3.55 GHz [see
inset of Fig. 2(a)]. Here, we define the enhancement as the
ratio of the field intensity of a transmitted EM wave
through an SRR and aperture to that through the aperture
only. It is noteworthy that the maximum enhancement was
obtained at the resonance frequency of SRR (A). Elec-
trically excited resonance of SRR causes the strong local-
ization of the electric field at the splits and gaps of an SRR
structure. The resonance of SRR is responsible for the
enhancement in the transmission. We performed additional
measurements to verify this evidence. We placed a closed
ring resonator (CRR) in front of the aperture. CRR is
comprised of two concentric rings without splits [17].
Evidently, the resonance behavior was no longer present
for CRR, and the transmission was comparable with that of
the aperture. We also checked the SRR (B) configuration
together with the aperture, for which E k x and H k y.
Since the resonant coupling of EM waves to the aperture
is not present for this configuration, transmission is not
enhanced through the aperture. In our approach, where we
utilized a bianisotropic SRR, the enhancement of micro-
wave radiation depends on the polarization of the incident
EM wave. It has been previously shown that apertures with
rectangular [10,11] and elliptical [6] shapes yield enhanced
transmission for a specific polarization of an incident EM
wave. Here, we achieved strong polarization dependence
of enhanced transmission through a circular aperture by
using a bianisotropic SRR structure. The inset of Fig.
2(a)
displays the transmission enhancement factors for four
different SRRs having different resonance frequencies.
These results show that the frequency of enhanced trans-
mission is governed by the resonance frequency of SRRs.
Numerical simulations were performed by using CST
MICROWAVE STUDIO. The dielectric constant and tangent
loss of the FR4 dielectric substrate were taken as " ¼ 3:6
FIG. 2 (color online). (a) Measured and (b) simulated inten-
sity of transmitted EM wave from only aperture, and aperture
covered with CRR, SRR (A) and SRR (B). Insets show the
enhancement factor obtained from SRR (A) and aperture. Inset
of Fig. 2(a) plots enhancement factor of 3 different SRRs.
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and ¼ 0:01, respectively. We modeled the waveguide
antennas that were used in the experiments to transmit
and detect the electromagnetic waves. Open boundary con-
ditions were applied along all the directions. Figure 2(b)
shows the numerical simulation results for only aperture,
CRR, SRR (A), and SRR (B) configurations. There was a
good agreement between the measured and simulated re-
sults. In the simulations, the maximum transmission
through SRR (A) and aperture is observed at 3.70 GHz
and the enhancement factor is calculated as 820. The
differences between the measured and simulated results
were attributed to the deviation from the ideal material
parameters and the sensitivity of the distance between the
SRR and aperture on the transmission.
The maximum transmission enhancement was obtained
when the outer split region of SRR coincided with the
center of the aperture. The highest field intensity is at the
outer split region of SRR due to strong localization of the
incident EM wave [19]. These strong localized modes then
couple to the radiative modes of the vacuum by exciting
waveguide modes in the aperture [11]. By placing the outer
split region at the center of the aperture, one can obtain the
highest coupling efficiency to yield maximum transmis-
sion enhancement.
The incident EM wave was propagating perpendicular to
the SRR; thus, the resonance is of electrical origin. One can
also achieve enhanced transmission through SRR and ap-
erture by exciting the magnetic resonance of SRR. A single
SRR is placed in front of the aperture perpendicularly, as is
schematically drawn in Fig. 1(b). SRR (D) is symmetric
with respect to the E field, whereas SRR (C) is antisym-
metric. The corresponding results are plotted in Fig. 3(a)
and 3(b). The orientation of SRR is shown in the figure
insets. Measurements (solid line) and simulations (dash-
dotted line) revealed that the transmission through the
subwavelength aperture was increased for both SRR con-
figurations. SRR (D) yielded higher transmission com-
pared to SRR (C). The magnetic field excited the
resonance in both orientations. However, due to the asym-
metry of the SRR (C) with respect to the incident E field,
there occurred an electric coupling to the magnetic reso-
nance, which was not present in SRR (D). The transmis-
sion through the aperture can be described, as a first
approximation, in terms of a parallel (with respect to the
screen) magnetic dipole moment and a perpendicular (with
respect to the screen) electric dipole moment [13]. Because
of the boundary conditions imposed by the perfect electric
screen, only the aforementioned dipole moments may play
a significant role. SRR (C) and SRR (D) both excite the
required magnetic dipole moment; the difference is in the
direction of the electric dipole moments excited across the
splits. In the SRR (C) case, the electric dipole moment is
parallel to the screen and, thus, does not contribute to the
transmission enhancement. On the other hand, the electric
dipole moment is perpendicular to the screen for the SRR
(D) case, where both the magnetic and the electric dipole
moments contribute to the transmission enhancement
mechanism. Enhancement was measured to be 38 for
configuration (D) at 3.84 GHz and 7.5 for configuration
(C) at 3.76 GHz as plotted in Fig. 3(c). The simulation
results were consistent with the experiments where we
calculated the enhancement factors of 34 and 7.8 for (D)
and (C) configurations.
We calculated the amplitude of the electric field at
frequencies where our simulations predicted the highest
transmission. Figure 4(a) plots the E
y
amplitude evaluated
at the y ¼ 0 plane, corresponding to the center of the
structure. The amplitude of the electric field of the incident
EM wave was unity. The EM wave was incident from z
and propagated along the þz direction. The aperture was
located at z ¼ 0, between x=r ¼1 and x=r ¼ 1. In the
figures, the x and z values were given in terms of the radius
of the aperture r. The solid rectangles show the positions of
the dielectric FR4 substrates on which the SRR and me-
tallic plate were deposited. The fields are localized strongly
at the split regions of the SRR. The coupling of fields to the
aperture via SRR is clearly seen. Figure 4(b) shows the E
y
amplitude for SRR (D) that is evaluated at the y ¼ 3 plane,
where the outer split of the SRR is located. The incident
EM wave was coupled through the aperture via the mag-
netic resonance of SRR. The amplitude of E
y
for the SRR
(C) and aperture at the y ¼ 0 plane is shown in Fig. 4(c).
Although the field enhancement is higher for the SRR (C)
structure compared to that of SRR (D), the transmitted
electromagnetic wave is apparently smaller.
In all of the experiments and simulations, the aperture
radius was 20 times smaller than the wavelength (r= ¼
0:05), whereas the typical aperture size utilized to achieve
enhanced transmission was r= ¼ 0:20 [3]. In our experi-
ments, we successfully showed that an enhanced trans-
mission was achieved around the resonance frequency of
SRR. Since the frequency of an enhanced transmission was
determined by the resonance frequency of SRR, one can
employ an active SRR medium [20] to tune the wavelength
FIG. 3 (color online). Measured and simulated intensity from a
single aperture covered with (a) SRR (D) and (b) SRR (C).
(c) Enhancement factors obtained from the measurements and
simulations for SRR (C) and (D).
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of maximum enhancement. Utilizing SRR also provides
design flexibility, in which one can easily fabricate SRRs to
resonate at a desired frequency.
Our experiments were carried out at microwave frequen-
cies, but similar results could also be obtained from THz
[21] to near IR [22] frequencies, since SRRs have already
been realized at these frequency regimes. To achieve an
enhanced transmission at optical frequencies, one can fab-
ricate an optical antenna close to a subwavelength aperture.
The fields were strongly localized in the antenna feed [23],
similar to the localization of fields at the SRR’s split
region. Note that metals are perfect conductors at micro-
wave frequencies, and therefore, surface plasmons do not
contribute to the enhancement process. Furthermore, sur-
face waves were not present in our approach, since we did
not use a grating structure to enhance the radiation.
To conclude, we successfully demonstrated the en-
hanced transmission of microwave radiation through a
single subwavelength aperture by placing a single resonant
element in the proximity of the aperture. The enhanced
transmission was achieved by exciting the electric and/or
magnetic resonance of SRR. A 740-fold enhancement was
obtained through an aperture that was 20 times smaller
than the wavelength. We also found that there was strong
polarization dependence for enhancing the transmission,
due to the bianisotropic nature of a split-ring resonator.
This work is supported by the European Union under the
projects EU-METAMORPHOSE, EU-PHOREMOST, EU-
PHOME, and EU-ECONAM as well as TUBITAK under
the Projects No. 105E066, No. 105A005, No. 106E198,
and No. 106A017. One of the authors (E. O.) also acknowl-
edges partial support from the Turkish Academy of
Sciences.
*To whom all correspondence should be addressed.
aydin@fen.bilkent.edu.tr
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FIG. 4 (color online). Simulated E
y
magnitude for an aperture
and (a) SRR (A), (b) SRR (D), and (c) SRR (C) structure.
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