Optical properties of gold nano-bowtie structures
Gang Bi
a
, Li Wang
b
, Li Ling
a
, Yukie Yokota
c
, Yoshiaki Nishijima
c
, Kosei Ueno
c,d
,
Hiroaki Misawa
c
, Jianrong Qiu
b,
n
a
Institute of Information and Electronics, Zhejiang University City College, Hangzhou 310015, China
b
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China
c
Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan
d
PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan
article info
Article history:
Received 12 November 2012
Received in revised form
18 December 2012
Accepted 25 December 2012
Available online 17 January 2013
Keywords:
Gold nano-bowtie
Optical properties
Surface plasmons
abstract
Gold-bowtie nanostructures are fabricated by electron beam lithography and lift-off techniques. Their
optical properties are investigated by both experimental studies and numerical simulations. The
influence of sharpness and gap of nano-bowtie on optical properties such as extinction cross section,
local electric field intensity, and surface resonance wavelength is explored. The experimental extinction
spectra agree well with the simulation results. This investigation may help us to design plasmonic
sub-wavelength gold-bowtie nanostructure with desired spectral properties.
& 2013 Elsevier B.V. All rights reserved.
1. Introduction
Nanoengineered particles of noble metals have unique physi-
cal and chemical properties due to resonant scattering, field
enhancement, and surface effect, which are quite different from
the bulk or atomic species. By controlling the size and shape of
nanoparticles and their surrounding dielectric medium, the reso-
nance wavelength can be tuned and that promise novel applica-
tions in nonlinear sub-wavelength optics [1–5], single molecule
detection [6,7], high-resolution microscopy [8,9], and bio-sensing
[10–15], etc.
The metallic bowtie nanostructures are configured with two
isosceles triangles which apexes face each other with a sub-
wavelength gap (shown in Fig. 1). Bowtie antennas strongly
concentrate the field at the extremities of the individual trian-
gular monomers, and can exhibit remarkably large local electric
field (E) enhancement [16,17]. Therefore there have been some
investigations and applications of nano-bowtie structures in
various fields such as optical antenna [18–20], optical waveguide
[21], and surface enhanced Raman scattering (SERS) [22].
However, there are few theoretical researches about the
influence of both sharpness and gap on the optical properties
and local electric field enhancement of gold nano-bowtie struc-
tures. In the present study, we prepare gold nano-bowtie struc-
tures by electron beam lithography (EBL) and analyze the optical
properties by using extinction spectrum measurements. We
discuss the influence of gold nano-bowtie sharpness and gap on
optical properties and electric field enhancement effect. For
shape-dependent optical properties, we keep the identical area
of triangle so as to eliminate the size effect. The tip-angle ranges
from 20–1401, including not only acute angle and right angle, but
also obtuse angle, whereas the previous researches usually con-
centrated on the acute and right angles. The gap distance used in
the previous works was usually equal or larger than 0 nm.
However, the gap distance in this study includes the minus gap
as well. The bowtie structure with minus gap means that the tip-
angles of two triangles overlap each other. Furthermore, we use
finite difference time domain (FDTD) simulation to examine the
optimum shape of the gold nano-bowtie structure for realization
of the maximized local field enhancement effect and the desired
localized surface plasmon resonance (LSPR) band.
2. Experimental
2.1. Sample preparation procedure and characterization
Gold nano-bowtie structures are fabricated on clear silica
glasses by electron beam lithography (EBL) system (ELS-7700H,
Elionix Co., Ltd., Japan). The geometric structure of gold nano-
bowtie is shown in Fig. 1. We fabricate three kinds of bowtie
structures of tip-angle (
y
)as301,601 and 901. All the bowties have
the same size (100 373 nm
2
) and thickness (50 nm). The height
of the bowtie which
y
equals to 301,601 and 901 is 373 nm,
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journal homepage: www.elsevier.com/locate/optcom
Optics Communications
0030-4018/$ -see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.optcom.2012.12.080
n
Corresponding author.
E-mail addresses: qjr@zju.edu.cn, big@zucc.edu.cn (J. Qiu).
Optics Communications 294 (2013) 213–217