Visualizing the Optical Field Structures in Metal Nanostructures - The

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Visualizing the Optical Field Structures in Metal Nanostructures Hiromi Okamoto*,† and Kohei Imura‡ †

Institute for Molecular Science and The Graduate University for Advanced Studies, 38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan ‡ Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Okubo, Shinjuku, Tokyo 169-8555, Japan ABSTRACT: Noble metal nanostructures yield confined optical fields on a nanometer scale due to plasmon resonances, and they have potentially novel applications in spectroscopic analysis, photochemical reactions, optical devices, bioimaging, and so forth. To design nanoscale confined optical fields for use in specific applications, visualization of the fields is needed to provide essential information. We adopted nearfield optical microscopy to visualize the optical fields in the present study. Examples of the direct visualization of optical fields in metal nanostructures are presented together with the analysis of the unique spectroscopic characteristics based on near-field imaging. For single nanoparticles such as gold nanorods, the plasmon standing wave functions and/or enhanced local fields near the particles were visualized depending on the particle shapes and sizes. In the assembled nanoparticles, the enhanced optical fields at the gap sites between the particles were visualized, thus elucidating the mechanism of surface-enhanced Raman scattering. The characteristic optical fields for various other metal nanostructures are also discussed. enhanced optical fields. The collective oscillation of the conduction electrons near the surface of the metal (surface plasmon resonance) is the essential origin of the localized optical fields.9,12−15 Spherical gold nanoparticles are the most wellknown example. Gold nanospheres with diameters of approximately 100 nm or smaller display strong extinction peaks in the wavelength region of 520−600 nm in colloidal solutions with spectral bands that originate from the surface plasmon resonances. A particle that is irradiated with photons that are resonant with the surface plasmon resonance yields a localized and enhanced optical field in its proximity. The radius of the particle determines the approximate size of the space in which the optical field is confined. The molecules in the proximity of the particle are then subjected to a stronger optical field than those in the free space. Although a spherical metal nanoparticle can create a confined optical field, the enhancement of the field is not large, and the spatial structure of the confined field is not flexible. The enhancement and the spatial structure of the optical field depend strongly on the metal species, the geometry of the material, and the wavelength of the radiation.12,14,15 By utilizing nanoparticles of various shapes and assemblies, we can construct confined optical fields that are greatly enhanced and highly flexible in their local structure. The ability to custom design the enhancements and local structures of the fields will permit wide application of confined

he confinement of optical fields in small spaces creates various novel possibilities for applications in physical and chemical research fields. The confined fields provide highly sensitive methods of spectrochemical analysis, such as surfaceenhanced Raman scattering (SERS)1−6 or surface-enhanced infrared absorption (SEIRA).3,7−9 The optical field confinement may also provide novel photochemical reaction fields. By squeezing strong optical fields into small spaces, unconventional reactions, such as multiphoton-induced reactions with low incident photon fluxes10 or reactions initiated by dipolar forbidden transitions,11 may occur. These unconventional reactions have the potential for application in new schemes of photolithography and photofabrication. The confined optical fields also play essential roles in the development of devices for nano-optical waveguides and information processing as well as the development of materials that have novel characteristics, such as meta-materials.12

T

To design nanoscale confined optical fields for use in specific applications, visualization of the fields is needed to provide essential information. In general, a material having boundaries with nanometerorder radii of curvature yields localized optical fields when the material is irradiated by light. Noble metal (gold and silver) nanostructures are particularly effective at creating localized and © 2013 American Chemical Society

Received: May 17, 2013 Accepted: June 19, 2013 Published: June 19, 2013 2230

dx.doi.org/10.1021/jz401023d | J. Phys. Chem. Lett. 2013, 4, 2230−2241

The Journal of Physical Chemistry Letters

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have been published, based on scattering-type near-field microscopy with a dielectric probe tip27 or even with metallic probe tips by the use of electric fields on the circumference of the tip22,31,32 or with a probe with a bent tip end.33) In recent years, there have been a number of reports for visualization of localized plasmon modes based on electron microscopy with cathodoluminescence detection34−37 or with electron energy loss spectroscopy (EELS).38−45 In these methods, spatial resolution the same as that of electron microscopy can be achieved in principle. Investigations of metal nanorods,34,36,40−42 triangular38 and circular44 nanoplates, and other metal nanostructured samples35,37,39,43,45 were reported. Various super-resolution far-field optical microscopic methods have been also developed. They have achieved high spatial resolution beyond the diffraction limit of light on the basis of nonlinear optical responses (such as stimulated emission depletion13,46), single fluorescent molecule detection (such as stochastic optical reconstruction microscopy: STORM47), and other principles. Application of the far-field super-resolution method to observation of localized fields on metal nanostructures has been reported very recently.48,49 Since some years ago, our group has reported observation of optical field structures on noble metal nanostructures by the use of aperture-type near-field optical measurements. One of the advantages of the method is facility of polarizationdependent measurements because the aperture probe yields in-plane polarized signals, as mentioned previously. We conducted research on the metal nanostructures based mainly on two methods of aperture-type near-field optical microscopy.50−57 One method is the transmission-type measurement performed through the aperture probe to obtain the nearfield extinction images of the samples. The other method is the near-field two-photon excitation probability imaging, in which femtosecond optical pulses are irradiated onto the sample through the aperture probe to detect the two-photon-induced photoluminescence from the gold or silver nanostructures as the signal. Both methods yield information that corresponds to the spatial structure of the electromagnetic mode density near the nanostructure, which can be viewed as measurements of an identical physical process observed from two different standpoints. In a transmission-type measurement, the phase of the scattered radiation relative to that of the incident light, in addition to its intensity, affects the detected signal, whereas in the two-photon excitation measurement, the electromagnetic mode density near the nanostructure, which is the intensity of the enhanced optical field, is obtained. Figure 1 summarizes the noble metal nanostructures that we have investigated using these methods. Concerning the polarization of the detected optical field, information on the in-plane component of the electric field (i.e., parallel to the sample substrate) is typically obtained via measurement with an aperture-type probe. It is possible to select the polarization direction of excitation and/or detection in the plane. By contrast, out-of-plane polarization components are detected in the scattering-type measurements with few exceptions that enable the detection of in-plane components.22,31−33 As typical measurement examples, near-field transmission and two-photon excitation images of gold nanorods29,30,58−63 are shown in Figure 2A and B, respectively. The detected polarization (for the transmission measurement) and the excitation polarization (for the two-photon measurement) are both parallel to the long axis of the rod. In the transmission image, the oscillating feature of the detected intensity is observed along the

By utilizing nanoparticles of various shapes and assemblies, we can construct confined optical fields that are greatly enhanced and highly flexible in their local structure. optical fields. To achieve the desired field design, the experimental observation of the confined optical field structures in the metal nanostructures must be highly informative to understand the basic features of the designed nanomaterials. The most direct technique to visualize optical field structures is to observe the system using optical microscopic methods. However, the spatial resolution of conventional optical microscopy on the basis of diffraction optics is restricted to the wavelength of the light for observation due to the diffraction limit.13,16 By contrast, the spatial scale of the confined optical fields with the metal nanostructures is intrinsically smaller (on the order of a few nanometers to a few hundred nanometers) than the wavelength of the light in a vacuum. Visualization of an optical field structure of this small size requires an optical imaging method beyond the limits of conventional optical microscopy. To visualize the confined optical fields, we adopted scanning near-field optical microscopy.13,17,18 The methods of near-field optical microscopy are categorized into two types, aperturetype and scattering-type. In the former type, a small aperture on a thin metallic film (aperture near-field probe) is utilized to gain the spatial resolution.13,17,18 The metallic film of the probe is irradiated with light from one side. The localized optical field (near-field) is generated close to the aperture on the other side of the film and does not propagate through space. The probe aperture approaches the sample surface, which is then excited by the localized field. An optical image of the sample can be obtained by raster-scanning the lateral sample position while detecting scattered, transmitted, or emitted light from the sample. The near-field probe can be also used to detect the optical field that is localized near the sample surface while nonlocally shining the sample with light from the outside of the probe. The spatial resolution of the observed images is determined by the aperture diameter of the probe. In scattering-type near-field microscopy, the radiation that is localized near the sample surface is scattered by approaching the sample with a sharpened probe tip made of a metallic or dielectric material, and the optical image is recorded by raster-scanning the sample while detecting the scattered radiation.9,13 The approximate spatial resolution is given by the radius of curvature of the tip. These methods allow the observation of the localized optical fields in the peripheries of the metal nanostructures. Both methods have been utilized to observe the optical fields in the peripheries of metal nanostructures. From the viewpoint of spatial resolution, it is easier to fabricate a sharpened tip with a small radius of curvature than to prepare an aperture with a small diameter, and mainly for this reason, the scattering-type has been used rather more frequently recently to visualize the optical fields.19−28 As for the electric near-field component observed, the aperture-type measurements principally yield the field component in the sample plane29,30 under a rough approximation, while in the scatteringtype, observation of the field normal to the sample plane is easier. (A few reports of in-plane components of electric fields 2231



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Figure 1. Schematics of noble metal nanostructures that we have investigated and the typical optical field distributions for the respective structures observed. The field distributions displayed are observed when the polarizations of the electric fields are set as the arrows indicate. (A) Nanorod (standing wave), (B) nanorod (lightning rod effect), (C) circular plate, (D) triangular plate, (E) dimer of spherical particles, (F) monolayered assembly of multiple spherical particles, (G) linear array of spherical particles, (H) nanorod joined with spherical particles, (I) circular voids and their chain structures opened on thin metallic film, (J) elongated rectangular void opened on thin metallic film.

Figure 2. Near-field optical images of gold nanorods. (A) Transmission images of a nanorod (510 nm l × 20 nm ϕ) at various wavelengths. (B) Two-photon excitation image of a nanorod (540 nm l × 20 nm ϕ) at 785 nm. (C) Two-photon excitation image of a nanorod (565 nm l × 21 nm ϕ) at 785 nm. Scale bars: 100 nm. (Reproduced with permission from refs 54 and 57. Copyright 2008 and 2013, The Chemical Society of Japan.)

mode in which all of the electrons in the rod move in the same direction; this mode does not have a node. Among longitudinal modes, conversely, there are many higher-order modes that have nodes70−72 (i.e., the direction of the collective electronic oscillation depends on the position on the rod). The higher-order modes have larger wavenumbers and higher oscillation frequencies. These modes yield large electric fields along the rod axis at the antinodes of the wave function (in which the electronic oscillation amplitude is large), resulting in a large extinction or a high two-photon excitation probability in the near-field images. The observed images and their wavelength dependences are thus reasonably explained. Therefore, the plasmonic standing wave function can be optically visualized by near-field microscopy when the observation wavelength is resonant with the longitudinal plasmon mode of the nanorod. Because of the dispersion relation of plasmon resonances, the plasmon wavelength is always shorter than that of the light that is resonant with the plasmon.12,70−72 We wish to stress here that optical visualization of the plasmonic wave function is possible only with the near-field measurements beyond the diffraction limit of the light. While the benefit of near-field microscopy is the high spatial resolution, one drawback of the method is that close contact of a probe to the sample is a prerequisite; we cannot exclude the possibility of serious effects of the probe on the plasmon resonances. It may be necessary to theoretically investigate the probe effects in detail and systematically. However, comparisons between the near-field experimental data and the results of electromagnetic density-of-states calculations show that the near-field images and spectra represent well the plasmon-mode structures and spectral characteristics, respectively,30,61,73 at least qualitatively. In a recent report on aperture-type near-field measurements and theoretical analysis for gold nanorod, it was proposed that the observed images reflect the lateral magnetic fields of the plasmon resonances.74 Further detailed and systematic studies on this interpretation are desired, including those for other nanostructures than single nanorods. Scattering-type and aperture-type near-field measurements as well as EELS- and cathodoluminescence-detected electron microscopy of finite metal nanorods were reported later by several other groups.24,25,34,36,40−42,74 Although these methods

Our group has reported observation of optical field structures on noble metal nanostructures by the use of aperture-type near-field optical measurements. One of the advantages of the method is facility of polarization-dependent measurements because the aperture probe yields in-plane polarized signals. rod axis. Similarly, the excitation probability oscillates along the rod axis in the two-photon excitation image. The images do not reflect simply the shapes of the rods. This result indicates that the longitudinal polarization component of the optical field (having the electric field parallel to the rod axis) is not uniform. Strong and weak fields appear alternatively along the rod axis. The separation between the bright and dark parts becomes wider as the wavelength of observation (i.e., the detection wavelength of the transmission measurement or the excitation wavelength of the two-photon excitation measurement) becomes longer. This observation implies that the images obtained correspond to the wave functions of the excited states of the rods. We found that the images were successfully interpreted as square moduli of standing wave functions of plasmons based on analysis using electromagnetic calculations.64−69 These results are explained pictorially in Figure 3. The plasmon modes that interact with the electric field parallel to the long axis of the rod are the longitudinal plasmons in which the collective electronic oscillations occur along the rod axis. The plasmonic wave function is given as the amplitude of the collective electronic oscillation as a function of the position. The fundamental longitudinal mode is a dipolar 2232



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Figure 3. Longitudinal plasmon modes of a metal nanorod and the expected optical images observed at wavelengths resonant with the respective modes.

enhancement effect (sometimes called the “lightning rod effect,” the confinement of a strong electric field at the ridges and apexes of metal structures).75−79 Near-field images of gold nanorods sometimes show similar enhanced field structures, which depend on the relationship between the resonance wavelengths of the longitudinal plasmon modes of the rod and the observation wavelength or on the microscopic structures of the ends of the rod. The same experimental methodology can be applied to the nanoparticles of two-dimensional structures. In the case of a triangular nanoplate, the two-photon excitation image presents the enhanced optical fields at a slightly inner position relative to the apex.73 If the lightning rod effect is at work, then strong enhancements immediately outside of the apex are expected,75,80 whereas the present result suggests that this expectation is not valid when the incident wavelength is resonant with the plasmon modes of the particle. This tendency was reproduced via electromagnetic theoretical simulation, and similar results were demonstrated in later years using electron energy-loss microscopy for triangular nanoplates.38 The field distribution depends, of course, on the resonant mode being excited, which is exemplified by other experimental results22,32 where enhanced fields appeared near the apexes of the triangles. As for the signal intensities, examination of the two-photon excitation signal intensities reveals relatively strong signals for nanoplates compared with those of nanorods and nanospheres as a general tendency. This result suggests that nanoplates have a higher efficiency for near-field enhancement in the periphery of the particles than other nanoparticles.73,81 The near-field measurements of disk-shaped gold nanoparticles revealed a phenomenon that presumably is closely related to the near-field enhancement characteristics described above.82 Figure 5B shows the near-field transmission spectra of the gold nanodisks (thickness, 35 nm; diameters, 50−200 nm) that were measured using a near-field probe with an aperture diameter of ∼100 nm. In this figure, the I/I0 = 1 (100%) level corresponds to the normalized transmission intensity at a probe position that is distant from any of the gold structures (i.e., on a bare substrate). Disks with diameters exceeding 100 nm fully cover the aperture of the probe. In addition, the thickness of the disk is a few fold larger than the penetration depth of the material (gold) in this wavelength region. Consequently, the transmission intensity through the disk should be much lower than that through a bare substrate based on conventional macroscopic optics. However, we found the enhancement of the transmission intensity (transmittance higher than 100%) in

yielded slightly different plasmon-mode images depending upon the respective image-forming principles, the images were found basically to reflect the electromagnetic local density of states. Figure 4 shows, as a representative example, plasmon

Figure 4. EELS-detected electron microscopy images of a silver nanorod (diameter ∼26 nm, length ∼450 nm). (Reproduced with permission from ref 40. Copyright 2011, American Chemical Society.)

maps for a silver nanorod observed with EELS-detected electron microscopy.40 In a similar manner as in Figure 2A, the spatial oscillation period becomes short with increasing energy loss, which reflects the dispersion relation of the plasmon resonances. Spatial resolution close to 10 nm was achieved by the scatteringtype near-field microscopy, and the electron microscopic methods enabled even higher resolution. Visualization of higher wave vector modes may be achieved in the future, based on these emerging technologies. Under some circumstances, the rod gives a near-field twophoton excitation image with a high excitation probability near both ends of the rod, as shown in Figure 2C.54,58 This image appears to reflects the spatial distribution of the confined optical fields at both ends of the rod arising from the quasi-static surface 2233



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aperture is thus efficiently scattered to the far-field, which causes an apparent enhancement of the transmitted light.83 This result, as well as the previously mentioned two-photon excitation signal enhancement in the triangular nanoplate, indicates that the plate nanoparticles achieve high performance in the conversion between propagating waves and near-fields, therefore yielding highly enhanced local optical fields near the particles.

Plate nanoparticles achieve high performance in the conversion between propagating waves and near-fields, therefore yielding highly enhanced local optical fields near the particles. As described previously, the spatial features of the locally enhanced optical fields near the single metal nanoparticles are directly visualized by near-field optical imaging. Assemblies of nanoparticles are considered to be more efficient than single nanoparticles in producing locally enhanced optical fields.75,80,84−88 The enhanced optical fields on the assembled nanoparticles are believed to be essential for SERS,4−6 and many studies have investigated the mechanism of the enhancement. The dimers of spherical particles are prototypes of metal nanoparticle assemblies that yield enhanced optical fields. Extensive theoretical studies have been performed on these dimers. A highly enhanced optical field (up to 103-fold in the electric field amplitude, depending on the wavelength and the polarization) is generated in the interstitial site when the dimer is irradiated by light. Because the size of the nanoparticle assembly is smaller than the diffraction limit of the light that is resonant with the plasmons, optical measurement with a resolution higher than that of conventional optical microscopy is a prerequisite to experimentally investigate the structures of the confined optical field. We visualized the localized optical field by applying near-field two-photon excitation probability imaging to dimers of gold nanospheres.89,90 As shown in Figure 6, a strong optical field is observed in the gap between the two particles when the incident polarization is Figure 5. Transmission spectra of the gold nanoplates. The thickness and diameters of the plates are 35 and 50−200 nm, respectively (the diameters are indicated near the traces). (A) Conventional far-field spectra. (B) Spectra observed through a near-field aperture with a diameter of ∼100 nm. The optical arrangement of the measurement is shown in the inset. (C) Simulated spectra of the near-field spectra. (Reproduced with permission from ref 82. Copyright 2011, American Chemical Society.) Figure 6. Near-field observation of the enhanced fields in the gold nanosphere dimers. (A) Topography of the sample. Scale bar: 500 nm. (B,C) Near-field two-photon excitation images. The excitation wavelength was 785 nm. The arrows indicate the polarization of the excitation fields. The circles denote the approximate positions of the particles. (Reproduced with permission from ref 55. Copyright 2008, The Japan Society of Applied Physics.)

the wavelength region longer than the plasmon resonances (Figure 5A). Therefore, the transmitted light intensity through the aperture increases when the aperture is covered with a disk. In addition, the peak transmittance becomes higher as the disk diameter increases. We analyzed this result using theoretical model calculations and found that the observed spectroscopic features can be explained semiquantitatively as originating from the near-field scattering property of the nanodisks (Figure 5C). According to the calculations, the gold nanodisks efficiently scatter the near-field radiation into propagating waves in the wavelength region longer than the plasmon resonances shown in Figure 5A. The nonpropagating field confined near the probe

parallel to the interparticle axis of the dimer. The enhancement is low when the polarization is close to the axis that is perpendicular to the dimer axis. In the same image, only very weak optical fields are observed for the isolated particles. 2234



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two-dimensional monolayered assembly composed of 100 nm diameter gold nanospheres.92 The assembly consisted of partially close-packed structures as well as defects. The scanning electron micrograph image of the sample is superimposed on the near-field image in this figure to facilitate comparison between the nanostructure and the optical field distribution. From the image, we find that the optical field in the inner part of the assembly is weakly enhanced. The highly enhanced fields are localized in the outer boundary of the island-like structure, as observed at a wavelength of 785 nm. Strong enhancements in the boundary area of the assemblies have been observed as common features of the chain-like array and two-dimensional island structures. The fields are enhanced in the inner parts of the assemblies to a certain extent but not maximally, and the field enhancements are not uniform in the assemblies. What is the origin of such nonuniform distributions of the optical fields? We assumed that propagation of the plasmon that was excited on an individual particle through the interaction between the particles is the major origin of the characteristic field distributions. On the basis of this idea, we discussed the origin of the nonuniform field distribution using electromagnetic theoretical simulations and simplified model calculations.95 In the simplified model, the plasmon on each particle was regarded as an oscillating point dipole. We accounted for the potential of the dipole−dipole interaction and considered the whole assembly of particles as a coupled oscillating dipole system. The local density of the squared oscillation amplitude was estimated by solving a secular equation to discuss the spatial characteristics. The calculations were performed for a slightly lower oscillation frequency range than the plasmon resonance frequency of the isolated particle, which corresponds to the laser excitation frequency of our experiment. The results from both the electromagnetic simulations and the model calculations illustrate that the electric field or the dipole oscillation amplitudes are localized in the outer boundaries and/or in the defects of the island-like assembly. This feature qualitatively reproduced the observed feature of the image in Figure 8. Similar features were also obtained for the linear array structures. At the lower-frequency region, a different tendency was found; the stronger fields become localized in the central areas of the arrays or assemblies.94,95 This tendency may be comparable to the recent theoretical reports on metal particle chains.96,97 These results strongly suggest that the highly enhanced optical fields in the peripheral parts of the nanoparticle assemblies

The observed feature is consistent with the theoretical prediction for the localized optical fields on metal nanoparticle dimers,75,80,84−88 and this result demonstrates that the nearfield two-photon excitation image displays the spatial distribution of the optical fields. The near-field excited Raman scattering experiment revealed that the Raman activity in the dimer has essentially an identical spatial feature to that of the field distribution described previously. Similar results have been reported recently for scattering-type near-field measurements with a higher spatial resolution.26,27 Note that the experimental visualization of the localized optical fields for assembled nanoparticles, whose sizes are smaller than the optical wavelength, has been made possible based on the high spatial resolution of the near-field measurement. Next, we investigated optical field distributions for the assemblies of many gold nanoparticles using the same imaging method.91−94 In Figure 7, the near-field two-photon excitation

Figure 7. Plots of near-field two-photon excitation probabilities for the linear array structures of spherical gold nanoparticles (diameter ≈ 100 nm) along the chain axis. The excitation wavelength was 785 nm, with the polarization direction being along the chain axis. The scanning electron micrographs for the corresponding arrays are indicated below the plots.

probability distributions are compared with the scanning electron micrographs for the linear array structures of 100 nm diameter gold nanospheres94 using an excitation wavelength of 785 nm. The variation in the interparticle separations in the arrays may affect the optical field distribution. However, as a general tendency, higher-intensity optical fields are found in both ends of the arrays rather than in the central parts; this tendency is pronounced in longer arrays. In relatively short arrays, by contrast, we found strong fields in the central parts. The optical field distributions in the two-dimensional assemblies of nanoparticles were also examined. Figure 8 shows a typically observed two-photon excitation image of an island-like

Figure 8. Near-field two-photon excitation image of an island-like gold nanosphere assembly. The scanning electron micrograph image of the sample (in gray scale) is superimposed onto the two-photon excitation image (in color scale). The line profile of the two-photon excitation probability along the dashed line in the image is indicated in the left panel. (Reproduced with permission from ref 92. Copyright 2008, American Chemical Society.) 2235



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perpendicular to the rod axis are generated at the nodes radially around the rod. The fields perpendicular to the rod axis interact with the plasmons of the spherical nanoparticles adjacent to the rod to yield coupled modes,108 which produce an enhanced field perpendicular to the rod axis (Figure 9). The Raman

resulted from the propagation of plasmon excitations through the interparticle interactions. The plasmon excitation at an inner part of the island-like assembly is propagated in all directions toward the outside, and it may be trapped by the localized oscillation modes at the boundaries or in the defects of the assembly. The localized oscillation modes are generally formed at the boundary areas in a finite coupled oscillator system composed of multiple identical elementary oscillators.98 The localized vibrational modes at the boundary areas of polymer molecules and the localized molecular orbitals appearing in the boundary areas of conjugated double bond systems are believed to be of the same physical origin. If we consider the plasmon resonance of each particle to be equivalent to an atomic orbital of the constituent atom of a molecule, then the resonance plasmon mode of the entire assembly corresponds to a molecular orbital. It has been recently reported that a metal nanoparticle assembly can be viewed as a “plasmon molecule” that gives resonant coupled plasmon modes.28,99−101 We can regard the images of the enhanced optical fields in the nanoparticle assemblies described here as optically visualized molecular orbitals of the plasmon molecules. The enhanced optical fields in the nanoparticle assemblies, in particular, the two-dimensional monolayered assemblies, are important to the application of SERS. To develop substrates for highly sensitive SERS, a number of efforts have been made to design particle arrays of close-packed regular structures with area sizes as large as possible.102−105 However, the enhanced optical fields in the particle assemblies tend to be localized in close-packed structures opposed to being uniform, as described previously, which suggests that the large-area close-packed arrays are not advantageous for the enhancement of Raman scattering. We propose that the nanostructures with many dispersed small assemblies (such as dimers or trimers) are favorable. However, this expectation may be valid only for the excitation with 800 nm radiation sources; the result may be different when the excitation is in a longer wavelength range. To increase the degree of freedom in the design of optical field structures, assemblies consisting of particles of different shapes may be important. We examined silver nanorods joined with silver nanospheres attached on the side walls via analyses with SERS measurements and near-field imaging.106 In the nanoparticle assemblies, the induced polarizations generally occurred along the incident electric fields. Some works reported an enhanced electric field nearly parallel to the incident field for the rod−sphere joined system.107 In the report, the researchers analyzed the polarization characteristics of Raman scattering from the molecules in the gap between the rod and the sphere. A locally enhanced optical field was generated when the system was irradiated by light with polarization perpendicular to the long axis of the rod. We found, on the contrary, that the enhanced electric field (or the induced polarization) is perpendicular to the incident field under a certain condition. In our case, the rod was excited by the radiation in resonance with a longitudinal plasmon mode of the rod. The Raman scattering from the molecule in the gap between the rod and the sphere was enhanced when the incident polarization was parallel to the long axis of the rod. This result indicates that the strong polarization was generated at the interstitial site between the rod and the sphere; thus, the enhanced electric field perpendicular to the rod axis was induced locally. When the longitudinal mode of the rod is excited, electric fields parallel to the long axis of the rod are generated at the antinodes of the resonant plasmon wave functions near the rod, whereas those

Figure 9. Schematic view of the electric field for the rod−sphere joined system when the system is irradiated by light resonant with the longitudinal mode of the rod polarized along the rod axis. Generation of the confined optical field with a polarization perpendicular to the rod axis is expected if the sphere is attached near the node of the longitudinal plasmon mode of the rod.

enhancement of the molecules in the gap sites upon the excitation of the longitudinal mode of the rod can be thus explained by this assumed mechanism, which suggests the generation of enhanced fields perpendicular to the rod axis. The metal nanoparticle assemblies provide a wider variety of optical characteristics compared with the isolated nanoparticles, and they have the potential to create unique optical field structures. The optical measurements using nanometric spatial resolution provide fundamental and indispensable information for the application of the design and control of the field characteristics. We have discussed the optical field distributions in the metal nanoparticles and assemblies. The optical fields for the inverted metal nanostructures, that is, the structures with the metal and space portions inverted to each other, are also of interest.20,109−114 We investigated near-field images of the circular and elongated square openings on the metallic thin films (called voids in this Perspective) on glass substrates by means of near-field microscopy.115,116 Figure 10 shows typical near-field two-photon excitation images of the linear arrays of 400 nm diameter circular voids on a 20 nm thick gold film.115 The sample assembly may be generally regarded as inverted structures of arrays of spherical nanoparticles. In these images, the localized optical fields are observed in the “gap” area between the circular voids when the incident light polarization is nearly parallel to the chain axis. This optical field confinement was not observed when the incident polarization was perpendicular to the chain axis. This feature is apparently common in the assemblies of spherical nanoparticles and suggests that we can utilize the void assemblies as well as the particle assemblies to confine the optical fields. According to the electromagnetic analysis results, however, the mode characters of the confined fields observed for the void assemblies differ substantially from those for the particle assemblies. The diameter of the void (or the aspect ratio, defined as diameter/thickness) is very different from that of the spherical particle described previously, and consequently, the plasmon resonance frequencies of the isolated particle and the void are very different from 2236



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Figure 10. Near-field two-photon excitation images of a dimer (A,B), trimer (C,D), hexamer (E,F), heptamer (G,H), and decamer (I,J) of the circular nanovoids (diameter ≈ 400 nm) on a gold film (thickness ≈ 20 nm). The arrows indicate the excitation polarization directions. The excitation wavelength was 785 nm. (Reproduced with permission from ref 115. Copyright 2011, American Chemical Society.)

As the development of this methodology continues, a combined visualization approach using near-field microscopy and detection methods based on various optical processes is expected, in addition to quantitative improvements in spatial resolution, wavelength range, and other variables. Currently, we are developing near-field ultrafast measurement systems.51 Time-resolved near-field images of gold nanorods were observed with a time resolution of ∼100 fs, which allows the analysis of dynamics after dephasing of the plasmons.29,117 Recently, time resolution higher than 20 fs was achieved with the aperture-type apparatus, which enables measurements of dynamics before the dephasing of plasmons.118 Other groups reported that a similar time resolution is attainable using scattering-type near-field microscopy.119,120 We are also testing the near-field imaging of local circular dichroism for the metal nanostructures.121 In addition, we may consider various other developments of near-field microscopy combined with secondharmonic generation,122 coherent anti-Stokes Raman scattering,123 and other nonlinear optical processes, as well as other techniques such as near-field photocurrent detection.124 Developments in theoretical methods are also desired because the available theoretical frameworks are not always useful for the interpretation of observed near-field images. Currently, numerical methods based on classical electromagnetism, such as the finite difference time domain (FDTD) method,125 the discrete dipole approximation (DDA) method,126 and the Green dyadic method,64,65,69 are frequently used for the analysis of the optical characteristics of metal nanostructures. Extension of the Mie scattering theory14,16,127 allows mode analysis of plasmon resonances.128,129 However, the analysis requires a large computational resource and a long computational time, based on these presently available frameworks, to produce simulated results that can be compared directly with observed near-field images of the metal nanostructures. This current limitation in analysis of near-field images is caused by the close proximity of the measurement device to the sample, which results in a strong interaction between the two. As mentioned before, we found that the near-field images and spectra represent well the electromagnetic density-of-states of the plasmonic systems,30,61,73 at least qualitatively. However, for the purpose of quantitative discussion or to understand the circumstances where particular care should be taken to interpret the observed images, further theoretical assessment may be necessary. Construction of convenient and sufficiently accurate theoretical models for near-field imaging that yield a method of data analysis with realistic calculation resources is highly desirable. Developments in

each other. The modes observed at the same wavelength for the particle and the void are thus essentially of different characters. Similarly, the observed collective modes of the assemblies should also be of different character from each other. The images of the optical field distributions in Figures 6 and 10 were observed at the same wavelengths and are therefore attributable to the plasmon modes of different origins. We consider that the particle assemblies and the void assemblies accidentally exhibited similar optical field structures. The confined fields with similar mode characters to those observed in nanosphere dimers (Figure 6) might give resonances in much longer wavelengths for the void arrays. The optical field structures for the elongated rectangular voids, which can be generally regarded as inverted structures of single nanorods, were also visualized using the same method.116 We found that the standing waves of the electromagnetic modes arising from the plasmons localized in the voids were observed in the long rectangular voids in a similar manner as the nanorods described previously. This result indicates that the voids have the potential to create optical field structures with similar features to the particle plasmons. These examples reveal that both the metal nanoparticles and the voids corresponding to the inverted structures of particles are useful as components of another category to design confined optical fields. Near-field microscopy is highly effective for visualizing optical field structures arising from plasmon resonances, and the visualization is indispensable for correctly elucidating the plasmon fields, as we have described here. We have demonstrated that some of the ideas concerning optical field enhancement are in need of revision based on the experimental visualization of the optical fields. We believe that the nanoscale optical field visualization method provides a valuable tool for developing techniques using the optical characteristics of the metal nanostructures. The nanoscale design of the optical field structures and the enhancements based on the plasmon resonances have the potential to revolutionize photochemistry and photophysics. In addition, near-field optical microscopy may impact various applications, such as high-sensitivity analytical methods, the creation of novel chemical reaction fields, photofabrication, photoenergy conversion, and photoinduced materials conversion. Effects of this technique on photophysics and its applications as well as on bioimaging studies are also anticipated. We consider that visualization of plasmon-induced optical fields has significance as a basic methodology for applied research. 2237



Perspective

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theories that rigorously account for the quantum electronic structures of the sample systems are also desired to explore novel nano-optical characteristics.130,131

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Hiromi Okamoto received his B.S. (1983) and M.S. (1985) degrees from the Department of Chemistry, Faculty of Science, The University of Tokyo. He received his Doctoral degree in 1991 from The University of Tokyo. He was appointed as a Research Associate at the Institute for Molecular Science and The University of Tokyo and then as an Associate Professor at The University of Tokyo. Since 2000, he has been serving as a Professor at the Institute for Molecular Science. His present major research interests are near-field optical and spectroscopic studies of nanomaterials, particularly excited-state dynamics. Kohei Imura received his B.S. (1995) and M.S. (1997) degrees from the Department of Chemistry, Faculty of Science, Osaka University. He received his Doctor of Science degree in 2000 from Osaka University. He was appointed as an Assistant Professor at the Institute for Molecular Science in 2001. Since 2009, he has been serving as an Associate Professor at Waseda University. His present major research interests are near-field optics, plasmonics, and ultrafast phenomena in nanomaterials.

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ACKNOWLEDGMENTS We are grateful to the collaborators who contributed to this research project, in particular, Dr. T. Nagahara, Prof. J. K. Lim, Prof. Y. Jiang, Dr. T. Narushima, Dr. S. I. Kim, Dr. M. K. Hossain, Dr. T. Shimada, Prof. M. Kitajima, Prof. K. Ueno, Prof. H. Misawa, Ms. Y. C. Kim, Mr. S. Kim, M. Lee, Prof. D. H. Jeong, and Prof. S. Kim. The authors are also indebted to the Equipment Development Center and the Laser Research Center for Molecular Science of the Institute for Molecular Science. This research was supported by Grants from the Ministry of Education, Culture, Sports, Science and Technology (Nos. 17034062 and 19049015), the Japan Society for the Promotion of Science (Nos. 16350015, 17655011, 18205004, 18685003, 21655008, 22225002, 24655020, and 24350014), the Japan Science and Technology Agency PRESTO, and the Kurata Science Foundation. The project was also supported in part by the Extreme Photonics Project, the Consortium for Photon Science and Technology, the Nanotechnology Platform Program, and the Cooperative Research Program of Network Joint Research Center for Materials and Devices.

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REFERENCES

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