2015-3

121110-2 Nasir et al. Appl. Phys. Lett. 107, 121110 (2015)
diameter via post processing of the pores while keeping both
the inter-rod spacing and rod length constant.
The optical properties of plasmonic nanorod metamaterials
are determined by the coupling of cylindrical surface
plasmons supported by individual nanorods in the array. On
the macroscopic level, this is well described by the Maxwell-
Garnett type EMTs which can be validated through comparison
with the experimental and microscopic numerical modelling.
19 The effective permittivities in both the in-plane and
out of plane directions are given by
ð1 þ pÞe Au þ ð1 pÞe h
e xy ¼ e h
ð1 pÞe Au þ ð1 þ pÞe h
e z ¼ pe Au þ ð1
pÞe h ; (1)
where p ¼ p(r/d) 2 is the nanorod concentration with r being
radius of the nanorods and d being distance between the
nanorods, e Au and e h are the permittivities of Au and the host
medium (Al 2 O 3 ), respectively. This model is valid away
from the Brillouin zone edge of the nanorod array with
k 0 p d , where k 0 is the wavevector of the incident light in
the plane of the metamaterial slab. 11 The effective plasma
frequency of Au nanorod metamaterials, which is determined
by Re(e z ) ¼ 0, is affected by both the host medium properties
and the nanorod concentration. 11 This frequency separates
the elliptic dispersion regime, where the metamaterial
behaves as a strongly anisotropic dielectric, and the hyperbolic
regime where the metamaterial supports bulk plasmonpolaritons
due to its metallo-dielectric behaviour.
In order to fabricate the metamaterial, Au nanorods are
electrodeposited in porous anodic alumina oxide (AAO) templates,
synthesized by a two-step anodization process. An aluminium
film of 800 nm thickness was sputtered on a
multilayer substrate comprised of a glass slide with a 10 nm
thick adhesion layer of tantalum pentoxide and a 7 nm thick
Au film acting as a weakly conducting layer. Tantalum pentoxide
is deposited by sputtering tantalum using a 20% oxygen/80%
argon mixture. The porous alumina structures were
synthesized by a two step-anodization in 0.3 M selenic acid
at 40–48 V at 0 C. The temperature of the sample and electrolyte
was controlled throughout the anodization process.
After an initial anodization step, the porous layer formed
was removed by etching in a solution of H 3 PO 4 (3.5%) and
CrO 3 (20 g l –1 )at70 C leaving an ordered, patterned surface.
The samples were then subjected to a second anodization
step under the same conditions as in the first step and
subsequently etched in 30 mM NaOH to tune the diameters
of pores from 15 nm to 50 nm. Gold electrodeposition was
performed using a three-electrode geometry and a noncyanide
solution. The length of the nanorods was controlled
by the electrodeposition time.
In general, the geometrical parameters of the porous alumina
template used for nanorod metamaterial fabrication are
determined by the electrolyte used for anodization and by
the applied anodizing potential. The pore diameter and interpore
spacing of the porous alumina template is proportional
to the applied voltage with proportionality constants
k d ¼ 1.29 nm V 1 and k int ¼ 2.52 nm V 1 , respectively. 20–24
Therefore, for a chosen electrolyte, the diameter of the pores
can be increased in a pore widening process for a given
voltage, resulting in an increase in the nanorod concentration
p. However, due to the limitations of currently used electrolytes,
a different approach is required in order to provide
ENZ-related functionalities in the infrared, including at telecom
wavelengths.
Self-ordered porous alumina is typically obtained in
acidic electrolytes, such as sulphuric, oxalic, phosphoric,
malonic, and tartaric acids. 25–28 In these electrolytes, the diameter
of pores is determined by the applied voltage with a
proportionality constant equal to or greater than 1 nm/V and
the interpore distance determined by a proportionality constant
2.52 nm V 1 . In selenic acid, 24 the pore diameter of
the self-ordered porous alumina template has a weaker dependence
on the applied voltage (a proportionality constant
is 0.3 nm/V) while the interpore distance has about the
same dependence (proportionality constant 2.33 nm V 1 ).
Figure 1(b) shows the SEM image of the self-ordered porous
alumina template formed in 0.3 M selenic acid at 48 V,
keeping the temperature at 0 C. The regular pore arrays
have been observed within micrometre sized domains. The
diameter of the pores is 15 nm with an interpore separation
of approximately 112 nm. The porosity, determining nanorod
concentration p, of the anodic alumina obtained in selenic
acid is 3.4%, much lower than achievable with sulphuric
acid (down to 12.6%). 22,29 It is very interesting to note that
both sulphuric acid and selenic acid possess similar chemical
structures (Fig. 1(c)) but totally different anodization behaviour.
In the case of acids having similar chemical structure,
acid strength decreases as the size of the central atom
increases. The atomic size of selenium is bigger than sulphur;
therefore, selenic acid is less soluble than sulphuric acid
under the same anodization conditions. This low solubility
accounts for the much smaller pore diameter observed here.
With a decrease in anodization temperature, the solubility
of selenic acid decreases leading to the unusual selfordered
porous alumina with smaller than expected pore
diameter, while the period remains unaffected. This unique
porous alumina structure has enabled the metamaterials’
plasma frequency to be tuned throughout the visible and
near-infrared spectral ranges. Additionally, since the geometry
of the gold nanorods is determined by the porous alumina
template, the diameter of nanorods may be varied by changing
the pore diameter in a pore widening process while keeping
the inter-rod spacing the same. Alternatively, we can
also tune the pore diameter while keeping the interpore distance
constant by raising the temperature during the anodization
process. Figure 1(e) shows the effect of varying the
nanorod diameter while maintaining constant nanorod spacing
and length. With a reduction in nanorod concentration, a
monotonous long-wavelength shift of the dominating extinction
peak is observed, accompanied by a broadening of the
peak due to the red-shift of the effective plasma frequency
(as the concentration of metal becomes smaller the interaction
between the nanorods becomes weaker).
Metamaterials comprised of Au nanorods embedded in
an AAO matrix were designed with the effective plasma frequency
in the infrared spectral range (Fig. 2). The extinction
spectra show the two typical dominating resonances, which
are well separated from each other spectrally. A short wavelength
resonance is associated with plasmonic excitations by