Nanooptical Studies on Physical and Chemical Characteristics of Noble Metal Nanostructures.pdf

8/13/2019 Nanooptical Studies on Physical and Chemical Characteristics of Noble Metal Nanostructures.pdf

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Award Accou ntsThe Chemical Society of Japan Award for Creative Work for 2011

Nanooptical Studies on Physical and Chemical CharacteristicsofNoble Metal Nanostructures

Hiromi Okamoto1,2

1Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585

2The Graduate University for Advanced Studies, Myodaiji, Okazaki, Aichi 444-8585

Received October 2, 2012; E-mail: [email protected]

Studies on physicaland chemicalproperties ofnoble metalnanostructures, mainly by near-field optical microscopyand spectroscopy, are described. Near-field optical microscopy provides optical observation methodology with a spatial

resolution in nanometer regime beyond the diffraction limit of light. We developed near-field imaging systems equipped

with various light sources including ultrashort pulsed lasers, which enables advanced nonlinear and ultrafast near-field

measurements as well as conventional near-field imaging. In particular, near-field two-photon excitation imaging was

shown to provide a convenient toolto visualize the enhanced optical fields in the vicinities ofmetalnanostructures. With

these methods we demonstrated that nanoscale optical field structures in metal nanostructures can be directly visualized.

For single noble metal nanoparticles, such as gold nanorods as typical examples, plasmon standing wave functions were

visualized. Ultrafastimaging revealed that sub-picosecond relaxation was reflected on the plasmon wavefunctionimages

through thermallyinduced dielectricfunction changes ofthe metal. In some cases optical field distributionfeatures arising

from thelightning rod effects were observed, depending on the resonance conditions ofthe incident wavelengths with the

plasmon modes. We also found anomalous near-field transmission phenomenonfor nanoapertures blocked by nanodisks

near the plasmon resonance wavelengths, which arisefrom the efficient near-field to propagating-field conversion ability

of the nanodisks. In assembled nanoparticles, enhanced optical fields at the gap sites between the particles werevisualized, which elucidates experimentally the mechanism ofsurface-enhanced Raman scattering. The characteristicfield

distributions in many particle assemblies were also observed and analyzed. Through these studies, we established a

valuable methodology to investigate optical and spectroscopic properties of metal nanostructures, and the information

obtained is available only by optical measurements with high spatial resolution.

1. Introduction

My research group started our project of the optical prop-

erties ofnoble metalnanoparticles rather by chance some years

ago. When I was given a chance to launch a research group at

the Institute for Molecular Science, I decided to carry out a

research plan that I had cherished for several years. That is,I planned to develop new experimental methodologies based

on combination of near-field optical microscopy1-8 and laser

molecular spectroscopy,9 and to applyit to materials that attract

interests from a physical chemistry point ofview. I had some

research background of studies on excited-state molecules by

time-resolved spectroscopy and vibrational spectroscopy, and

hence had knowledge of advanced laser molecular spectros-

copy. However, I did not have any experience with treating

nanomaterials and probe microscopy. I had to launch a new

project in a research area that I was totally not familiar with.

I was not sure whether we could develop novel methods and

apparatuses that could provide sufficiently high-level research

achievements, and whether I would encounter any interesting

materials to study. For the first few years we concentrated on

developing the near-field apparatus. As a result, we succeeded

in constructing a sufficiently competent near-field microspec-

troscopicimaging system,10-13 as describedin Sections2 and 7

ofthis account. I believe that the most successfulachievements

of this project originate in the efforts toward the apparatus

development.At the beginning, we conducted the research project not

bearing strongly metalnanoparticlesin mind. In the discussion

of various possibilities of studies utilizing the uniqueness of

near-field imaging, we considered that a new research area

might be expanded by combining near-field microscopy with

the confinement ofoptical fields by noble metal nanoparticles.

As a basisfor that, we decided to perform near-fieldimaging of

metal nanoparticles to reveal the basic nanooptical character-

istics. We began with the studies of near-field characteristics

ofspherical gold nanoparticles as the most basic case, and we

extended the study to rod-shaped particles (gold nanorods). The

first observation of the steady-state wave functions of surface

plasmon resonance in gold nanorods11,14,15

made me aware of

2013 The Chemical Society ofJapan

Published on the web April 15, 2013; doi:10.1246/bcsj.20120268

Bull. Chem. Soc. Jpn. Vol. 86, No. 4, 397-413 (2013) 397
http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268http://dx.doi.org/10.1246/bcsj.20120268


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possible development ofa new research field in nanoscience.

Then we applied the method to several other types of noble

metalnanoparticles16,17 to promote the plasmon-modeimaging

and studies on nanoopticalcharacteristics ofthem. In Sections

4 and 5 of this account I will describe mainly the research

achievements related to this topic.

The unique characters and functions of noble metal nano-

structures are not sufficiently clarified only by studying iso-

lated single nanoparticles. Characteristics ofassembled nano-

particles are sometimes essentialto understand thefunctions. In

thelast two decades studies on mechanism ofsurface-enhanced

Raman scattering have been greatly advanced,18-24 and it has

been revealed that assembled noble metal nanoparticles are

indispensable for efficient Raman enhancement. Optical char-

acteristics of noble metal nanoparticle assemblies attracted

much attention partly because ofthis reason. We applied near-

field imaging to assembled gold nanoparticles and succeeded

in experimental visualization of localized enhanced optical

fields for the first time.25,26 I felt that this finding might also

stimulate a novel research area, as in the standing wave obser-

vation of a single nanoparticle mentioned above, and made

some efforts to develop the study. As a result we have derived

some guidelines for the structures of nanoparticle assemblies

to get efficientfield enhancement.27-30 Section 6 is devoted to

this topic.

My originalpurpose ofthis research project was to develop a

unique method ofhigh spatial-resolution spectroscopicimaging

by combining dynamic/nonlinear spectroscopies with near-

field microscopy, and to apply it to scientifically interesting

nanostructured materials. To this end we constructed ultrafast/

nonlinear near-field microscopy systems by introducing ultra-

short (tens of femtoseconds) pulsed laser sources.10-12,31 We

developed advanced dev

ices to ach

ieve very h

igh t

ime reso

lu-tion under the near-field microscope, and made measurements

of near-field ultrafast imaging/spectroscopy for gold nano-

particles. Section 7 summarizes this topic. We have found by

using these systems that two-photon-induced photolumines-

cence from noble metal nanoparticles provides a valuable

detection method for near-field imaging. We applied this

method in the near-field imaging studies of plasmonic mate-

rials.14,16,25-29,32,33 Since then two-photon-induced emission

from gold has been utilized in many studies.

I wish to stress in this account the validity and the unique-

ness ofnanooptical measurements in the studies ofcharacter-

istics ofnoble metal nanostructures.

2. Near-Field Optical Microscope Setup

In this section, the experimental methods of near-field

optical microscopy are described, mainly on that equipped

with an apertured optical fiber probe, which we have adopted

in this project. The methods ofnear-field microscopy presently

utilized are roughly classified into two types, aperture1,3,4,7,34

and nonaperture (or scattering) types.2,8,35-39 In the aperture

type, a tiny aperture with a diameter smaller than the wave-

length of light opened on an opaque thin film (usually a

metallic film) is installed. A nonpropagating radiation field

localizedin a space ofapproximately the aperture size (optical

near-field)is generated when the apertureisirradiated bylight.

The sample to be observed approaches the aperture, and is

illuminated by light through the aperture. Since the localized

light near the aperture does not propagate, the samp le is never

photoexcited unless the aperture locates sufficiently close to it.

The sample interacts with light only when it enters into the

area ofthe near-field radiation, to give absorption, scattering, or

emission of photons. By detecting the optical responses while

the aperture nears the surface ofthe sample and scanning the

position laterally relative to the sample, we can observe an

optical image of the sample with a spatial resolution deter-

mined approximately by the aperture size. This is the basic

principle ofnear-field microscopy. In the scattering type near-

field microscope, a sharpened metallic tip is installed and is

externally illuminated. A confined radiation field is generated

in the region near the apex ofthe tip, approximately within its

radius ofcurvature from the apex, due to the electromagnetic

effect. By the use ofthe confined optical field a high-resolution

optical image can be obtained with a method similar to the

aperture type method, which is the basic principle of the

scattering type near-field optical microscopy.

In a practical experimental aperture-type near-field micro-

scope, an aperture on an opaque metallic film is fabricated at

a sharpened end ofsingle-mode optical fiber core.1,4,7,34 Light

for the measurement is introduced from the other end of the

optical fiber, and optical near-field is generated at the aperture

probe tip. The height of the probe tip is maintained at several

nanometers to about 10 nm from the sample surface while the

lateralposition relative to the sampleis scanned and the optical

signal is recorded. In our apparatus we have adopted shear-

force feedback1,3,4 to maintain the probe tip height. As other

methods for tip height control, the mechanism of an atomic

force microscope or that ofa scanning electron microscope (for

conducting materials) can be also applied. To detect the optical

response, we can select one o

fthe

fo

llow

ing three measurementmodes.1,3

1) The sample is irradiated by light through the probe, and

transmitted, scattered, or emitted radiation is detected exter-

nally (this is called i llumination mode).

2) The sample is irradiated by a far-field excitation source,

and the optical near-field induced by the excitation light is

picked up by the near-field probe and detected (collection

mode).

3) The sample is irradiated by light through the probe, and

the optical response in the near-field regime is also picked up

by the same probe (illumination-collection mode).

The illumination-collection mode is advantageous over the

other two because of its fundamental merit that irradiationand detection are performedfor anidentical local area through

an identical probe, and in addition because of its practical

merit that it can be applied to opaque samples or samples on

opaque substrates. On the other hand, since the irradiation and

detection are done through a tiny low-throughput aperture, the

detection efficiency is much lower than the other two modes.

In our studies on metal nanostructures, we adopted in most

cases illumination mode for the samples on transparent glass

substrates (Figure 1).40,41 The radiation transmitted through

the probe and the sample is collected by an objective lens

wit h a high numerical aperture (NA) and detected by a

photodetector after passing through appropriate optical filtersif

necessary. We use a polychromator equipped with a charge-

AWARD ACCOUNTSBull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013)398


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coupled device (CCD) multichannel detector when we need to

measure spectra.

The light for the measurement was coupled into the fiber

from the other end ofthe near-field probe tip. Continuous wave

(cw) lasers are frequently used as light sources for near-

field measurements. In addition to them, we sometimes use

Xe discharge arc lamps for the transmission measurements.

Conventional light sources like discharge arc lamps are gener-

all

y considered unsu

itab

le

for coup

ling

into a s

ing

le modefiber because of their low focusability. However, for trans-

mission measurements, strong transmitted light is not neces-

sary. Coupling ofonly a smallportion ofthe focused spotinto

the fiber core is actually sufficient for the detection, and thus

the arc lamp is rather a convenient, easy-to-align light source

for this purpose. For nonlinear and ultrafast time-resolved

signal detection, we use mainly femtosecond mode-locked

Ti:sapphire lasers. When a femtosecond pulse propagates

through an optical fiber, the pulse duration is broadened to

several picoseconds due to the group velocity dispersion of

thefiber medium. This effect results in serious deterioration of

time resolution in time-resolved measurements, and in non-

linear optical measurements lowering of signal levels becausethe rate of nonlinear processes becomes lower as the pulse

peak power decreases. To avoid that, we install dispersion

compensation devicesfor the opticalbeam before couplinginto

the fiber, which are precisely adjusted to get the shortest pulse

duration at the apertured probe tip. The details of the disper-

sion compensation are described in Section 7. The polariza-

tion ofthe incident light on the sample is in general elliptical

due to the retardation effects in the fiber, even though the

light source provides perfectly linear polarization. We inserted

a half-wave plate and a quarter-wave plate to the opticalbeam

before the fiber, which allows us to get approximately linear

polarization at the exit of the fiber and to adjust the direction

ofpolarization.

3. Noble Metal Nanostructured Materials and Their

Fundamental Optical Characteristics

The noble metal nanoparticle samples for the near-field

measurements were prepared either by dispersing chemically

synthesized particles on glass substrates or by electron beam

lithography. Spherical gold nanoparticles were commercially

perchased. Gold and silver nanorods and triangular nanoplates

were synthesized as descr

ibed

in prev

ious reports.

42-50

Byelectron beam lithography,51,52 we prepared particles that are

difficult to prepare by other synthetic methods, such as circular

nanoplates.17 The samples ofcircular nanoapertures on metallic

thinfilms (we callthem voidsin this article) were fabricated

by polymer bead lithography.33,53

As anillustrative description of fundamental optical proper-

ties of the metal nanoparticles, typical extinction spectra of

aqueous colloidal solutions of spherical gold nanoparticles

and gold nanorods are shown in Figure 2. The solution of

spherical gold nanoparticles shows a strong extinction peak

at around 550nm when the particle diameter is smaller than

ca. 100 nm, which is attributed to the collective oscillation of

conduction electrons in the particle, that is, surface plasmonresonance (hereafter we callthis simply plasmon or abbreviate

as SPR).2,54-57 The extinction spectrum of the spheric nano-

particle is reproduced nearly quantitatively by the analytical

formula of Mie scattering theory,56,58,59 and the peak around

550nm is assigned to the dipolar electric oscillation on the

particle. Ifthe radius ofthe particleris smallenough compared

to the wavelength of light, which allows us to treat the system

with a dipolar approximation, the polarizability ofa spherical

particle, , is given in general by the following formula:2,56,57

4r3 m= 2m 1

where and m denote, respectively, (complex) dielectric con-

stants of the particle material and the medium. The polar-

fs Ti:sapphirelaser

780-920 nm

CW laser

Xe lamp

PZT

single channeldetector

polychro-mator

CCD

PZT xyz stage

sample

for dithering

opticalfibercore

metalcoat

aperture50-100nm

near-fieldprobe tip

microscopeobjective

sin

glemodefiber

Figure 1. Schematic diagram of a typical scanning near-field optical microscope setup the authors group adopted. PZT:

piezoelectric transducer; CCD: charge-coupled device multichannel photodetector.

H. Okamoto Bull. Chem. Soc. Jpn. Vol. 86, No. 4 (2013) 399


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izability shows a resonance peak at a wavelength giving

Re()= 2m, because the absolute value of the denominatorin the equation aboveis minimum at this wavelength. Since the

real part of is negative for metals, this resonance condi-tion is realized in ordinary dielectric media. This resonance

corresponds to SPR. In nanorods, we find a stronger extinction

peak at alonger wavelength,in addition to the peak ofa similar

character to that of

spheres.

60-63

The peak shif

ts towardl

ongerwavelength when the aspect ratio (= length/diameter)increas-

es. These two peaks for the nanorods are assigned to the

oscillation ofelectronsin a direction perpendicular (transverse

mode) and parallel (longitudinal mode) to the long axis of

the rod, respectively, for the peak around 550nm and that in

the longer wavelengths. For nanorods there is practically no

theoretical framework that allows us an analytical and quan-

titative treatment like Mie theory for spherical particles.

However, ifthe size ofthe nanorodis sufficiently small(which

permits the use of the dipolar approximation) and ifthe parti-

cle can be treated as an ellipsoid, the extended Gans theory

reproduces sufficiently wellthe observedfeatures ofthe extinc-

tion spectra.

60-63

4. Observation ofStanding Wave Functions

ofPlasmons in Noble Metal Nanorods

When we finished the initial construction of our near-field

imaging apparatus, we decided to investigate the optical

characteristics of gold nanorods to utilize the localized fields,

and began near-field transmission measurements for nanorods.

Before the experiment we expected to observe some shades

ofthe rod shapes with scatteringfrom the ends. However, what

we actually obtained for some of the rods was optical extinc-

tion images consisting of bumpy structures, in contrast to the

expectation. After repeated confirmatory experiments, it be-

came clear that we can observe spatially oscillating features

in the near-field transmission images of gold nanorods, with

sufficient reproducibility.11,15,40,64-66 Typical examples are

shown in Figure 3.11 The spatially oscillating features were

found when the image was recorded for the polarization

parallel to the long axis ofthe rod.

Atfirst we wondered what the physicalorigin ofthe features

were. After some time when I was thinking of a subject for

a grant application, I got an idea that the features might

correspond to the amplitudes of standing wave functions of

plasmon modes. The essentialpart ofthe ideais schematically

shown in Figure 4. The fundamental mode of the longitudinal

plasmon is the dipolar mode that gives the lowest resonance

frequencies (the mode ofm= 1 in Figure 4 where m denotes

the mode index). In this mode, the collective oscillation of

electrons

is essent

ia

lly un

iform

in

its phase throughout the rod.The amplitude of the collective oscillation gives the standing

wavefunction ofthe plasmon mode. In addition to the dipolar

mode, there are higher order longitudinal modes (the modes

with m > 1 in Figure 4) where the phase (or direction) of the

collective electronic oscillation is not uniform and depends on

the position along the axis. The wave functions ofthe modes

have (m 1) nodes. The resonant frequency becomes higher

withincreasingm. Radiation-matterinteraction, which absorp-

tion and scattering of light originates in, is strong when the

electronic oscillation amplitude is large. Then, the rod gives

strong extinction at the loops of the plasmon standing wave

function. In other words, the optical image of the rod corre-

sponds to the square modulus of the standing wave functionofthe plasmon mode resonant with the incidentlight. There is

a close analogy between this near-field wavefunction imaging

and scanning tunneling microscopyimaging ofelectronic wave

functions.67-69 In scanning tunneling microscopy local den-

sity of states of electrons70 is visualized, while in near-field

microscopy electromagnetic (or photonic) local density of

states70-76 is imaged. Because of the dispersion characteristics

ofSPR,15,77-84 the wavelength of the plasmon wave is shorter

than the wavelength of the resonant radiation (this is the

physical origin ofphoton confinement by SPR), and thus the

wave functions cannot be imaged by conventional far-field

opticalmicroscopy since the spatialresolutionislimited by the

wavelength. By the near-field measurement, however, the wave

Absorbance(normalized)

11001000900800700600500400

Wavelength / nm

Figure 2. Extinction spectra ofcolloidal aqueous solutions

of spheric gold nanoparticles (diameter 15-25 nm, dotted

curve) and gold nanorods with a low aspect ratio (solidcurve) and a high aspect ratio (dashed curve). Extinction

is normalized at ca. 520 nm (Reproduced with permis-

sion from Ref. 40. Copyright 2006, Royal Society of

Chemistry.).

A B

100 nm 100 nm

Figure 3. Near-field transmission images ofa golfnanorod

(diameter 30 nm, length 180 nm). (A) At 780 nm, polar-

ization parallel to the long axis ofthe rod, and (B) at 530

nm, polarization perpendicular to the rod axis (Reproduced

with permission from Ref. 11. Copyright 2004, American

Chemical Society.).



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function was visualized thanks to the intrinsic high spatial

resolution of the method. The image in Figure 3 is interpreted

as comingfrom the plasmon mode ofm =2 that was visualized

with the polarization parallel to the rod axis.

As f

or the transverse modes where the electrons osc

illatealong the direction perpendicular to the rod axis, they are also

indexed with m according to the number of loops, but the

resonantfrequency does not strongly depend onm56 and all of

them show an extinction peak around 550 nm. The image in

Figure 3 was observed withlight polarized perpendicular to the

rod axis, at a wavelength ofaround 530 nm, anditlookslike a

shadeofthe rod shape. This observedimage isinterpreted as

an overlap of many transverse modes with differentms.

Figure 5 shows near-field transmission images of a gold

nanorod wit h a higher aspect ratio (diameter 20 nm, length

510nm) observed with light polarized along the rod axis at

various wavelengths.40,64 In these images further higher order

longitudinal modes are visualized. At 647 nm there are sevendark spots aligned along the rod axis, which coorespond to the

loops ofthe plasmon wave function, while the number of the

dark spots decreases one by one as the observation wavelength

gets longer, and finally four spots are observable at 830 nm.

The wavenumber increases with increment of the resonant

frequency, reflecting the dispersion relation of the longitu-

dinal plasmon in the rod.40,41,62,65,66 This result also supports

the idea that the obtained near-field images correspond to

optically visualized standing wave functions (square moduli)

ofplasmons.

The high-resolution opticalobservation ofplasmon standing

wave functions can be regarded as a special case of local

excitation ofpolarization waves in nanomaterials. We consider

here the nanorod as a symmetric one-dimensional system

and give a simple formalism of optical measurements of the

polarization waves to discuss the polarized excited state.41,66

To discuss the spatial features of the excitation, the nanorod

is divided into N equivalent volume elements as shown in

Figure 6, and introduce local (elementary) polarization as a

function of the position. The total polarization on the rod is

Transverse modes

Longitudinal modes

m=1

m=2

m=3

m=1

m=2

Optical images

Figure 4. Schematic illustration for the plasmon modes of a nanorod (left) and expected optical images corresponding to them

(right). The dotted curves in the left represent the standing wave functions ofplasmons.

625 nm 647 nm 679 nm

730 nm 830 nm

Figure 5. Near-field transmission images ofa gold nanorod

with a high aspect ratio (diameter 20 nm,length 510nm), at

various wavelengths. The polarization of the observation

was parallel to the long axis of the rod.



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given by the summation of elementary polarizations pi on

volume elements i= 1, 2, +, N. The ground states ofall the

volume elements are assumed to be described by a state vector

of symmetric character j0ii and unpolarized (h0ijpij0ii 0).

The polarization on a volume element i is considered to be

induced by minimal hybridization of an excited state of

asymmetric characterj1iito the ground statej0ii. The polarized

excited state ofthe volume elementjxii is then given as

jxii j0ii ij1ii 2

where i has been introduced here as a coefficient of theasymmetric state (i 1), whileit gives the wavefunction ofthe polarized excited state ofthe rod as will be clarified later.

The expectation valuefor the excited state polarization ofthe

volume element i is given as

hxijpijxii 2Reih0ijpij1ii iRe2p01 3

p01 h0ijpij1ii corresponds to the transition moment for the

volume element i and we assume here that its value does not

depend on i. The ground state G] and the excited state X] of

the whole rod is given by the following equation.

jG jg1ijg2i jgNi ij0ii 4

jX jx1ijx2i jxNi ij0ii

iij1iij6ij0ji jG iij1i 5

where

j1i j1iij6ij0ji 6

corresponds to thelocally excited state ofthe volume elementi.

From this equation, we may understand thati gives a wavefunction of the state X] as a function of the position i, with

{1i]} as a basis set. The polarization at a position i in the

ground and the polarized excited states, respectively, and the

total polarization Pofthe whole rod are given as

GjpijG h0ijpij0ii 0 7

GjPjG ih0ijpij0ii 0 8

XjpijX hxijpijxii Re2p01i 9

XjPjX iXjpijX Re2p01ii 10

From eq 9 we can understand that the wave function i isdirectly related to the local polarization amplitude pi in the

excited state.

Now we discuss optical transition probabilities for this rod.

The probability ofthe optical transition from the ground state

to the polarized excited state is proportional to [GPEX]2

whereEdenotes theincident electricfield. ElectricfieldEisin

generalposition dependent and thus can be expressed asE(i)=

E0e(i), where e(i) is a function that describes the position

dependence of the electric field amplitude. For conven-

tional far-field irradiation, the field amplitude is assumed to

be uniform and independent of the position. In this case we

may set e(i) t o b e u nity and the transition probability is

expressed as, based on the equation above,

jGjPEjXj2 E02jiGjpiij1ij

2 E02jp01iij

2 11

The transition probabilityis thus determined by the summation

(integral) of the wave function (or the polarization wave) over

the whole rod, and gives a conventional selection rule ofopti-

cal transitions. For example, antisymmetric modes of polar-

ization waves with respect to the rod center yield null integral

values of the wave functions, which results in null transition

probabilities (i.e., forbidden transition) from the ground state.

In contrast, underlocalized near-field irradiation, e(i) is not

uniform and has nonzero values only at some specific posi-

tions. Ifa sample system is irradiated by light only at a posi-

tion k (here we assumed for the simplicity that the irradiated

area has a similar size to the volume element), then we may set

e(i)= ik, andin this case the transition probabilityis given as

jGjPEjXj2 E02jiGjpiikij1ij

2 E02jp01j

2k2 12

This means that the transition probability is proportional to

the square modulus of the wave function at the position k.

This situation is totally different from the far-field irradiation

case, and gives the basis for visualization of a polarization

wave (wave function) by near-field measurements. It is worth

noting that, in the near-field excitation, the transition proba-

bility is given not by the integral ofthe wave function but by

thelocalamplitude ofthe wave function, and hence the dipole

forbidden modes can be also excited by near-field irradia-

tion.85,86 Infact,in Figures 3 and 5, the dipoleforbidden modes

with nodes at the center ofthe rod (giving null integral values

of wave functions) are visualized, indicating that these modes

were optically excited.

The modelana

lys

is descr

ibed above can be extended to two-dimensional systems, and may be also applicable to polarized

excited states ofnanomaterials other than plasmons. That is,

visualization of excited-state wave functions may be possible

in general by the near-field spectroscopic imaging under

appropriate conditions. Indeed, we succeeded in observation

of two-dimensional plasmon modes in metal nanoparticles,16

and by another research group wave functions of excitons in

semiconductor quantum wells were visualized.87,88

5. Spectral Characteristics ofNear-Field

Transmission near the Plasmon Resonance

and Anomalous Transmission Phenomenon

Prior to near-field imaging of metal nanorods, we inves-tigated near-field transmission characteristics of spheric gold

nanoparticles, to understand the basics ofnear-field spectra of

nanoparticles. Near-field transmission characteristics of gold

nanospheres were reported previously,89 but we extended the

wavelength region of observation, and clarified an important

fundamental feature of near-field measurements ofnanoparti-

cles. In some cases thefeature provides uniqueinformation that

is available only with the near-field measurement, butin some

other cases it may act as a drawback to make theinterpretation

of experimental results difficult. Figure 7 shows a near-field

transmission spectrum ofa spherical gold nanoparticle (diam-

eter 100nm).13 The horizontal line atT= 0 means that the

transmissionintensity detected through the near-field probe and

i

pi

1 N

Figure 6. Dividing a nanorod into N equivalent volume

elements (Reproduced with permission from Ref. 66.

Copyright 2008, The Japan Society ofApplied Physics.).



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the sample (particle+ substrate) is identical to that on a bare

substrate with no particle. The plasmon resonance ofthe parti-

cle locates around 550 nm. The near-field transmission spec-

trum shows a differential type feature around this resonance

wavelength. In particular, it shows an interesting feature that

the transmission through the particle is even stronger than that

on a bare substrate at wavelengthslonger than 700 nm.

We analyzed this feature based on Mie scattering theory.

With Mie scattering theory, we can estimate, in addition to

the extinction of propagating light (that includes contribu-tions ofabsorption and scattering), the near-field scattering

intensityfor a spherical particle.2,56,57 The near-field scattering

is the conversion ofpropagatinglightirradiated on the particle

into the localized radiation field in the vicinity ofthe particle

(near-field radiation), and is of very different character from

the conventional sense ofscattering in far-field radiation. We

considered that Mies formulation of the near-field scattering

also gives a description ofthe reverse process, i.e., conversion

of the near-field radiation near the particle into the far-field

propagating light. The near-field transmission spectrum was

simulated based on this idea. The radiation from the apertured

probe is regarded as composed of the major component of

localized near-field and the minor component ofpropagating

field. Then the observed signal in the far-field should b e a

superposition of the extinction ofthe propagating wave (i.e.,

far-field extinction) by the particle and the scattering from the

near-field localized at the probe tip to the propagating wave.

The simulated near-field transmission spectrum based on this

idea is shown in Figure 7. The simulated result reproduces

the observed spectrum well, atleast qualitatively. The far-field

extinction component is ofcourse the same as the extinction

spectrum of the colloidal solution, which shows a resonance

peak around 550 nm, while the near-field scattering component

has a resonance enhancement peak at a longer wavelength

than the far-field extinction peak, based on Mie theory. The

superposition of these two contributions gives differential

shape spectrum. It is worth noting, in particular, that the

enhancement of the transmitted light is observed at longer

wavelength than the plasmon resonance due to the near-field

scattering.

This is expected to be observed not only for the spherical

gold nanoparticles but also plasmon resonances of metal

particles in general. In fact, we examined near-field trans-

mission characteristics ofcircular nanodisks, andfoundfurther

interesting anomalous transmission phenomenon.17 Circular

gold nanodisks (diameter 50-200 nm, thickness 35 nm) were

fabricated by electron beam lithography, and near-field trans-

mission spectra were measured using an aperture probe with

a diameter of 100nm. The distance from the aperture to the

disk surface was 20-30nm orless, and hence the aperture was

blocked almost completely by the disk when the disk diameter

was 100 nm orlarger. Under the conventionaloptics of far-field

propagating radiation, the transmission lightintensity must be

reduced significantly when the aperture is blocked by the disk.

In Figure 8, observed near-field spectra of the nanodisks are

shown. The 100% li

ne corresponds to the transmittance

forthe bare substrate without particles. In the region above this

line the transmitted light through the aperture is enhanced by

the existence of the nanodisk. What we observed in reality

was against the intuitive expectation from the conventional

optics mentioned above: the transmission was even enhanced

by blocking the whole area of the aperture with a nanodisk.

Further surprisingly, the maximum transmission increased as

the diameter of the blocking disk gotlarger. The transmission

spectra showed differential shape features in a similar way to

the sphericalparticle case: the transmission enhancements were

found at thelonger wavelength ofthe disk plasmon resonances.

Consequently, we consider that the feature originates again in

the superposition of far-field extinction and near-field scatter-ing. In fact, simulation of the transmission spectra based on

an idea similar to the spherical particle case reproduced the

experimentally observed spectra qualitatively well. I n t his

simulation Gans theory (a dipolar approximation theory) for

spheroids56,62,90 was used to calculate the far-field extinction

and the near-field scattering.

The observation of remarkably strong anomalous trans-

mission in nanodisks with large diameter indicates that noble

metal nanodisks yield highly efficient mutual conversion

between propagating fields (far-fields) and localized near-

fields, and this may provide valuable information in designing

enhanced optical fields. This example demonstrates that the

near-field transmission spectra give some unique information

Crosssection/arb.units

400 500 600 700 800 900 1000

-4

-2

0

2

4

-4

-2

0

2

4

Wavelength / nm

Transmission/arb.units

A

400 500 600 700 800 900 1000-0.4

-0.2

0.0

0.2

0.4

Wavelength / nm

B

T

Figure 7. (A) Near-field transmission spectrum of a gold

spheric nanoparticle (diameter 100 nm). (B) Simulation of

the near-field transmission spectrum based upon Mie

scattering theory. Long-dashed curve: far-field extinction

component; short-dashed curve: near-field scattering com-

ponent; solid curve: simulated curve considering both

components (Reproduced with permission from Ref. 13.

Copyright 2004, Elsevier.).



8/17

on the localized optical near-fields in the vicinities of nano-

structures. On the other hand, for this feature the near-field

transmission characteristics cannot be in general interpreted

simply in a similar way as the far-field cases, and we have

to keep in mind that special care must be taken for correct

understanding of the near-field experimental results.

6. Optical Field Enhancement in

Assembled Metal Nanoparticles

It has been known for a few decades that efficient surface-

enhanced Raman scatteringis often observedin metalsurfaces

with nanoscale roughness or in assembled metal nanoparti-

cles.20-24 At the end of the 1990s it was reported that surface-

enhanced Raman scattering with single molecule level sensi-

tivity is possible with gold and silver nanoparticle assem-

blies.22,23 Since thisfinding, many experimentaland theoretical

studies have been devoted to reveal this phenomenon and to

extendits relevant applications.24,91-97 Sphericalparticle dimers

are thoroughly studied theoretically as a prototypicalassembly

of metal nanoparticles.22,23,91-93,95,97 As mentioned before, the

spheric gold nanoparticles show plasmon resonances around

550nm if the diameter is 100nm or smaller, and they show

coupled plasmon modes when they form dimers. For polar-

ization parallelto the the dimer axis (thelinelinking the centers

of the two particles), low-energy dipole-allowed mode (we

call it parallel mode here) and high-energy dipole-forbidden

mode (antiparallel mode) are generated. When the parallel

mode plasmonis excited on a dimer with a narrow interparticle

gap, it is anticipated that a highly enhanced electric field is

induced in the gap. Based upon electromagnetic theoretical

calculations,91,95,97 the highest enhancement of electric field

at the gap relative to that ofthe incidentfieldin the free space

is predicted to be as high as a few thousands (i.e., 106-107 in

optical power), when two noble metal nanoparticles form a

dimer with a gap ofa few nanometers. When molecules enter

this gap area, those molecules are irradiated with an optical

field 106-107 times stronger than the normal situation. The

Raman scattered radiation from the molecule is also enhanced

by the plasmon resonance, and consequently 1010-1013 times

enhancement of the Raman intensity is possible as a whole.

This mechanism is believed to be the major origin of single-

molecularlevel sensitivity ofsurface-enhanced Raman scatter-

ing. The optical fields are also enhanced in the vicinities of

sing

le meta

l nanopart

ic

les, depend

ing on the structures andwavelengths, while the enhancements are generally not as high

as in the gaps ofdimers.

Thelocal enhancement ofoptical fields by metalnanostruc-

tures may provide potential applications not only in Raman

scattering but also in broad areas of spectroscopy, photo-

chemistry, photophysics, and so on. For this reason many

fundamental and applied studies have been devoted in the past

decade to the study ofdimers ofgold and silver spheric nano-

particles as well as various other assembled nanoparticles.91-97

To discuss possible applications of optical field confinement

in assembled nanoparticles, observation and analysis ofspatial

features ofthe confined fields give indispensable fundamental

information. Direct observation of optical field distributionwith a high-resolution optical method is straightforward and

effective to achieve that. While that is practically impossible

in conventional optical microscopy because of the diffraction

limit of light, it becomes feasible by the use of near-field

optical microscopy. We have indeed succeeded in the direct

observation of confined fields by applying near-field two-

photon excitation imaging to assemblies of gold nanoparti-

cles25-30 (The details of the near-field two-photon excitation

imaging is explained later in Section 9).

Figure 9 shows near-field two-photon excitation images

for gold nanosphere dimers using a femtosecond Ti:sapphire

laser (wavelength 780 nm) as an excitation source.25,26 In this

measurement the system is illuminated by the near-field radia-

50-200 nm

Figure 8. (Top) Experimental configuration of anomalous

transmission for a nanoaperture blocked by a gold nano-

disk. (Middle) Observed near-field transmission spectra of

gold nanodisks (thickness 35 nm) with various diameters.

(Bottom) Simulated near-field transmission spectra based

upon Gans theory (Reproduced with permission from

Ref. 17. Copyright 2011, American Chemical Society.).



9/17

tion through the aperture probe, and hence may actually not

coincident with the field distribution upon far-field irradi-

ation. However, the near-field images obtained in this way

can be regarded as essentially equivalent with the optical field

distribution upon far-field irradiation, based upon the consid-

eration ofelectromagnetic modes ofthe system mentionedlater

in Section 9. By comparing the two-photon excitation images

of the sample with the topographic image, we found that the

dimer generates a strong localized optical field in the gap site

between the particles when the incident polarizationis parallel

to the dimer axis. The confinedfieldis not observedforisolated

nanoparticles, indicating that the local field is not very much

enhanced at this excitation wavelength. Such features are very

consistent with the theoreticalpredictionsin the past. The near-

fie

ld two-photon exc

itat

ion

images obta

ined are thus attr

ibutedto the optical field distributions in the nanoparticle assemblies.

Similarly, we measured near-field Raman excitation images

for the dimer samples very lightly doped with a Raman active

compound (dye molecules), by detecting Raman scattered

signals from the dye molecules while the sample is excited by

light through the near-field aperture. The spatialdistribution of

the Raman activity and its polarization dependence basically

coincided with those for the two-photon excitation images.

This result strongly supports the idea that the optical fields

confined in the interparticle gaps make major contributions to

the surface-enhanced Raman scattering.

This experimentalmethod was also applied to the assemblies

ofmany spheric gold nanoparticles.

27-30

From the applicationviewpoint, high-sensitivity surface-enhanced Raman scattering

is ofgreat use, and hence many studies have been reported to

date on bulk preparation of noble-metal nanoparticles-fixed

substrates, to get highly efficient enhanced optical fields and

Raman scattering.98-102 We observed near-field two-photon

excitation images for island-like28 and band-like29 assemblies

composed of spheric gold nanoparticles (diameter 100nm)

at the excitation wavelength of780nm, to investigate optical

field distribution. A typical example is shown in Figure 10.

In this Figure, the SEMimage ofthe sample (in gray scale)is

superimposed on the near-field two-photon excitationimage (in

color scale). Although optical field enhancements are found to

a certain degree in the inner part ofthe assembly compared to

the bare substrate, the enhancement factors are not very high.

Much higher enhancements are ratherlocalizedin the rim parts

of the island-like assembly. In the studies to develop highly

efficient Raman substrates with noble metal nanoparticles,

many researchers have been trying to preparelarge-area particle

arrays as closely packed as possible.99 However, the present

results show that this strategy is not effective to get highest

possible enhancement. We performed a model analysis and

electromagnetic calculations to getinsightinto the origin ofthe

characteristic spatial features ofthe optical fields.30 The result

of model analysis suggested that the plasmons excited in the

inner parts ofassemblies propagate to the outer rim through the

interact

ions between the part

ic

le p

lasmons, and that p

lasmonsform coupled modes localized in the boundary area, which

may be the basic origin ofthe characteristic spatial features of

the optical fields. Localized modes are sometimes found at

boundaries and defects also for electronic and atomic motions

in crystals.103 The localized electromagnetic fields mentioned

above may be of a similar physical origin. We consider that

closely packed arrays of particles do not yield efficient

enhancements but fabrication of arrays with fluctuations and

defects is essential to obtain efficient enhanced optical fields

(the situation might be differentifthe wavelength ofexcitation

is different, however, according to our model analysis30).

We investigated also the optical field structures for assem-

blies ofcircular void apertures opened on thin metallicfilms onsubstrates, by applying near-field two-photon excitation.33 For

instance, wefoundforlinear arrays ofcircular voids (diameter

ca. 400nm) on a 25-nm thick gold film that optical fields are

confined in gaps between the voids. This indicates that not

only assemblies of particles but also arrays of voids on thin

metallic films can be utilized for spatial design of localized

optical fields.

7. Time-Resolved and Nonlinear Measurements

in Near-Field Microscopy

The original motivation of our near-field research project

was to develop a new microspectroscopic method by com-

bining near-field optical microscopy and laser spectroscopic

E E

A B C

Figure 9. Near-field observation ofenhanced fields in gold

nanosphere dimers. (A) Topography ofthe sample. (B) and

(C) Near-field two-photon excitationimages ofthe sample.

The excitation wavelength was 785 nm. The arrows indi-

cate polarization of the excitation fields for respective

panels. The white circlesindicate approximate positions of

the particles (Reproduced with permission from Ref. 66.

Copyright 2008, The Japan Society ofApplied Physics.).

Figure 10. Near-field two-photon excitation image of an

island-like gold nanosphere assembly. The scanning elec-

tron micrograph image of the sample (in gray scale) is

superimposed on the two-photon excitationimage (in color

scale). The line profile of two-photon excitation proba-

bility along the dashed line in the image is indicated in

the left panel (Reproduced with permission from Ref. 28.

Copyright 2008, American Chemical Society.).



10/17

techniques and to apply it to the studies of nanomaterials.

In particular I intended to perform ultrafast time-resolved and

nonlinear optical measurements in the near-field regime. We

thus set up a near-field imaging system by introducing a

mode-locked Ti:sapphire laser to a near-field optical micro-

scope.10-12,31,104 Figure 11 shows the experimentalarrangement

for time-resolved measurements. There are two possible modes

of measurements: the collection-mode arrangement where the

ultrashort pulsesilluminate the sample as propagating far-field

radiation and the system response is collected by the near-field

probe;105

and theillumination-mode setup where the pulses are

illuminated through the near-field aperture.106-110 We adopted

the illumination-mode setup because we preferred to specify

the location of photoexcitation by the position of the near-

field probe, although the system arrangement is more compli-

cated than that for the collection-mode measurement. As the

ultrashort optical pulses transmit through the fiber for quite a

long distance in the illumination-mode measurements with an

apertured fiber probe, the pulse width suffers from serious

broadening due to the dispersion effect of the fiber material.

The pulse broadening results in lowering of time resolution

in the t ime-resolved measurements, and lowering of signal

efficiency in the nonlinear optical measurements. To avoid

that, the pulse is passed through a dispersion compensation

device before coupling into the fiber to give a negative dis-

persion.10,106,107 The dispersion compensation deviceis adjust-

ed to yield the shortest possible pulse width at the exit of the

aperture probe tip. A grating pair deviceis adopted typically as

a dispersion compensation device. We achieved ca. 100 fs time

resolution in the near-field ultrafast imaging by introducing a

grating pair device that nearly recovers the originalpulse width

of the laser output (ca. 80 fs) after ca. 1-m optical fiber.10,11

The principle of the near-field ultrafast measurement is

basically the same as that of transient absorption correlation

method10,106,110 in the far-field experiment thatis often used in

ultrafast spectroscopy, except that the dispersion compensation

isindispensable.10,106,110 The opticalbeamfrom the pulsedlaser

source was divided into pump and probe beams with approx-

imately the same intensity, and the two beams passed through

respective optical delay lines to give a variable delay time

between the pump and probe pulses. The two beams were

collinearly combined againinto one beam, which was coupled

into the fiber after the dispersion compensation device men-

tioned above. Here, the pump and probe pu

lses have essent

ia

llythe same characteristics (equal pulse correlation method),111

and there is n o discrimination between them except for the

differencein the pulse timing. We can also perform two-color

experiments if necessary by converting the probe pulse wave-

length using a photonic crystal fiber or other nonlinear media,

but in this case the time resolution usually becomes worse

than the equal pulse correlation method.12,104 The pump and

probe beams were modulated by a mechanicalchopper, and the

light intensity after transmission through the aperture probe

and the sample was lock-in detected to extract the signal of

pump-induced transmission change for the probe beam. With

this setup, near-field measurements with transient absorption

changes were feasible.We have performed also near-field nonlinear optical imaging

measurements with a similar setup. For the two-photon excita-

tion probabilityimaging14,16,25-29,32,33,112,113 that has been used

as the major technique to visualize enhanced optical fields, no

pump-probe delayis necessary, and thus the opticaldelaylines

were removedfrom the setup. The signaldetected was the two-

photon-inducedluminescencefrom gold nanostructures (wave-

length around 500 to 700 nm). We can also perform near-field

nonlinear optical imaging by detecting second-harmonic gen-

eration signalwith a one-color experimentalsetup, that ofsum-

or difference-frequency generation or coherent Raman scatter-

ing signalwith a two-color experimentalsetup, and soforth, by

minor modifications of the apparatus.

Ti:sapphire laser

grating

chopper

optical delay line

detector

sample

near-field probe

optical fiber

coupler

microscope objective

grating pair

device

(A)

(B)

Figure 11. Schematics of ultrafast near-field measurement

setups. (A) A setup for measurements with ca. 100-fs time

resolution. (B) A setup for measurements with


11/17

Figure 12 demonstrates the time-resolved measurement of

energy relaxation in a gold nanorod after photoexcitation, as

a typical example of near-field ultrafast time-resolved imag-

ing.11 Since the plasmon has already dissipated in a very fast

dephasing time (less than ca. 20 fs for gold nanorods), the

dynamics observed here are electron-electron scattering,

electron-lattice scattering, and cooling processes, whichfollow

after the dissipation of the plasmon.114-120 A characteristic

spatial feature is observed at several hundred femtoseconds

after photoexcitation, where the sign ofthe transient absorption

in the center ofthe nanorod is oppositefrom that at both sides

of

the nanorod. We analyzed the or

ig

in o

f th

is w

ith the a

idofelectromagnetic theoretical simulations, and found that this

reflects the transient change of the plasmon modes (plasmon

waves) due to the electronic temperature rise induced by the

pump pulse.121 This finding suggests potential for develop-

ment of a new way to control the plasmon waves in metal

nanostructures.

As mentioned above, we can observe dynamic behavior

of the metal nanostructured materials after the dissipation of

plasmon resonance, when the time resolution of the measure-

ment is of the order of 100 fs. However, this time resolution

is still not sufficient to investigate the essential function of

plasmons from a dynamic point of view, and time resolution

even higher than the plasmon dephasing is necessary. For thispurpose, near-field ultrafast measurements with time resolu-

tion higher than 20 fs is required, since the dephasing times of

plasmons in gold nanostructures are in the range between a

few femtoseconds to ca. 20 fs.122 Very recently scattering type

ultrafast near-field measurements with


12/17

field nonlinear optical measurement setup described before,

with the near-field illumination mode. The excitation wave-

length was 785 nm. The spectral features depend on the geo-

metries ofthe gold nanostructures measured, butin most cases

two peaks (or shouldersin some cases) appear around 550 and

650 nm regardless of the geometries.14,32,112 According to the

band theory calculation of gold, these emission wavelengths

correspond wellwith that ofrecombination ofan electronin the

sp band and a hole

in the d band nearL (

for the peak around550nm) symmetry points of the first Brillouin zone and that

near X point (650 nm), respectively.127,135 The polarization

measurement of the emission show that the emission around

650nm (we call it hereafter X emission) is strongly polarized

along the rod axis, while that around 550 nm (L emission) is

only weakly polarized along the axis.32,112 These observed

polarization characteristics are consistent with the assignment

ofthe emission to the electron-hole recombination nearXand

L points. Based on the polarization characteristics of the

luminescence and analysis of the X- and L-emission intensity

ratios for various nanorods, we presently believe that the

luminescence is attributable to the sp-d electron-hole recom-

bination emission resonantly enhanced with the plasmons.

112

Each nanoparticle shows its own characteristic X and L

intensity ratio, probably because the plasmon resonance

condition strongly depends on the geometry of the particle.

Dulkeith et al. reported, on the other hand, that the lumines-

cence from gold is attributed to radiative decay of a plasmon

that is excited by the electron-hole recombination, from the

analysis ofdiameter dependence ofthe emission quantum yield

for spherical gold nanoparticles.128 This interpretation of the

emission mechanismlooks similar to that mentioned above, but

is different in its basic physical picture. Further investigation

is necessary to clarify the luminescence mechanism of gold

nanoparticles. In either case, appearance of strong plasmon

resonances in the wavelength region longer than ca. 600 nm

(corresponding to the X emission region) may be the major

factorfor the strong luminescence from gold nanostructures.

Thisluminescent property suggests a potentialutility ofgold

nanoparticles as probe materials for two-photon imaging, and

some researchers indeed attempted application of gold nano-

particles to bioscience.132,133 Our group found that triangular

plate nanoparticles of gold, among various single gold nano-

particles, show particularly strong two-photon-induced lumi-

nescence,16 and proposed that this material may be applied to

bioimaging.134

Silver nanoparticles also show two-photon-induced lumi-

nescence in the visible to near-infrared region when excited

with ultrashort near-infrared pulses. However, each particle

shows a different luminescence spectrum with different peak

wavelengths, un

like go

ld nanopart

ic

les.

113

Interband transi-tions are not expected in this wavelength regionfrom the band

structure ofsilver. Meanwhile, the surfaces ofsilver nanostruc-

tures are easily oxidized compared with that ofgold, and thus

the surface is thought to be covered with silver oxide under

ambient conditions, which gives rise to photoluminescence.

It is therefore likely that the two-photon excitation energy

imposed on silveris transferred to silver oxide on the surface,

and the luminescence from the silver oxide is resonantly

enhanced by the plasmon modes ofthe nanoparticle. Thisis the

most probable origin of the two-photon-inducedluminescence

from silver nanoparticles.113

For both gold and silver nanostructures the two-photon-

induced photoluminescence shows sufficiently strong intensitythat we can record near-field two-photon excitation images by

detecting the luminescence as a signal, as mentioned in the

previous sections. Figure 14 shows two typical near-field two-

photon excitation images ofgold nanorods.14 The sample was

irradiated by femtosecond near-infrared (wavelength 780nm)

pulses through an apertured optical fiber probe, and the inten-

sity of the emission from gold was detected while raster-

scanning the sample, to construct theimage. The dimensions of

the respective rods were 40 nm 230nml and 35nm 440

nml. These two images look very different. Obviously the image

in Figure 14B corresponds to the square modulus ofthe wave

function of the plasmon mode resonant with the incident

light, as we discussedin Section 4. In contrast, in Figure 14A

400 500 600 700

0

100

200

300

400

500

PLIntensity/arb.units

Wavelength / nm

(B)

(A)

(A)

(B)

Figure 13. Typicaltwo-photon-inducedluminescence spec-

tra for a gold nanorod. Excitation wavelength was 780 nm.

Black curve (A): polarization ofthe luminescence perpen-

dicular to the rod axis; red curve (B): polarization parallel

to the axis of the rod (Reproduced with permission from

Ref. 112. Copyright 2009, American Chemical Society.).

A B

Figure 14. Near-field two-photon excitationimages ofgold

nanorods (A: diameter 40 nm,length 230 nm; B: diameter

35nm, length 440 nm). Incident wavelength was 780 nm,

and the polarizations of excitation were approximately

parallel to the long axes of the rods. Scale bars: 200nm

(Reproduced with permission from Ref. 14. Copyright

2004, American Chemical Society.).



13/17

the excitation probability is confined near both ends of the

rod. Such an observed image corresponds well with the field

distribution associated with the lightning rod effect, where the

electricfields are confined near sharpened edges and apexes of

metal structures.93,136-139 We introduced in Section 4 an inter-

pretation ofthe plasmon modes as polarization standing waves.

Since the longitudinal polarization wave is accompanied by

an electric field along the rod axis, the wave observed in

Figure 14B corresponds to the optical field distribution arising

from the stand

ing wave o

fthe p

lasmon mode. In F

igure 14B,the plasmon mode was resonant with the incident light and

thus the image was reflected by the optical field distribution

arisingfrom the mode. Theimagein Figure 14A,in contrast,is

interpreted as the optical field due to the lightning rod effect

being visualized because theincidentlightis not resonant with

any standing wave modes ofplasmons. Thelightning rod effect

occurs not only under oscillating electric field but also under

direct current (DC) field, and shows only weak frequency

dependence. This effectis hence observable atfrequencies not

in resonance with any standing wave plasmon mode.

9. Correlation between the Near-Field Excitation Image

and the Localized Field Distribution

As illustrated in the previous section, the near-field two-

photon excitation imaging method provides a valuable tool to

visualize the localized optical fields in nanostructures and we

utilized this method in our studies of assembled nanoparticles

as describedin Section 6. To rationalize this way ofobserving

field distributions, it is necessary to ensure that the near-field

excitation probability corresponds to the field intensity upon

far-field irradiation of light to the system. We discuss here

qualitatively the correlation between the near-field excitation

images and the optical field distributions.

What is obtained directly by the two-photon excitation

image is basically the distribution ofexcitation probability and

not that of field enhancement in the system. The optical field

distributionis a map ofthe fieldintensity when the whole area

of the system is irradiated by light. These two (the excitation

probability distribution and the field distribution) are related

to different physics and are not necessarily equivalent to each

other. However, we believe that a kind of reciprocity relation

holds roughly among these two as mentionedin thefollowing,

and in that case the excitation probability image gives the

optical field distribution.140

As a model system to discuss the physical picture, we con-

sider osc

illat

ion o

fa str

ing. An osc

illat

ing

loca

l fie

ld

is app

liedto the string, which excites a wave on the string (Figure 15). If

the excitationis at thefundamentaloscillationfrequency ofthe

string 1, the string may be excited at any position on the stringbut particularly efficiently at the center. On the other hand, if

the excitationis at thefirst overtonefrequency 2, the overtonemode cannot be excited at the center where the oscillation

amplitudeis null. Instead, strong excitation is possible at both

sides ofthe center ofthe string. Based on this consideration, we

may expect that a plot of oscillation amplitude against the

excitation position gives the spatial structure ofthe oscillating

wave on the string (i.e., mode) that is in resonance with the

excitation frequency. In this model, the local excitation source

corresponds to thelocalizedirradiation of light with an apertureprobe in near-field optical microscopy. The oscillating string

corresponds to the electromagnetic field of the system. This

qualitatively explains the physical picture that the observation

ofexcitation probability distribution gives the spatial structure

of the electromagnetic modes of the system, and as a con-

sequencelocalized optical fields are visualized. In other words,

the near-field excitation probability gives the local density of

electromagnetic modes, and a strong electromagneticfield exist

ifthe density ofmodes is high at the position.

10. Summary

In this account, I described near-field microscopic studies

on noble metal nanoparticles and related topics. On noble

VU

filterstrong speaker

resonantly excited

moderate

VU

weak

VU

VU

strong

near-field (localized) excitation far-field excitation

fundamental overtone

fundamental

frequency

Figure 15. Excitation ofmechanical oscillation waves in strings as analogies to near-field and far-field excitation ofopticalmodes

ofmaterials. See the textfor the details.



14/17

metal nanoparticles and assemblies, we showed that plasmon

waves and enhanced optical field structures can be visualized

by near-field optical microscopy. Development of near-field

ultrafast and nonlinear optical measurement methods, which

played essential roles in visualization of plasmon fields,

was also described. Through these studies, we established

a valuable methodology to investigate optical properties of

metal nanostructures, and succeeded in obtaining informa-

tion that is available only by optical measurements with high

spatial resolution beyond the diffraction limit of light. I hope

that the methodology we developed and the information

we got provide foundations for basic and applied studies of

plasmon resonances in the future. We may expect not only to

deepen understanding of fundamental physical and chemical

properties of plasmons in metal nanostructures, but also to

contribute to broad research areas, whichinclude,for example,

ultrasensitive analytical methods on the basis of enhanced

spectroscopy, luminescence properties of metal nanoparticles

and application of that to bioimaging materials, application

of enhanced optical fields to energy and chemical conversion

systems and photolithography, design of novel nanooptical

devices based upon the characteristics ofplasmon modes, and

so forth.

We are now planning (or partly executing) the following

projects as the next step of this study. One is to perform

near-field measurements of enhanced field distributions in a

wide wavelength range for metal nanostructures (nanoparticle

assemblies in particular), to verify the validity of the inter-

particle coupling model we proposed in Section 6 . This may

provide valuable designing guidelines of enhanced optical

fields. Next is application of the near-field ultrafast method

to various systems and development ofspace-time manipula-

tion methodo

logy

for p

lasmon

fie

lds. By the pu

lse shap

ingtechnique for the excitationlight combined with proper design

of metal nanostructures, potential utilities ofdynamic charac-

teristics ofmetalnanostructures may be increased. Another one

is to pursue potentials of polarization dependent near-field

measurements, in particular circular dichroism measurements.

This methodology provides novel and valuable techniques for

the study ofchirality ofnanomaterials, and may develop a new

research area in photo- and materials sciences.

For further progress in application of near-field optical

methods, development in theoretical and calculation methods

tointerpret the observedimagesis essential, in addition to that

in experimentalmethods. Thefar-field optics theory has a suffi-

cient accuracy to interpret images obtained by conventionalopticalmicroscopes,58 while the near-field theoryis much more

complex and not so accurate as the far-field theory at least

at present.4,71-76,141-143 One of the reasons is that the probe is

almost in contact with the sample in the near-field measure-

ments and hence the perturbation ofthe probe to the sample is

sometimes not negligible. Another difficulty in the theoretical

treatment of near-field imaging is that the wide range of the

spatial scale (nano to macro) must be involved in the system,

since the material system treated is ofsubwavelength spatial

scale while the photodetection is usually done in the far-field

macroscopic regime. Ifthese difficulties can be overcome and

the theoretical framework is established to interpret near-field

images of various samples from a unified viewpoint, it will

be highly useful for the studies of nanooptical properties of

materials.

In recent years, various other experimentalmethods to study

excited-states of materials in spatial resolutions beyond the

diffraction limit of light have been highly advanced.144-146

Typical examples are electron/X-ray diffraction147-150 and

electron microscopy144,145,151 combined with ultrashort pulsed

light sources. Nanooptical methods are expected to provide

valuable information complementary to that by the advanced

methods mentioned above, andfurther developments are highly

desired to advance nano-photosciences.

I am very much grateful to collaborators who contributed

to this research project, in particular Prof. K. Imura (now

at Waseda University), who was the first member of my

laboratory at the Institute for Molecular Science and made

major contributions to most ofthis research project, and many

other members (present and former) of my laboratory as

well: Dr. T. Nagahara (Kyoto Institute of Technology), Prof.

J. K. Lim (Chosun University), Dr. N. N. Horimoto (Tohoku

University), Dr. Hui Jun Wu, Prof. Y. Jiang (South China

Normal University), Dr. T. Narushima, Dr. Y. Harada (The

University of Tokyo), Dr. S. I. Kim (Samsung), and Dr. Y.

Nishiyama. I also thank many collaborators from other insti-

tutions: Dr. M. K. Hossain (King Fahd University ofPetroleum

and Minerals), Dr. T. Shimada (Hirosaki University), Prof.

M. Kitajima (National Defense Academy), Prof. K. Ueno

(Hokkaido University), Prof. H . Misawa (Hokkaido Univer-

sity), Ms. Y. C. Kim (Seoul National University), Mr. S. Kim

(Seoul National University), Minwoo Lee (Seoul National

University), Prof. D. H. Jeong (Seoul National University),

Prof. K. Sawada (Shinshu University), Prof. H. Nakamura

(Nationa

l Inst

itute

for Fus

ion Sc

ience), Pro

f. T. Sa

ik

i (Ke

ioUniversity), Dr. K. Matsui (National Institute for Physio-

logical Sciences), Prof. R. Shigemoto (National Institute for

Physiological Sciences), and Prof. S . K im (KAIST). The

author is also indebted to the Equipment Development Center

and the Laser Research Center for Molecular Science of

Institute for Molecular Science. This research was supported

by the Grants from Ministry of Education, Culture, Sports,

Science and Technology (Nos. 17034062 and 19049015), from

Japan Society for the Promotion of Science (Nos. 16350015,

17655011, 18205004, 21655008, and 22225002), and from

Kurata Science Foundation. The project was also supported in

part by the Extreme Photonics Project and by the Consortium

for Photon Science and Technology.

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