Nanoimprint fabrication of gold nanocones with
È10 nm tips for enhanced optical interactions
Juha M. Kontio,1,* Hannu Husu,2Janne Simonen,1Mikko J. Huttunen,2Juha Tommila,1
Markus Pessa,1and Martti Kauranen2
1Optoelectronics Research Centre, Tampere University of Technology, FIN-33101 Tampere, Finland
2Department of Physics, Optics Laboratory, Tampere University of Technology, FIN-33101 Tampere, Finland
* Corresponding author: firstname.lastname@example.org
Received March 20, 2009; accepted May 8, 2009;
posted May 27, 2009 (Doc. ID 108985); published June 24, 2009
We show that nanoimprint lithography combined with electron-beam evaporation provides a cost-efficient,
rapid, and reproducible method to fabricate conical nanostructures with very sharp tips on flat surfaces in
high volumes. We demonstrate the method by preparing a wafer-scale array of gold nanocones with an av-
erage tip radius of 5 nm. Strong local fields at the tips enhance the second-harmonic generation by over 2
orders of magnitude compared with a nonsharp reference. © 2009 Optical Society of America
OCIS codes: 220.4241, 190.2620.
Metal nanostructures are under intense investiga-
tion especially in the fields of plasmonics  and
metamaterials . They act as optical antennas ,
which couple light between the near and the far
fields, allowing light to be manipulated beyond the
diffraction limit. The resulting strong nanoscale elec-
tromagnetic fields can enhance optical interactions,
such as surface-enhanced Raman scattering . The
nanoscale localization of light arises from plasmon
resonances of metal nanoparticles and can be further
enhanced by nanoscale gaps between particles . In
addition, sharp tips can lead to very strong local
fields through geometrical effects (lightning rod ef-
fect) , which has many applications in tip-
enhanced near-field microscopy , sensing , and
nanofocusing of light . Strong local fields are par-
ticularly important for nonlinear optical interactions,
which scale with a high power of the fields, as dem-
onstrated by second-harmonic generation (SHG) from
nanodimers , sharp tips [7,11], four-wave mixing
, and high-harmonic generation . Sharp me-
tallic tips are also in wide use outside optics acting as
efficient Spindt-type electron emitters , for ex-
ample, in the emerging applications of surface con-
duction electron emitter displays (SED) .
It is challenging to fabricate metal nanostructures
reproducibly over large areas while maintaining good
structural quality. Individual particles can be made
byfocused ion beam(FIB)
dimensional particle arrays by electron-beam (e-
beam) induced deposition, but the sample dimensions
are limited by reasonable lithography time to 0.1–1
mm. Both methods have also been used to demon-
strate conical structures with sharp tips (nanocones),
but the processing is slow and quite expensive .
Conical shapes can also be prepared by self-assembly
and etching  or by nanotransfer printing .
These methods do not simultaneously produce truly
sharp features (i.e., a few nanometer tip radii) and al-
low accurate control of the placement of the particles.
Sharp low aspect ratio nanostructures have been pro-
duced by a soft lithography-based approach , but
the shape is limited to pyramids because of the crys-
tallographic etching in mold fabrication.
In this Letter, we show that UV-nanoimprint li-
thography (UV-NIL), combined with e-beam evapora-
tion, can overcome the problems of other fabrication
techniques. Our method enables the fabrication of
large arrays of gold nanocones with sharp 10 nm
scale tips, good structural quality, and high reproduc-
ibility. The technique is fast and relatively cheap and
provides accurate control of the particle positions. We
characterize the nanocones by extinction spectros-
copy, which identifies a plasmon resonance along the
cone axis and SHG, which verifies the existence of a
strong local field polarized along the cone axis.
We prepared arrays of conical gold nanocones of
130 nm base diameter, organized in a square array
with a cone-to-cone period of 300 nm (Fig. 1). A mas-
ter template with a lattice of cylindrical holes was
first prepared by laser-interference lithography (LIL)
(AMO GmbH) on a silicon wafer. The nanopatterns
on the master were copied to a stamp made of poly-
(dimethylsiloxane) (PDMS). A fused-silica wafer used
as a substrate was coated with a 600 nm polymethyl
methacrylate (PMMA) film and a germanium inter-
mediate layer, followed by spin coating of a thin
nanoimprint lithography (NIL) resist layer (Amonil,
AMO GmbH). The nanoimprinting was performed by
an EVG 620 mask aligner using a PDMS stamp [Fig.
2(a)]. Reactive ion etching (RIE) was then used to
etch through the lift-off structure to expose the sub-
strate and to form deep cylindrical holes in the resist
base diameter of 130 nm, and an average cone height of 291
nm on a fused-silica substrate.
Array of gold nanocones with a period of 300 nm, a
July 1, 2009 / Vol. 34, No. 13 / OPTICS LETTERS
0146-9592/09/131979-3/$15.00 © 2009 Optical Society of America
mask [Fig. 2(b)]. The metal layers forming conical
nanostructures were defined by depositing titanium
adhesion (10 nm) and gold (300 nm) layers in an
e-beam evaporator until the holes in the resist mask
were completely filled with gold [Fig. 2(c)]. Finally,
lift-off was applied in acetone using ultrasonic agita-
tion [Fig. 2(d)].
In traditional lift-off processes the gold patterns
would have vertical sidewalls similar to those of the
mask. Therefore one would expect to grow cylindrical
nanorods, but this did not occur in our case. The
holes in the resist mask were narrow (130 nm diam-
eter) and deep (600 nm), giving them a high aspect
ratio. The structures grew conically because the top
of the hole shrank in diameter during gold evapora-
tion, similar to the process for Spindt-type emitters
. Owing to mutual collisions, the gold atoms ar-
rived at the sample from slightly different directions
during evaporation. Thus, the atoms tended to stick
on the top edge of the hole rather than the bottom
edge of the hole, leading to the preferred deposition
to the center part of the bottom. The end result was
that the particle grew sharper during the deposition
until the hole was completely covered with gold as
shown in Fig. 2(c).
The shape of the gold nanocones was verified by an
atomic force microscope (AFM) (not shown) and a
field-emission scanning electron microscope (FE-
SEM) (Fig. 1). The yield of an unoptimized nanocone
process was approximately 95% on a 4 cm2area,
which consisted of 4.4?109nanocones. The size of
the patterned area is only limited by the size of the
NIL stamp and could be as large as 150 mm.
The main advantages of our process arise from the
use of NIL. After the initial master pattern is pro-
duced, for example, by expensive electron-beam li-
thography (EBL), it can be replicated hundreds of
times cost effectively. Compared with EBL, our NIL-
based process is superior in time and repeatability.
Moreover, the final cone height, together with sharp-
ness, may be accurately controlled contrary to nan-
otransfer printing . Our method is also less dam-
aging than FIB milling. The most damaging process
step for the substrate in our method is the O2etching
of the PMMA layer in RIE. This is an advantage if
nanocones are to be prepared, for example, over com-
pound semiconductor quantum wells. Furthermore,
FIB is not suitable for large-volume production. The
disadvantage of our method compared with FIB is
that the deposited gold structures are grainy, unlike
features produced by removing material from bulk
metal. However, in many applications the metal
structures will in any case be deposited on a sub-
strate, so our cones will be of similar quality.
To identify the plasmon resonance of the nano-
cones, we measured their extinction spectrum using
a fiber-optic spectrometer for wavelengths from 450
to 950 nm. For TE polarization the spectra are fea-
tureless in this range. However, for TM polarization
and at oblique angle of incidence, a strong resonance
is located at 615 nm [Fig. 3(a)]. This result, therefore,
suggests that the resonance is associated with the
longitudinal particle plasmon of the cone, which os-
cillates along the cone axis.
To demonstrate the strong local fields at the tips of
the cones, we utilized optical SH generation with an
ultrafast pulsed Nd:glass laser (wavelength of 1060
nm, pulse length of 200 fs, repetition rate of 82 MHz).
Note that the fundamental wavelength is nonreso-
nant with the plasmon peak, but the second-
harmonic (SH) wavelength of 530 nm is rather close
to the resonance. The diameter of the focal spot in the
experiment was about 3 ?m, so there were approxi-
mately 80 nanocones within the spot area. The SH
signal was detected by a photomultiplier tube com-
bined with a single-photon counting system. To
couple the incident beam with the direction of the
cone axis, a polarization component along this direc-
tion is needed. A simple way to do this is to tilt the
sample away from normal incidence. However, this
can lead to two problems. First, because the array is
periodic, a propagating diffraction order appears for
the SH wavelength at a certain angle of incidence.
Second, the incident field can also couple to off-
diagonal components of the nonlinear response be-
cause of polarization components in the plane of the
sample, complicating the analysis. These problems
are avoided by using a focused radially polarized fun-
damental beam, which has a strong longitudinal elec-
tric field component at the focal plane, while the lat-
eral polarization components cancel each other.
Simulations using the finite-element method (Com-
sol) show that this situation leads to a strongly en-
hanced field [white area in Fig. 3(b)] at the cone tip.
To quantify the enhancement of the SH response
from the nanocones, we had as a reference sample a
Fig. 2. Fabrication principle. (a) The lift-off structure. The
top layer is the NIL patterned resist layer. (b) Etching by
RIE to expose the substrate. (c) Metal evaporation. Arrows
mark the growth direction. (d) After lift-off in acetone.
angle of incidence for nanocones with (solid curve) and
without (dashed curve) sharp tips. (b) A 3D finite-element
simulation of electric field distribution in a gold nanocone
with a 5 nm tip radius of curvature, illuminated from above
by a focused radially polarized beam at 1060 nm wave-
length. The scale is logarithmic and is cropped at 85% for
(a) Extinction spectra for TM-polarized light at 50°
OPTICS LETTERS / Vol. 34, No. 13 / July 1, 2009
similar array of half-cones with 88 nm height. The Download full-text
reference thus lacks the sharp tip. Its longitudinal
plasmon is shifted to a slightly shorter wavelength of
525 nm [Fig. 3(a)]. Therefore our reference sample is
actually closer to the two-photon resonance with our
laser than the sample with sharp tips.
Figure 4 presents the SH signals from the two
samples as they were scanned in a transverse direc-
tion in the focal plane. There is a significant fluctua-
tion in the SH signal for the sharp nanocones,
whereas the signal from the reference sample is more
uniform. This is significant because the measured
signals represent averages over about 200 individual
cones. During the scan nanocones are moving into
and out of the focal area. The SH signal is therefore
very sensitive to the SH response of single nano-
cones. Such sensitivity arises from the fact that the
SH intensity is proportional to the fourth power of
the local fundamental field and to the second power
of the local SH field. Thus, even small differences in
the features of the individual cones and of their local
fields can lead to large differences in SH signals. To
quantify the enhancement, we averaged the SH sig-
nals over the whole transverse scan. The SH inten-
sity from the sample with sharp nanocones was en-
hanced by a factor of 150 compared with the half-
cones. Clearly, sharp nanocones are efficient in
producing high local electric fields.
In conclusion, we have shown that nanoimprint li-
thography enables cost-effective fabrication of large-
area arrays of gold nanocones. Gold evaporation into
deep holes spontaneously forms nanocones with good
reproducibility, uniformity, and sharp tips. We have
also shown, using SHG, that the nanocones enhance
local electric fields polarized along the cone axis.
Such structures could prove useful in applications
where a large number of regularly spaced sharp nan-
otips are needed, such as plasmonic sensors, nanoe-
mitters, nanofocusing, and metamaterials.
This work is supported by the Academy of Finland
(projects 114913 and 123109), the Finnish Funding
Agency for Technology and Innovation (project 40149/
08), and the Finnish Ministry of Education (The Re-
search and Development Project on Nanophotonics).
Timo Lehto is acknowledged for technical help in op-
tical measurements. J. M. Kontio and H. Husu ac-
knowledge the graduate school of the Tampere Uni-
versity of Technology. J. M. Kontio also acknowledges
the Vilho, Yrjö, and Kalle Väisälä Foundation and
the Emil Aaltonen Foundation for financial support.
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direction. The measurement is done for sharp nanocones,
with a height of 291 nm, and half-cones, with a height of 88
nm. Dashed lines illustrate the average SHG intensities.
SHG signal as the sample is scanned in transverse
July 1, 2009 / Vol. 34, No. 13 / OPTICS LETTERS