Near-field optical response of a two-dimensional grating of gold nanoparticles
M. Salerno, N. Fe
´
lidj, J. R. Krenn, A. Leitner, and F. R. Aussenegg
Institute for Experimental Physics, Karl-Franzens-University Graz and Erwin Schro ¨dinger Institute for Nansoscale Research,
Universita ¨tsplatz 5, A-8010 Graz, Austria
J. C. Weeber
Laboratoire de Physique, Optique Submicronique, Universite ´ de Bourgogne, Boite Postale 47870, F-21078 Dijon, France
Received 1 August 2000; published 4 April 2001
This article reports on the near-field optical response of a small square grating of gold nanoparticles tailored
by electron-beam lithography. The investigation of the grating is aimed at a deepened understanding of
electromagnetic interaction among particles due to scattered light fields. Therefore, a photon scanning tunnel-
ing microscope is applied to acquire near-field optical images. Two different incident wavelengths are used to
characterize the intensity and the spatial localization of the electromagnetic near field both in and out of
resonance for the excitation of particle plasmons. Furthermore, the near-field enhancement resulting from the
plasmon excitation is evaluated.
DOI: 10.1103/PhysRevB.63.165422 PACS numbers: 78.66.-w, 07.79.Fc, 42.25.Fx
I. INTRODUCTION
Nanoscale metal structures are well known to manifest
strong local enhancement of the electromagnetic field with
respect to the incident light, due to the excitation of coherent
resonant electron plasma oscillations particle plasmons.
1
The resonance wavelength of this phenomenon depends on
the geometry of the structure as well as on the dielectric
functions of both the structure and the surrounding medium.
2
Since metal nanostructures are promising candidates for such
nano-optical applications as controlling and guiding light
fields on the submicrometer scale,
3–6
they are the subject of
extensive fundamental research. In this context, specifically
designed nanostructures obtained by electron-beam lithogra-
phy EBL represent ideal samples for a fundamental sys-
tematic investigation. EBL is a powerful tool for the fabrica-
tion of highly reproducible nanostructures with a great
variety of geometries and arrangement patterns.
7
In particu-
lar, it allows matching of the particle plasmon frequencies to
defined values.
As for many practical applications large ensembles of
nanoparticles are of interest,
8
, a better understanding of elec-
tromagnetic coupling effects among nanoparticles is of great
importance. Coupling can arise both from very short distance
interactions on the order of some tens of nanometers, near-
field coupling and from long range interactions dipolar far-
field coupling
9
. Recently, direct observations of the near-
field response of an isolated gold nanoparticle and of a one-
dimensional 1D chain of gold nanoparticles by means of
near-field optical microscopy were reported.
10
It was shown
that the individual particle gives rise to a rather broadly dis-
tributed plasmon field, while a strong confinement of the
field was observed above the particle chain due to plasmon
near-field coupling. On the other hand, electromagnetic far-
field coupling was found to be particularly strong among
nanoparticles arranged in regular 2D particle gratings.
11
When the interparticle distance is increased for a given light
wavelength, dipolar interaction between the particles in-
creases dramatically whenever the light field corresponding
to a particular grating order changes from evanescent to ra-
diative in character. As a consequence, a strong dependence
of the plasmon resonance wavelength and the plasmon life-
time on the interparticle distance was reported. This phenom-
enon was investigated by conventional far-field spectro-
scopy. Since the physical origin of the observed effects is
expected to be connected to the local electromagnetic fields
acting on the individual nanoparticles, direct measurement of
these local field intensities by near-field optical microscopy
is a promising task.
In this article we examine the optical near-field zone of
regularly arranged gold nanoparticles by means of a photon
scanning tunneling microscope
12
PSTM operated in
constant-height mode. Previous work has established that the
PSTM is capable of measuring the near-field intensity distri-
bution around nanoscale metal samples.
10,13–16
We investi-
gate a small regular 2D grating consisting of 3 3 gold par-
ticles with the interparticle distance grating constant fixed
to 1 m. We chose this small grating for the investigation
of the near-field zone of regularly arranged nanoparticles as
it constitutes the transition case between the individual par-
ticle and the infinitely extended grating.
II. EXPERIMENTAL AND THEORETICAL BACKGROUND
Samples consisting of gold particles with a diameter of
100 nm and a height of 40 nm are produced by EBL per-
formed on a modified scanning electron microscope SEM.
As the substrate an indium tin oxide ITO doped glass plate
is used to provide the weak electric conductivity required for
EBL and SEM in order to avoid charging effects. Further
details on EBL are provided elsewhere.
7
After fabrication, a
standard characterization of the samples is performed. The
SEM is used to check overall quality of the samples, and
shape and lateral size of the particles, while atomic force
microscopy AFM allows a cross-check and measurement
of the particle height. Optical extinction spectroscopy at nor-
mal incidence is accomplished to find the resonance wave-
lengths of the particle plasmons. As the overall size of
PHYSICAL REVIEW B, VOLUME 63, 165422
0163-1829/2001/6316/1654226/$20.00 ©2001 The American Physical Society 63 165422-1