4(a) also shows the Fabry–Perot peak at about 4
which was also seen in the experimental data and in
the simulation result of a single bowtie aperture [Fig.
3(a)]. The resonant peak is blueshifted from the case
of the isolated aperture. This is due to inductive cou-
pling between adjacent apertures. Figures 4(b) and
4(c) show the electric and magnetic ﬁeld structures
for this resonance extracted from an eigenmode
simulation. As was the case for the isolated aperture,
the electric ﬁeld remains well conﬁned in the gap re-
gion in each aperture. The magnetic ﬁeld is aligned
with the z direction within the aperture, indicating a
mode, and circulates between adjacent aper-
tures, further reducing its conﬁnement. This mag-
netic coupling greatly increases the effective area of
the aperture, permitting more incident radiation to
transmit through the array. The spacing between the
apertures is critical to maximize this effect.
The isotropic nature of the transmission from an
isolated aperture suggests the possibility of grating
phenomena when or
(in the IR,
since the imaginary part of the permittivity of metal
is sufﬁciently large). The simulation results do show
a weak RWA/SPP feature at =2.85
m [inset of Fig.
4(a)]. Figure 4(d) shows the ﬁeld distribution corre-
sponding to this mode. The magnitude of the RWA/
SPP is small compared with the waveguide mode at
m. This is further exacerbated by the roughness
of the ﬁlm, which serves to scatter the surface mode,
making it difﬁcult to detect in the experiment.
In summary, this work demonstrates extraordinary
IR transmission through a bowtie aperture array.
The high transmission is shown to be the result of
coupling to and from resonant waveguide modes for
both a bowtie array and a single bowtie aperture
with surface-mode phenomena playing a negligible
role. The mode structures of the apertures are shown
to be inductively coupled to each other when the ap-
ertures are placed in the array, which contribute to
the extraordinary transmission of the aperture array.
Support to this work by the Air Force Ofﬁce of Sci-
entiﬁc Research STTR program (contract FA9550-09-
C-0058, Program Manager Dr. Gernot Pomrenke),
National Science Foundation (NSF) (DMI-0707817),
and Defense Advanced Research Projects Agency
(DARPA) (grant N66001-08-1-2037, Program Man-
ager Dr. Thomas Kenny) is gratefully acknowledged.
The authors also thank John Coy of the Purdue Uni-
versity Birck Nanotechnology Center for assistance
in taking the FTIR measurements and Dr. Axel Reis-
inger of QmagiQ LCC and Prof. W. J. Chappell of
Purdue University for helpful discussions.
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