In this study, the optical properties of multilayer MoS2 at various temperatures and incident powers were studied. The multilayer MoS2 exhibits two excitonic bands and an indirect band gap transition. We found from the incident power dependence of photoluminescence that the bands exhibit a redshift with increasing incident power. For high-power excitation, a new band emerges near the indirect band and intensity ratio between the excitonic band and indirect band shows peculiar incident power dependence. We conclude that the emergence of a new band and incident power-dependent intensity ratio originate from the elevation of temperature and the morphological change, respectively.

A bright spot is observable in the center of Bull's eye plasmonic pattern with a fluorescence microscope due to the plasmonic nanoantenna effect. In this effect, a propagating wave of surface plasmon resonance concentrates in the center. This study focused on the relationship between the center structure of Bull's eye pattern and the nanoantenna effect in four fabricated Bull's eye-type plasmonic chips with centers of different sizes (full- or half-pitch diameter) and shapes (convex or concave). The fluorescence intensity of the fluorescent nanoparticles adsorbed to these plasmonic chips was measured with an upright-inverted microscope to evaluate the plasmonic chip enhancement factor composed of the product of the excitation and emission enhancement and individual factors. When the emission enhancement factor was investigated under nonresonance excitation conditions, by the disappearance of a bright spot, excitation enhancement was found to contribute to the plasmonic nanoantenna effect. The concave Bull's eye structure with a half-pitch diameter demonstrates the highest nanoantenna effect due to the formation of a larger constructive wave in the superposition of the diffraction wave of incident light under resonance conditions. In addition, the electromagnetic field intensity simulated by discrete dipole approximation agrees with the microscopy results. Overall, the results indicate that the plasmonic nanoantenna effect could be controlled depending on the resonance condition and center structure. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

The geometrical shape of a metal nanostructure plays an essential role in determining the optical functionality of plasmonic cavity modes. Here, we investigate the geometrical modification effect on plasmonic cavity modes induced in two-dimensional gold nanoplates. We perform near-field transmission measurements on triangular and tip-truncated triangular nanoplates and reveal that the plasmonic cavity modes are qualitatively consistent with each other as long as the snipping size is not significant. To elucidate the tip-truncation effect on plasmonic cavity modes in detail, we carry out numerical simulations for nanoplates with various snipping sizes and find that tip truncation affects not only the optical selection rules but also the energy relation for the plasmonic cavity modes. These findings provide a foundation for the rational design of plasmonic cavities with desired optical functionality.

The electronic properties of a substance are perturbed by interactions of elementary excitations. The optical properties of the interacting states have been extensively studied and revealed to be correlated with the eigenfunctions of the isolated systems. On the other hand, the spatial characteristics of the states have been little studied because of the diffraction limit of light. In this study, we examine plasmon and exciton interactions in silver nanoplate and organic J-aggregate hybrid structures using scanning near-field optical microscopy. We reveal that the light transmission is enhanced when the plasmon and exciton resonantly interact with each other. We visualize the spatial distribution of the interacting states and find that the interaction of the high-order plasmons with the exciton enables manipulation of the electronic states in a spatially resolved manner. This study demonstrates that the optical field can be spatially controlled via coupling of the elementary excitations.

Plasmonic optical fields have been applied for surface-enhanced spectroscopy, chemical sensing, and bioimaging. Spatial distributions of optical fields are critical for optimizing their functionalities. In plasmon-enhanced fluorescence, both incoming and outgoing fields excited by the plasmon should contribute to the enhancement of the fluorescence. Spatial characteristics of plasmons are critical not only for the fundamental understanding of the plasmon but also for their practical applications. Here, we investigate the spatial characteristics of the excitation and relaxation processes near the gold nanoplate using time-resolved near-field two-photon microscopy. We reveal from near-field optical microscopy that the incident field is locally enhanced by the plasmon resonance effect and the lightning rod effect. Near-field time-resolved fluorescence imaging demonstrates that the fluorescence decay is accelerated entirely over the surface of the plate regardless of the spatial distribution of the incident field. These results provide deep insight into plasmonic optical fields and are of great importance for designing plasmon-based substrates for surface-enhanced spectroscopy and photochemical reactions.

Gold nanoparticle assemblies significantly enhance optical fields and have been applied for nano-optical devices, biosensing, and chemical reactions. The optical properties of the assembly are, however, less controllable once the assemblies are fabricated on a solid substrate. An assembly prepared at the water-organic solvent interface overcomes this restriction and provides flexible photochemical reaction fields. Additionally, the physical and chemical properties of the assembly can be controlled by modification of the nanoparticle surface. In this study, we investigated the optical properties of the assembly using two-photon-induced photoluminescence and surface-enhanced Raman scattering, and demonstrated that the optical field enhancement and chemical environment near the gold nanoparticle assembly can be finely controlled by surface-modification of the gold nanoparticles.

Carbon dots (CDs) exhibit chemical stability and low toxicity, so they are promising for biomedical and imaging applications. The quantum yield of the photoluminescence is typically 10-20%, which limits practical applications. We fabricate carbon dot-gold nanoparticle photonic crystals (CD-GNP PCs) and demonstrate enhanced photoluminescence intensity from the carbon dots using the photonic and plasmonic double-resonant effects. A severalfold enhancement was obtained compared to the neat CD. The method developed in this study provides a universal scheme to enhance light-emitting materials, which is promising for the development of ultrahigh molecular sensing and bioimaging techniques.

Near-field optical microscopy visualizes spatial characteristics of elementary excitations induced in metal nanostructures. However, the microscopy is not able to reveal the absorption and scattering characteristics of the object simultaneously. In this study, we demonstrate a method for revealing the absorption and scattering characteristics of silver nanoplate by using near-field transmission and reflection spectroscopy. Near-field transmission and reflection images show characteristic spatial features attributable to the excited plasmon modes. The near-field refection image near the resonance shows a reversed contrast depending on the observed wavelength. Near-field reflection spectra show unique positive and negative resonant features. We reveal that the optical characteristics and the wavelength dependency of the optical contrast originate from the scattering and absorption properties of the plasmons, with the aid of the electromagnetic simulations.

Carbon dots are attracting much attention because of their low toxicity, biocompatibility, and photostability. In this study, we fabricate carbon dot-metal nanoparticle composites using electron-beam-induced chemical reactions to improve the photoluminescence characteristics. We investigate the spectral characteristics of the composites by linear and nonlinear optical spectroscopy. The composites show plasmon resonance depending on the material used. The linear and nonlinear photoluminescence from the composites are enhanced by plasmon excitations, and the enhancement reaches nearly several times and 20 times, respectively. We also fabricate nanowire structures containing the composites. The structures show very intense photoluminescence and unique polarization characteristics depending on the wire width. We reveal that the polarization characteristics originate from the plasmonic coupling along the short axis of the wire. The technique developed in this study is promising for the polarization control of light-emitting elements and the creation of new functional nanomaterials.

We visualize plasmon mode patterns induced in a single gold nanorod by three-dimensional scanning near-field optical microscopy. From the near-field transmission imaging, we find that 3rd and 4th order plasmon modes are resonantly excited in the nanorod. We perform electromagnetic simulations based on the discrete dipole approximation method under focused Gaussian beam illumination and demonstrate that the observed near-field spectral and spatial features are well reproduced by the simulation. We also reveal from the three-dimensional near-field microscopy that the 4th order plasmon mode confines optical fields more tightly compared with the 3rd order mode. This result indicates that the even-order plasmon modes are promising for enhancing the light-matter interactions.

Optical recording on organic thin films with a high spatial resolution is promising for high-density optical memories, optical computing, and security systems. The spatial resolution of the optical recording is limited by the diffraction of light. Electrons can be focused to a nanometer-sized spot, providing the potential for achieving better resolution. In conventional electron-beam lithography, however, optical tuning of the fabricated structures is limited mostly to metals and semiconductors rather than organic materials. In this article, we report a fabrication method of luminescent organic architectures using a focused electron beam. We optimized the fabrication conditions of the electron beam to generate chemical species showing visible photoluminescence via two-photon near-infrared excitations. We utilized this fabrication method to draw nanoscale optical architectures on a polystyrene thin film.

© 2018 American Chemical Society. We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5-20 nm while keeping the diameter constant (∼2 nm) by changing the relative concentration of OA and Au(I). It is proposed on the basis of time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (<2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near-IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is red-shifted with the AR. The NIR bands of AuUNRs with AR < 5 were blue-shifted upon the ligand exchange from OA to thiolates, in sharp contrast to the red shift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single-electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ∼2 nm significantly affects the optical response. The red shift of the LSPR band can be ascribed to the increase in the effective mass of electrons in AuUNRs.

We investigated the spectral and spatial characteristics of plasmons induced in chemically synthesized triangular gold nano- and microplates by aperture-type scanning near-field optical microscopy. Near-field transmission images taken at plasmon resonance wavelengths showed two-dimensional oscillating patterns inside the plates. These spatial features were well reproduced by the square moduli of calculated eigen functions confined in the two-dimensional triangular potential well. From the irreducible representations of the eigen functions, it was found that both the out-of-plane modes and in-plane modes were clearly visualized in the near-field images. We compared near-field transmission images of a triangular nanoplate to those of a truncated one with a similar dimension and revealed that the fine details of the geometrical shape of the apex on the plate strongly influence the experimentally observed eigen mode structures. We also performed near-field transmission measurements of micrometer-scale triangular plates and found that wavy patterns were observed along the edges of the plates. The wavy features can be interpreted as the superposition of eigen modes with similar eigen energy. These findings prove that near-field transmission imaging enables one to directly visualize plasmonic eigen modes confined in the particle and provide fruitful information not only for a deeper understanding of plasmons but also for the application of the design and active control of plasmonic optical fields.

We developed an aperture-type near-field reflection spectral imaging method to obtain optical properties of materials with a nanometer spatial resolution. We adopted a phase-stepping technique to extract genuine near-field signals of a sample from the observed intensity spectra dominated by the large background, in a multiplex manner. We performed near-field reflection spectral imaging of a single silver nanoplate to evaluate the developed system and also to examine near-field optical properties of the nanoplate. The near-field reflection spectrum of the nanoplate shows multiple resonant features that are not observable by conventional methods. The near-field reflection image taken at the resonance shows unique spatial features attributable to the plasmon mode resonantly excited. The developed system is applicable to transparent samples as well as opaque ones and enables spectral as well as spatial characteristics of the samples to be revealed.

We examine the far-field and near-field properties of complementary screens made of nanostructured gold thin films, a rectangular nanowire and a nanovoid, using an aperture-type scanning near-field optical microscope and electromagnetic field calculations, and discuss the applicability of Babinet's principle in the optical region. The far-field transmission spectra of the complementary screens are considerably different from each other. On the other hand, genuine near-field extinction spectra exhibit nearly complementary characteristics. The spatial features of the observed near-field images for the complementary screens show little correlation. We have found from the Fourier analysis of the simulated images that high spatial-frequency components of the electromagnetic fields show mutual spatial correlation. These results suggest that Babinet's principle is applicable to the high spatial-frequency components of electromagnetic fields for the complementary screens. (C) 2017 Optical Society of America

Interstitials to be active or inactive depend on the characteristics of nanoaggregate. In this perspective, near-field scanning optical microscopy has been utilized to probe well-defined monomer, dimer, tetramer, and small assembly of gold nanoparticles of 60 nm diameter with spatial resolution of similar to 35 nm. Confinement of localized surface plasmon resonances (LSPRs)-mediated near electromagnetic (EM) field distribution was observed through two-photon induced photoluminescence (TPI-PL) since TPI-PL emission is proportional to the fourth power of induced EM-field. It was revealed that the intensity of TPI-PL observed in closely connected nanoparticles was very strong with reference to those obtained in the presence of isolated nanoparticles of the same size. Among the closely connected nanoparticles, interparticle gap and interparticle axes played a vital role in TPI-PL emission enhancement, as was evident and observed directly through TPI-PL intensity distribution. Correlated local geometry and emission distribution confirmed that interstitials along the active interparticle axes exhibits confined and enhanced TPI-PL emission. The emission intensity for dimers was found higher than that of tetramer of gold nanoparticles. In case of small assembly of the same gold nanoparticles, intensity was observed to be the highest of all along with wider full width at half-maximum of 188.09 nm, more than double of those obtained at dimers and tetramer. The observations were further demonstrated as a function of EM near-field distribution, as it is well-known that TPI-PL emission is enhanced by LSPRs-assisted EM-field localization. Model systems were designed along with geometries and parameters very similar to those under investigation. Finite-difference time-domain analysis was carried out to deduce near EM field distributions at various deterministic factors such as size, interparticle gap, incident polarization, etc. A plausible explanation of enhanced and confined TPI-PL emissions in archetype interstitials of well-known plasmonic systems was highlighted.

We propose photobleaching-assisted near-field optical spectroscopy to obtain the absorption properties of nanomaterials. We used bis(3-sulfopropyl)-5,5', 6,6'-tetrachloro-11'-dioctyl-benzimidacarbocyanine (C8S3) double-wall tubular J-aggregates for demonstrating this method. We photobleached the J-aggregate by photoirradiation. We obtained the absorption spectrum of the J-aggregates by subtracting the transmission spectra observed before and after the photobleaching. The spectrum showed an intense asymmetric peak near 595 nm, which was assigned to the J-bands. These features were well reproduced by theoretical model calculations. We estimated the oscillator strength per molecule from the observed absorbance, and we found that the determined oscillator strength is comparable to that of the monomer. These results indicate that the proposed method is applicable for obtaining near-field absorption spectra of nanomaterials in a semi-quantitative manner.

Optical properties of a gold nanoparticle trimer assembly coupled with gold-coated hexagonally close-packed polystyrene microspheres were investigated by linear and nonlinear spectroscopy. The observed reflection spectrum shows multiple peaks from the visible to near-infrared spectral regions. The spectroscopic properties were also examined by a finite-difference time-domain simulation. We found that the optical response of plasmons excited in the gold nanoparticle trimers was significantly modulated by strong coupling of the plasmons and the photonic mode induced in the gold-coated polystyrene assembly. Two-photon induced photoluminescence and Raman scattering from the sample were investigated, and both signals were significantly enhanced at the gold nanoparticle assembly. The simulations reveal that the electric fields can be enhanced site-selectively, not only at the interstitial sites in the nanoparticle assembly but also at the gaps between the particle and the gold film due to plasmonic interactions, by tuning the wavelength and are responsible for the strong optical responses.

We performed dye-assisted two-photon-excitation imaging of plasmon modes in single gold nanoplates using a scanning near-field optical microscope. We observed that the fluorescence from the dye molecules was excited by the two-photon process and was enhanced several-fold on the nanoplate compared to the bare substrate. Two-photon excitation images of the single nanoplate show unique spatial features that differ from the morphology of the sample. Using electromagnetic simulations, we assigned the observed spatial features to the plasmon mode resonantly excited by the dye fluorescence. (C) 2016 Elsevier B.V. All rights reserved.

We investigated the surface-enhanced Raman scattering activity and spatio-temporal characteristics of plasmons excited on a rough gold film using ultrafast near-field optical microscopy. Near-field nonlinear excitation images showed a number of optically amplified spots on the sample surface. Raman scattering from molecules dispersed on the sample was enhanced at the amplified fields in a space-selective manner. Time-resolved near-field measurements revealed that the dephasing time varies depending on the excited plasmon. We evaluated the resonant mode volume from the Raman signal enhancement and the dephasing time of the plasmon, and found that the volume is on the same order as the one estimated from the topography of the sample.

The generation and dynamics of plasmon wave packets in single gold nanorods were observed at a spatiotemporal scale of 100 nm and 10 fs via time-resolved near-field optical microscopy. Following simultaneous excitation of two plasmon modes of a nanorod with an ultrashort near-field pulse, a decay and revival feature of the time-resolved signal was obtained, which reflected the reciprocating motion of the wave packet. The time-resolved near-field images were also indicative of the wave packet motion. At some period of time after the excitation, the spatial features of the two modes appeared alternately, showing motion of plasmonic wave crests along the rod. The wave packet propagation was clearly demonstrated from this observation with the aid of a simulation model. The present experimental scheme opens the door to coherent control of plasmon-induced optical fields in a nanometer spatial scale and femtosecond temporal scale.

We studied optical properties of hexagonally arranged gold nanoparticle trimer assembly using optical microscopy. We found that the scattering spectrum showed multiple resonances in the visible region. We simulated the optical properties by assuming a pair of trimers as a model structure. The simulated spectrum reproduced the resonance features observed, and were assigned to plasmon-modes excited. Surface-enhanced Raman Scattering (SERS) activity of the assembly was found to be dependent on the excitation wavelength. The highest SERS enhancement was achieved with 633 nm excitation, reaching 108. The wavelength-dependency of the activity indicates that plasmonic fields are responsible for the enhancement. (C) 2015 Elsevier B.V. All rights reserved.

We applied time-resolved near-field optical microscopic measurements with ultrashort light pulses of similar to 16 fs duration to observe plasmon dephasing processes in single gold nanorods. The correlation widths of the time-resolved signals obtained at each position on the nanorods were broadened compared with the autocorrelation width of the pulse because of the plasmon lifetime. The correlation width maps of the rods showed spatially oscillating patterns that look similar to the plasmon mode structures observed in the static near-field optical images. The spatial variation of the correlation widths was explained as arising from the position-dependent contribution of the resonant plasmon excitation in the time-resolved signals relative to that of the nonresonant excitation. This finding indicates that the dephasing times of the resonant plasmon modes were constant regardless of the excitation position. This result is understood to be a consequence of the spatial coherence of the plasmon mode that causes the local excitation to be immediately delocalized across the rod after irradiation. A comparison between the time-resolved signals of the inner parts and the outer parts of the nanorods suggests that the nonresonant contribution to the time-resolved signals may be driven by the lower-order plasmon modes having resonances in a much longer wavelength region.

Near-field scanning optical microscopy was used to probe single nanoparticles, close-packed dimer, well-defined trimeric nanoaggregate, and separated dimer of gold nanoparticles. Surface plasmon (SP)-coupled electromagnetic (EM) field confinement at interstitials was confirmed through two-photon-induced photoluminescence (TPI-PL) imaging. A fairly well correlation between topography and metrology in nanoscale was reported with sufficient high spatial resolution. A strong excitation of SP-assisted TPI-PL was observed at gold dimer and trimeric nanoaggregate, whereas single particles and separated dimer showed negligible TPI-PL excitation. A confined distribution of TPI-PL was revealed in close-packed dimer with reference to those at isolated nanoparticles and trimer. Strong optical distribution of TPI-PL was observed at trimer, but the distribution was found broadened with reference to those at dimer. A plausible explanation of such broadened distribution was interpreted as the coalescence of surface plasmon-assisted confined optical signal of TPI-PL. Such a direct and distinguished correlation between local topography and optical field distribution with high resolution is indispensable and further addition to realize surface plasmon coupling at the interstitials. Finite different time domain analysis was carried out to elucidate the results observed herein. For a small model of three nanoobjects keeping the geometry and parameters very similar to that of the close-packed trimeric nanoaggregate under investigation, multiple confined EM fields were found at the out-of-plane polarization to the interparticle axes. With the step sizes (i.e., 50 nm), these nearby localized EM near-field-mediated optical distributions open up a possibility to coalescence and thus get broadened.

The unique optical characteristics of noble metal nanostructures have their origin principally in surface plasmon resonances. To exploit and design the unique characteristics arising from plasmons, an investigation of optical field structures adjacent to the nanostructure is of fundamental importance. As the spatial scale of the optical field structures is essentially smaller than the radiation wavelength in resonance with the plasmon, optical imaging methods that achieve spatial resolution beyond the diffraction limit of light are necessary to visualise the fields. In this article, we review the studies of direct experimental visualisation of plasmon resonances using near-field optical microscopy. We briefly describe the method of near-field optical microscopy used to study noble metal nanoparticles and show with several typical single gold nanoparticles that the spatial features of plasmon resonances, in particular the standing wave functions of the plasmons, can be directly visualised by near-field imaging. We then describe our recent efforts to visualise ultrafast dynamics in metal nanostructures following plasmonic excitation, which are based on near-field ultrafast imaging measurements. Another notable aspect of metal nanostructures that has attracted attention recently is the chirality of plasmons. Here, we describe a method and examples of near-field optical imaging and analyses of chiral plasmons excited on metal nanostructures.

Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundamental modes inside and outside of the disc were performed. Simulated images reproduced the observed spatial structures excited in the disc. Mode-analysis of the simulated images indicates that the spatial features observed in the transmission images originate mainly from a few fundamental plasmon modes of the disc. (C) 2014 Optical Society of America

Near-field scanning optical microscopy (NSOM) is known to be a technique of choice to investigate nanometric materials and their properties beyond far-field diffraction limit resulting high spatial, spectral and temporal resolution. Here in this report, a state of art facility, aperture-NSOM was used to probe single nanoparticle, dimer, trimer and small nanoaggregate of gold nanoparticles. Shear force topography and two photon induced photoluminescence (TPI-PL) images captured simultaneously by the system facilitated to clarify the correlation between the local geometry and the emitted photon of TPI-PL. Small gold aggregates including trimer showed strong optical confinement of TPI-PL with reference to that of single gold nanoparticles. It was also evident that the interparticle gap does have a great influence in localized electromagnetic (EM) field mediated optical confinement of TPI-PL. Such observations were also supported by finite different time domain (FDTD) analysis keeping the simulation parameter nearly identical to that of experiment. FDTD simulation demonstrated that incident excitation parallel to the interparticle axis induces strong near-field distribution at the interstitials whereas out of plane excitation modifies such confinement depending on the nanometric geometry of the nanoaggregates. Such an observation is indispensable to understand the localized EM field-mediated optical confinement in surfaceenhanced spectroscopy. © (2014) Trans Tech Publications, Switzerland.

We developed an advanced near-field optical method by combining an ultrafast near-field optical microscope with a prism-based pulse shaping system. We used this apparatus to visualize plasmonic optical fields and to measure the lifetime of plasmons excited on a rough gold film. We also studied the influence of the phase-modulation of the excitation pulse on the spatial distribution of the optical fields. We found that the spatial distribution of the optical fields can be controlled by a negatively chirped pulse. ©2013 Optical Society of America.

We developed an advanced near-field optical method by combining an ultrafast near-field optical microscope with a prism-based pulse shaping system. We used this apparatus to visualize plasmonic optical fields and to measure the lifetime of plasmons excited on a rough gold film. We also studied the influence of the phase-modulation of the excitation pulse on the spatial distribution of the optical fields. We found that the spatial distribution of the optical fields can be controlled by a negatively chirped pulse. (C) 2013 Optical Society of America

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 near-field 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. © 2013 American Chemical Society.

A top-to-bottom joined system consisting of a silver nanowire and nanospheres was fabricated by embedding silver nanospheres on a glass or silicon substrate on which 3-aminothiophenol as an analyte molecule was adsorbed, and then placing silver nanowires on the substrate to make gap sites between a nanowire and nanospheres. In the far-field Raman measurements, the sphere under the wire exhibited more than 60 times higher Raman enhancement than isolated spheres. The surface enhanced Raman scattering (SERS) spectra obtained by the 647.1 nm excitation showed highly polarized feature, exhibiting ca. 4 times higher SERS intensity for the electric field parallel to the wire axis than that perpendicular to the wire axis while those by the 514.5 nm excitation showed non-polarized feature against the incident electric field direction. The polarized feature by the 647.1 nm excitation is explained in terms of optical coupling in a vertical direction to the substrate plane, between the silver nanosphere and the longitudinal surface plasmon mode of the nanowire. The longitudinal plasmon of the nanowire functions as an antenna of the incident radiation field in this type of coupled plasmon mode, to yield the confined field. Near-field two-photon excitation imaging measurements as well as numerical calculations of the localized electric field around the system support this idea and indicate that the coupling between the surface plasmon of silver nanospheres and the longitudinal mode of silver nanowires is site-selective. © 2013 the Owner Societies.

We studied optical properties of single elongated rectangular nanovoids opened on a thin gold film by scanning near-field optical microscopy. We found that the luminescence is induced via two-photon absorption of near-infrared pulses when the sample is locally photoexcited inside the void as well as on the film. The luminescence spectrum of the void shows spectral features very similar to those observed for gold nanorods and is attributable to that from gold. On the basis of the near-field two-photon excitation imaging by detecting the luminescence, we visualized the optical fields near the voids. The optical image shows a characteristic spatial feature depending on the dimension of the void. From the dose inspection of the topography and the optical images, we found that excitation probability becomes high at the metal void boundary as well as inside the void. We found an oscillating feature along the long axis of the void. The observed spatial features were qualitatively reproduced by photonic local density-of-states based on electromagnetic calculations, and we conduded that the oscillation observed inside the void is attributed to the electromagnetic modes in the void coupled with the plasmon excitations.

We visualized the enhanced optical field distributions in one-dimensional linear array structures of gold nanospheres by using scanning near-field optical microscopy. The characteristic field distribution depends on the chain length of the one-dimensional structures. The distribution of optical field is reproduced qualitatively by model calculations based upon finite-difference time-domain (FDTD) method. From the analysis, we have found that the characteristic distribution of the enhanced field arises from interparticle interactions, which cause propagation of the plasmon excitation in the inner part of the linear array, and trapping of it by a localized mode at the edges.

We have constructed an ultrafast near-field optical microscope system with near-field pump/probe pulses with a 17 fs duration at the probe tip. The dispersion effects arising from the optical components, especially from the optical fiber, were removed by a pulse-shaping system that consists of passive and adaptive group-velocity dispersion compensators. With this apparatus, we succeeded in measuring dephasing in a single gold nanostructure with a time constant on the order of 10 fs. (C) 2012 The Japan Society of Applied Physics

Modification of photovoltaic devices with metallic nanoparticles is expected to be one of the key methods for development of high performance devices as a future energy source. To clarify the mechanism of photocurrent changes induced by surface plasmon resonances of metal nanoparticles, we measured near-field photocurrent excitation images for a GaAs photodiode modified with gold nanospheres (diameter 100 nm) with a spatial resolution higher than 100 nm. The relationship between the photovoltaic efficiency and the plasmons of the gold nanospheres was investigated through the measurements of incident wavelength and polarization dependence of the near-field photocurrent images. Isolated nanospheres deposited on the GaAs active surface caused local photocurrent suppressions at the plasmon resonance wavelengths. In the case of assemblies (dimers and trimers) of the spheres, a remarkable decrease of photocurrent at the gap site between the spheres was observed. From the results, it turned out that the enhanced optical fields created via the plasmons on the metal nanostructures do not improve the photovoltaic efficiency and that forward scattering of photons by the gold nanoparticles is considered to be more important than the enhanced field effect at the particles for the GaAs photovoltaic device studied. (C) 2011 American Institute of Physics. [doi:10.1063/1.3662114]

Near-field two-photon excitation images of assemblies of many gold nanospheres show characteristic feature that enhanced optical fields are confined at the rim parts of the assemblies. In the present report we analyzed the origin of this feature based on finite-difference time-domain (FDTD) approach as well as a simple point dipole model that incorporates the interparticle interaction with the dipole-dipole potential. It has been found that the simple point dipole model is useful for qualitative discussion on the optical field distribution in the metal nanoparticle assemblies. From the analysis, we have found that the interparticle interaction, which causes the propagation of the plasmon excitation in the assemblies, seems to be essential for the localization of the enhanced field at the rim. We propose that regular close-packed assemblies do not yield efficiently enhanced optical fields in visible to near-infrared region, and rather assemblies with large fluctuation are more advantageous to get highly enhanced fields. (C) 2011 Elsevier B.V. All rights reserved.

We have succeeded in nanoscale polymerization of Langmuir-Blodgett (LB) films of cadmium 10,12-pentacosadiynoate (CdPA) by near-field two-photon excitation with femtosecond pulses. The LB films of CdPA were excited locally through an aperture of a near-field optical fiber probe by femtosecond-pulsed near-infrared radiation. The formation of polydiacetylene (PDA) films has been LB film confirmed by two-photon induced and ordinary (linear) luminescence and Raman measurements by using a scanning near-field optical microscope. The polymerization of CdPA to PDA was induced by the femtosecond-pulsed near-infrared radiation but not by continuous wave radiation. This result confirms that the polymerization was not induced by heat arising from the near-field irradiation. The number of photons involved in the polymerization reaction was estimated to be 2 from the dependence of photoluminescence intensities of PDA on the power of incident light for polymerization.

:We report an anomalous light transmission phenomenon for a nanoaperture on an opaque screen when the aperture is covered with an opaque cap. In conventional optics, light transmission must decrease when the aperture is capped. However, we found that light transmission is enhanced when the nanodisk is in close proximity to the aperture at a wavelength close to the plasmon resonance. This effect even occurs when the disk is larger than the aperture.

Linearly aligned nanovoid chain structures on Au thin films were fabricated on glass substrates. Two-photon excitation imaging techniques using an aperture-type near-field scanning optical microscope revealed localized optical field distributions due to the individual structures. Confined optical fields due to the void chains were observed at each interstitial gap between voids with excitation polarization parallel to the chain axis. Electromagnetic field simulations of dimeric voids qualitatively matched the experimental results. Under excitation polarization perpendicular to the chain axis, only weak optical fields were observed. Further detailed characteristics of localized optical fields in these systems, including those in relation to Babinet's principle in optics, were discussed. Our study may open up new possibilities in molecular sensing and photochemistry.

The optical properties of a gold nanorod were investigated by Imura et al. [J. Chem. Phys. 122, 154701 (2005)] using an apertured-type scanning near-field optical microscope (SNOM). The observed transmission image showed an oscillating pattern along the long axis of the nanorod. We obtain the image using the finitedifference time-domain (FDTD) method. Our model includes a nanorod on a glass substrate, a SNOM, and current as a light source. We develop a simple method for including the Drude-Lorentz dispersion relation of Vial et al. [Phys. Rev. B 71, 085416 (2005)] for gold in the FDTD. The oscillating pattern is explained by the total current in the nanorod, tip of the SNOM, and light source.

We investigated properties of photoluminescence from single gold nanorods induced by near-field two-photon excitation, using scanning near-field optical microscopy. Photoluminescence spectrum showed two peaks around 550 and 650 nm in wavelengths regardless of the rod dimensions, whereas the intensity ratio of the two spectral components varies from rod to rod. We discussed the observed spectral features of photoluminescence based on electromagnetic calculations and found that the observed spectral features can be explained in terms of local field enhancement in the vicinity of the gold nanorod due to plasmon-mode excitation. © 2009 American Chemical Society.

In this article, studies on noble metal nanostructures using near-field optical microscopic imaging are reviewed. We show that near-field transmission imaging and near-field two-photon excitation imaging provide valuable methods for investigation of plasmon resonances in metal nanostructures. The eigenfunctions of plasmon modes in metal nanoparticles are directly visualized using these methods. For metal nanowire systems, wavevectors of the longitudinal plasmon modes can be estimated directly from the wave-function images, and the dispersion relations are plotted and analyzed. Using ultrafast transient near-field imaging, we show that the deformation of the plasmon wave function takes place after photoexcitation of a gold nanorod. Such methods of plasmon-wave imaging may provide a unique basic toot for designing plasmon-mode-based nano-optical devices. We also demonstrate that the near-field two-photon excitation probability images reflect localized electric-field enhancements in metal nanostructures. We apply this method to gold nanosphere assemblies and clearly visualize the local enhanced optical fields in the interstitial sites between particles (hot spots). We also show the contribution of hot spots to surface enhanced Raman scattering. The methodology described here may provide valuable basic information about the characteristic enhanced optical fields in metal nanostructures as well as on their applications to new innovative research areas beyond the conventional scope of materials. (C) 2009 Elsevier Ltd. All rights reserved.

Cell imaging with two-photon-induced photoluminescence is demonstrated using gold nanoparticles (either triangular nanoplates or nanosphere aggregates) as imaging agents. On being conjugated to yeast cells, gold nanoplates exhibit visible two-photon excited luminescence that can be detected by two-photon laser scanning microscopy. The cells can be either dried (in the air) or alive (in water).

We report a near-field two-photon excitation imaging technique for silver nanostructures to visualize optical fields in the vicinity of nanostructures. We have found that the excitation image of an isolated wire shows an oscillating feature along the long axis of the wire. The period of the oscillation is in good agreement with that of plasmon wavefunctions resonant with the excitation light. The oscillation was thus attributed to the resonant plasmon wavefunction. We also found that bundles of wires exhibit more intense photoluminescence signals than do isolated wires, particularly at junctions between wire ends and the shafts of other wires, which was attributed to the amplified optical fields at the ends of nanowires or interstitial gaps between the wires. We discuss the mechanism of the photoluminescence from silver nanowires. The photoluminescence is most probably attributed to emission from the oxidized surface layer of the silver nanowires, which is enhanced in resonance with plasmons on the wires.

We investigated the enhancement and quenching of dye fluorescence by single gold nanoparticles using an aperture-type scanning near-field optical microscope to evaluate the shape dependence. Enhancement or reduction of fluorescence was found to be strongly dependent on the shape of the particle. Gold nanoplates showed significant fluorescence enhancement, as did aggregated nanoparticles for the most part. On the other hand, gold nanorods with high aspect ratios showed quenching effects. The results suggest that the electromagnetic near-field in the vicinity of the surface of nanoparticles, not just near the apex or the tip but the whole surface, is important for understanding fluorescence enhancement and quenching. © 2008 Elsevier B.V. All rights reserved.

Spatial distribution of surface enhanced Raman activity is visualized for two-dimensional (2D) nearly close-packed and well-ordered monolayer array of gold nanoparticles by using scanning near-field optical microscope. The 2D arrays exhibit highly nonuniform enhancement in Raman scattering, i.e., the regions along the edge of the 2D array are preferentially enhanced. We demonstrate that the spatial distribution of the localized electric field is also nonuniform and agrees well with that of the Raman enhancement.

Near-field optical microscopy enables spectroscopic and imaging measurements with a spatial resolution far beyond the diffraction limit of light. We show in this review that spatial structures of plasmonic wavefunctions and those of enhanced optical fields are visualized by near-field spectroscopic imaging for gold nanoparticles and their assemblies. A simple formulation for the optical observation of wavefunctions of oscillating polarization modes in nanomaterials is given. We will introduce experimental methods for near-field linear transmission and nonlinear two-photon excitation measurements. We will demonstrate that the wavefunctions of surface plasmons resonant with the incident wavelength are visualized by the near-field methods for gold nanorods. In addition, we will also show for aggregated gold nanospheres that an enhanced optical field is localized in the interstitial gaps between the particles by near-field two-photon excitation measurements. © 2008 The Japan Society of Applied Physics.

We have developed novel methods of near-field microspectroscopy by combining near-field optical microscopy with linear and non-linear optical techniques. The developed near-field microscope achieves high spatial resolution and high time resolution, and is applied to studies of local excitations and wave functions of single noble metal nanoparticles. We demonstrate that the plasmon wave functions and the optical fields in the vicinity of nanoparticles are visualized by the near-field methods. Based on these results, we clearly show that localized electromagnetic field enhancement in the vicinity of nanoparticles is one of the most important factors in surface-enhanced spectroscopies. We also extended the methods to ultrafast optical measurements, and show space-resolved ultrafast transient response of single gold nanorods. The characteristic spatio-temporal features observed in the nanorods are revealed to be arising from the changes of the plasmon-mode wave functions upon elevation of photo-induced electronic temperature of the nanorod.

We. clarified the relationship between near-field optical properties and the local structure in monolayer assembly of gold nanoparticles by using scanning near-field optical microscopy (SNOM) and scanning electron microscopy (SEM). The success of superposition of the SNOM image with the SEM image allows us to find that the intense electromagnetic field is localized at the rim part, especially the interstitial site of the dimerlike structure.

We have studied electromagnetic field on the surface of two-dimensional nanostructure of gold nanoparticles through two-photon-induced photoluminescence images by using scanning near-field optical microscope and far-field surface-enhanced Raman scattering measurements. The near-field two-photon-induced photoluminescence image shows that strong inhomogeneous enhancement in two-photon-induced photoluminescence occurs over the two-dimensional nanostructure, representing localized surface plasmon excitations. The results suggest that the local structure of the two-dimensional nanostructure influences the distribution of localized surface plasmon excitation.

Ultrafast transient responses in single gold nanorods are visualized in real space by femtosecond pump-probe measurements using a near-field optical microscope. The observed transient images show characteristic features, and the features are strongly dependent on the size of the rod. It is revealed that the features arise from changes in the plasmon-mode wave function upon elevation of the electronic temperature of the nanorod induced by photoexcitation. © 2008 The American Physical Society.

We studied single gold nanoparticles and aggregates by linear and non-linear optical methods combined with near-field optical microscopy. Near-field images of gold nanorods obtained show peculiar spatial patterns, and are strongly dependent on wavelength observed. The images are attributed to spatial characteristics of plasmon modes (wavefunctions), and are consistent with theoretical simulations. We also studied spatial distributions of optical fields by using near-field two-photon-induced photoluminescence and Raman imaging techniques. We revealed that optical fields are highly confined at interstitial sites in dimers of spherical gold nanospheres, and such an amplified electromagnetic field is one of the most important origins in single molecular sensitivity surface enhanced Raman scattering.

We report in this paper the near-field microscopic studies on localized plasmon resonances in gold nanoparticles and their assemblies (nanorods, triangular nanoplates, and assembled nanospheres). We have utilized near-field measurements of linear transmission/scattering, as well as nonlinear two-photon excitation, to enable spectroscopic imaging of local electric field or local density of electromagnetic states. We show that the wavefunction of the plasmon excitation in the nanoparticle is visualized by the near-field methods. For single nanorod, many plasmon resonances were observed in the near-field transmission spectrum. At each resonant peak wavelength, the near-field image of the nanorod gave a characteristic spatially oscillating feature along the long axis of the nanorod. The feature is attributable to the square modulus of the resonant plasmon-mode wavefunction. In the assembled nanoparticles, strong electric-field enhancement localized in the interstitial sites ("hot spot"), which was theoretically predicted previously, was clearly imaged by the near-field two-photon excitation method. Major contribution of the hot spots to surface enhanced Raman scattering is also shown for the samples weakly doped with Raman-active dye molecules, by the near-field excited Raman spectra and images.

We investigated near-field optical properties and images of single gold nanorods by using a near-field optical microscope. Observed transmission spectra show distinct transverse and longitudinal surface plasmon resonances. Transmission images observed near the surface plasmon resonances agree qualitatively with calculated maps of optical local density-of-states, and are assignable to plasmon wavefunctions. Ultrafast temporal responses in the single gold nanoparticles were observed by combining a near-field microscope with time-resolved techniques. Observed transient transmission images of the single nanorods show characteristic optical features, and are in good agreement with a calculated map of variation of local density of states arising from the elevation of electronic temperature in the nanorod induced by photoexcitation.

We demonstrate visualization of localized intense electromagnetic fields in real space in well-tailored dimeric and trimeric gold nanospheres by using near-field optical techniques. With two-photon induced luminescence and Raman measurements, we show that the electric field is confined at an interstitial site in the aggregate. We also demonstrate optical switching operations for the electric-field localized sites in the trimer structure.

We experimentally investigated the reciprocity of scanning near-field optical microscopy between illumination and collection modes. Near-field transmission images of single gold spheres and nanorods observed by the two modes are found to be equivalent to each other in the region from visible to near infrared. This result shows that reciprocity holds for the near-field scattering problems. We found that the conventional optical selection rule for far-field excitations does not apply not only under illumination mode but also with collection-mode arrangements. The possible origin of this observation might be the near-field probe. (c) 2006 Optical Society of America.

Scanning near- field optical microscopes enable optical measurements and observations with a spatial resolution far beyond the diffraction limit of light. This allows us to spatially resolve the positions in nanoparticles and at the same time to investigate spectroscopic characteristics point by point. In this feature article, we focus on near- field microscopic studies on surface plasmons in anisotropic gold nanoparticles and aggregates ( nanorods, triangular nanoplates, and aggregated nanospheres). We have utilized linear transmission/ scattering as well as nonlinear two- photon excitation measurements in the near field, to observe the local electric field or electromagnetic local density of states. We show that the wavefunction of the plasmon excitation in the nanoparticle resonant with the incident radiation field is visualized by the near- field methods. In the aggregated nanoparticles, strong electric- field enhancement localized in the interstitial sites, which was theoretically predicted previously, is clearly imaged, and its major contribution to surface enhanced Raman scattering is also shown by the near- field images.

We have investigated two-photon-induced photoluminescence (PL) properties of single gold nanoplates by using an apertured scanning near-field optical microscope. The nanoplates show PL much stronger than nanorods. The near-field PL images show characteristic spatial features, which depend on the incident polarization. These PL images are in good agreement with the calculated spatial distribution of the electric fields adjacent to the particles at the excitation wavelength. We attribute the observed images to spatial characteristics of plasmon modes.

We observed near-field images of aggregates of gold nanospheres, by detecting TPI-PL from gold and Raman scattering from Rhodamine 6G adsorbed oil the nanoparticles. The results were analyzed in relation to the localized Raman active site (hot Spot) of surface-enhanced Raman scattering,.

We fabricated porphyrin thin films on mica surfaces from acidic aqueous solutions of the preorganized H-aggregates of amphiphilic porphyrins by the simple spin-coating method. The morphological and spectroscopic properties of the film were investigated by scanning near-field optical microscopy. The results obtained in this study demonstrate that the preorganized structure in solution can be transferred as a thin film with a thickness of the monolayer level without losing their substantial structure and photophysical properties.

Surface plasmons of silver nanorods were investigated by using scanning near-field optical microscopy. Near-field transmission images showed spatially oscillatory patterns in nanorods. The oscillatory features of images are attributable to plasmon-mode wave-functions. From the near-field images and spectra, dispersion relations of surface plasmon modes for various silver nanorods are obtained. It is found that dispersion relation of a nanorod is dependent on its diameter. The spectral features obtained are compared with those for gold nanorods. (c) 2005 Elsevier B.V. All rights reserved.

We investigated the two-photon-induced photoluminescence properties of single gold nanorods by scanning near-field spectroscopy. The process was found to be initiated by a sequential one-photon absorption for creating a pair of an electron and a hole in the sp and d bands. Photoluminescence is then radiated when the electron near the Fermi surface recombines with the hole near the X and L symmetry points. The polarization characteristics of emitted photons from the X and L regions were found to be different. These characteristics can be understood by the crystalline structure and the band structure of the gold nanorod. We found characteristic spatial oscillatory features along the long axis of the nanorods in photoluminescence excitation images. The images were well reproduced by density-of-states maps of the nanorods calculated with Green's dyadic method and were attributed to the spatial characteristics of the wave functions of the plasmon modes inside the nanorods.

We have investigated optical properties of single gold nanorods by using an apertured-type scanning near-field optical microscope. Near-field transmission spectrum of single gold nanorod shows several longitudinal surface plasmon resonances. Transmission images observed at these resonance wavelengths show oscillating pattern along the long axis of the nanorod. The number of oscillation increases with decrement of observing wavelength. These spatial characteristics were well reproduced by calculated local density-of-states maps and were attributed to spatial characteristics of plasmon modes inside the nanorods. Dispersion relation for plasmons in gold nanorods was obtained by plotting the resonance frequencies of the plasmon modes versus the wave vectors obtained from the transmission images. © 2005 American Institute of Physics.

A novel apparatus for femtosecond pump-probe near-field optical microscopy is described. Probe pulses in visible to near-infrared regions are generated by focusing laser pulses in microstructure fiber. Excited-state dynamics of porphyrin J-aggregates are presented.

We investigated near-field optical properties of single gold nanorods. Transmission images observed near the surface plasmon resonances agree with calculated local density of states. Ultrafast temporal responses of single rods have been imaged by combining near-field microscope with time-resolved technique. It has been found that dynamic behavior in the center of the rod is different from that in the both ends.

We investigated near-field optical properties of single gold nanorods. Transmission images observed near the surface plasmon resonances agree with calculated local density of states. Ultrafast temporal responses of single rods have been imaged by combining near-field microscope with time-resolved technique. It has been found that dynamic behavior in the center of the rod is different from that in the both ends.

A novel apparatus for femtosecond pump-probe near-field optical microscopy is described. Probe pulses in visible to near-infrared regions are generated by focusing laser pulses in microstructure fiber. Excited-state dynamics of porphyrin J-aggregates are presented.

We investigate the interaction between evanescent wave and a single gold nanoparticle in near-field by utilizing an aperture-probe scanning near-field optical microscope. A characteristic spectral feature, consisting of transmission enhancement and absorptive parts, is found. Observed spectra are successfully simulated both by a model calculation based on extended Mie scattering theory of the near- and far-field scatterings as well as by a Green's dyadic method. (C) 2004 Elsevier B.V. All rights reserved.

Mesoscopic properties of J-aggregates of water-soluble [tetrakis(4- sulfonatophenyl)porphyrin] and water-insoluble [tetrakis(4-methoxyphenyl) porphyrin and tetraphenylporphyrin] tetraphenylporphyrin derivatives in thin films have been investigated by scanning near-field optical microscopy. From surface topography, the sample films were found to be composed of planar J-aggregate microcrystallines. The transmittance spectra of the water-insoluble samples are site specific, while those of the water-soluble samples are not. This observation might reflect the spatial inhomogeneity of the thin films in mesoscopic scales and is related to the broad J-bands of water-insoluble samples. Excited state lifetimes, obtained from time-resolved, pump-probe measurements in small domains, were in the 30-70 ps range for 4-methoxy substituted and in the 40-100 ps range for unsubstituted tetraphenylporphyrin, which are of the same order as the reported far-field results for 4-sulfonated compound in water.

A novel apparatus for time-resolved near-field optical microscopy is described. The apparatus consists of a mode-locked Ti:sapphire laser, a microstructure fiber, and a scanning near-field optical microscope equipped with an apertured optical fiber probe. The probe pulses in visible to near-infrared regions are generated by focusing laser pulses in the microstructure fiber. The broadband continuum can be used as a wavelength-tunable light source for fluorescence excitation as well as for probing absorption of excited states at arbitrary wavelengths by applying pump-probe scheme, in high spatial-resolution (similar to100 nm) predominantly determined by the aperture size of the tip. Time resolution obtained with 740-830 nm probe pulses was in 1-2 ps range, while that with 570 nm pulses was 5 ps without precompensation of the group delay dispersion for the probe pulses. Results on the excited-state dynamics of molecular aggregates are presented. (C) 2004 American Institute of Physics.

We investigated the near-field optical properties of single gold nanorods. Transmission images observed near the surface plasmon resonances agree qualitatively with the calculated maps of the optical local density of states and are reflected by spatial characteristics of plasmon wave functions. Ultrafast temporal responses have been observed by combining near-field microscopy with a time-resolved technique. It has been found that the behavior of the collective motion of electrons in the center of the rod is different from that in both ends.

Mesoscopic properties of J-aggregates of watersoluble [tetrakis(4-sulfonatophenyl)porphyrin] and water-insoluble [tetrakis(4-methoxyphenyl)porphyrin and tetraphenylporphyrin] tetraphenylporphyrin derivatives in thin films have been investigated by scanning near-field optical microscopy. From surface topography, the sample films were found to be composed of planar J-aggregate microcrystallines. The transmittance spectra of the water-insoluble samples are site specific, while those of the water-soluble samples are not. This observation might reflect the spatial inhomogeneity of the thin films in mesoscopic scales and is related to the broad J-bands of water-insoluble samples. Excited state lifetimes, obtained from time-resolved, pump-probe measurements in small domains, were in the 30-70 ps range for 4-methoxy substituted and in the 40-100 ps range for unsubstituted tetraphenylporphyrin, which are of the same order as the reported far-field results for 4-sulfonated compound in water.

Mesoscopic properties of J-aggregates of watersoluble [tetrakis(4-sulfonatophenyl)porphyrin] and water-insoluble [tetrakis(4-methoxyphenyl)porphyrin and tetraphenylporphyrin] tetraphenylporphyrin derivatives in thin films have been investigated by scanning near-field optical microscopy. From surface topography, the sample films were found to be composed of planar J-aggregate microcrystallines. The transmittance spectra of the water-insoluble samples are site specific, while those of the water-soluble samples are not. This observation might reflect the spatial inhomogeneity of the thin films in mesoscopic scales and is related to the broad J-bands of water-insoluble samples. Excited state lifetimes, obtained from time-resolved, pump-probe measurements in small domains, were in the 30-70 ps range for 4-methoxy substituted and in the 40-100 ps range for unsubstituted tetraphenylporphyrin, which are of the same order as the reported far-field results for 4-sulfonated compound in water.

The transition metal-benzene (1:1) complexes are synthesized by laser evaporation method followed by the size and structure selection with 2-m long electrostatic hexapole field. Electric dipole moments of half-sandwich type binary complexes for a series of the transition metals, i.e. Ti, V, Co, and Ni, with benzene molecule were determined by measuring the focusing curves, namely the dependence of the focused beam intensity upon the hexapole field strength. It is found that the dipole moments of Ti-C6H6 and V-C6H6 are larger than the ones of Co-C6H6 and Ni-C6H6. Such large dipole moments of Ti-C6H6 and V-C6H6 can be ascribed to the strong orbital interaction due to charge transfer between the metal atom and benzene molecule. (C) 2004 Elsevier B.V. All rights reserved.

Mesoscopic properties of water-insoluble tetrakis(4-methoxyphenyl)porphyrin J-aggregate in thin film have been investigated by time-resolved scanning near-field optical microscopy. The film was found to be composed of planar microcrystallines with lengths of a few micrometers, widths of similar to100 nm, and similar to20 nm heights. The transmittance spectra are site-specific. This observation might reflect inhomogeneity of the sample, and is related to the broad J-bands compared to those of tetrakis(4-sulfonatophenyl)porphyrin. Excited-state lifetimes, obtained from pump-probe measurements in small domains, were ranging from 10 to 50 ps that are of the same order with the far-field results. (C) 2003 Elsevier B.V. All rights reserved.

Supersonic beam of size-selected Ti-C6H6 metal-ligand binary complex was generated by a laser evaporation method followed by the size selection with 2-m long electrostatic hexapole field. The dependence of the beam intensity upon the hexapole field strength was measured in order to determine permanent dipole moment and complex symmetry. We find that the structure of Ti-C6H6 complex has nearly C-6v symmetry and its electric dipole moment is 2.4 +/- 0.3 D. The calculation with use of density functional theory substantiated its optimized structure as C-6v symmetric in accordance with the experimental result. The calculation also suggests that charge transfer interaction between Ti Atom and C6H6 plays a central role to stabilize the complex. (C) 2003 Elsevier Science B.V. All rights reserved.

Penning ionization of HCl upon collision with metastable He*(2(3)S) atoms was studied by two-dimensional (collision-energy/electron energy resolved) Penning ionization electron spectroscopy and by classical trajectory calculations. Collision energy resolved studies enable us to obtain important information about stereo dynamical aspects on this reaction; sideways approaches of the He* atoms on H-Cl molecular axis play a dominant role in the ionization event for producing HCl+(X (2)Pi(i), v' = 0, 1), and glancing collision is one of the dominant trajectories of the He* atoms to produce HCl+(A, (2)Sigma(+)). It was also found that the formation of vibrationally excited HCl+(X (2)Pi(i), v' &gt; 1) through the unidentified HCl** Rydberg state was influenced by a strong attractive interaction, whereas the formation of the HCl** Rydberg state, dissociating to H(1s) and autoionizing Cl**(D-1(2) nl) atoms, was found to be affected by a repulsive interaction around the Cl end of the HCl molecule.

Two-dimensional (collision-energy/electron-energy-resolved) Penning ionization electron spectroscopy (2D-PIES) have been applied to the reaction of 1,3,5-C6H3F3 and C6F6 with metastable He*(2(3)S) atoms. Collision energy dependence of the partial ionization cross sections (CEDPICS), which reflects interaction potential energy between the molecule and the He*(23S) atom, indicated anisotropic interaction around the molecules. Assignments of the Penning ionization electron spectra and He I ultraviolet photoelectron spectra have been made by the characteristics of the 2D-PIES. Furthermore, substituent effects on the reactivity of molecular orbitals and also on the interactions around the molecules for various fluorobenzenes were investigated. It was found that the reactivity of the molecular orbitals were closely related to the amount of F atomic orbital components in the orbital. Furthermore, an elucidation of the substituent effect on the interaction behaviors around the molecules gives us important insights on the dynamics of the colliding particles.

Ionization of dichlorobenzenes with metastable He*(2(3)S) atoms was studied by two-dimensional (collision-energy/electron-energy-resolved) Penning ionization electron spectroscopy. Collision energy dependence of the partial ionization cross sections (CEDPICS), which reflects the interaction potential energy between the molecule and He*(2(3)S), showed anisotropic interaction around the molecules. The pi and n (nonbonding) orbitals regions of the molecules showed attractive interactions. It was also found that the magnitude of lattractive interaction around the Cl atom lone pair region perpendicular to the phenyl ring was o- &gt; p- &gt; M-C6H4Cl2, Negative slopes of CEDPICS for sigma type bands indicate an attractive effect for ionization reaction by a substitution of the Cl atom.

Penning ionization of C2H5X (X = Cl, F) upon collision with metastable He*(2(3)S) atoms was studied by two-dimensional (collision-energy/electron energy resolved) Penning ionization electron spectroscopy. Partial ionization cross sections are found to be larger for ionization from orbitals having n(x) characters. Collision energy dependence of the partial cross sections, which reflects interaction potential energy between the molecule and He(2(3)S), indicates anisotropic interaction potentials around the molecule. As elucidate with the aid of calculated energy surfaces for the chemically related systems Li-C2H5X (X = Cl, F), a different trend was found in the interaction around C-X axis: for the former the attractive interaction was dominated around the perpendicular directions to the C-CI bond axis, while for the C-F bond the attractive interaction was localized around the collinear axis.

Penning ionization of difluorobenzenes upon collision with metastable He*(2(3)S) atoms was studied by two-dimensional (collision-energy/electron-energy-resolved) Penning ionization electron spectroscopy. Collision energy dependence of the partial ionization cross sections (CEDPICS), which reflects interaction potential energy between the molecule and He*(23S), showed anisotropic interaction around the molecules. Assignments of the Penning ionization electron spectra and He I ultraviolet photoelectron spectra were discussed on the basis of different behavior of CEDPICS and calculated ionization potentials by the outer valence Green's function (OVGF) method. Furthermore, the ordering of the reactivity of the n(//) and sigma (CF) orbitals and the magnitude of attractive interaction around the F atoms with the metastable atom was found to be o- &gt; m- similar to p-C5H4F2 It indicates that the wider potential well brings the larger attractive effect for Penning ionization reaction.

Penning ionization of C6H5X (X = F, Cl, Pr, I) upon collision with metastable He*(2(3)S) atoms was studied by two-dimensional (collision-energy/electron-energy-resolved Penning ionization electron spectroscopy. Partial ionization cross sections are found to be larger for ionization from orbitals having n and Jr characters. Interaction potentials between the target molecule and the He* atom were found to be highly anisotropic. Attractive interaction was dominated around the collinear access of He*(2(3)S) to C-F axis in C6H5F On the other hand, attractive interaction was localized around the out-of-plane perpendicular approach of He* atom to C-X bond (X = Cl, Pr, I). Attractive interaction for these compounds increases on going from C6H5Cl to C6H5I. Furthermore, the electronic factor due to the size of the halogen p orbitals and the conjugation between the benzene ring and the halogen atoms and also the steric factor due to the shielding effect were found to be important.

Penning ionization of Ne(3P) metastable atom with size-selected HCl dimer was studied by using an electrostatic hexapole field and chemiluminescence detection. We found that the HCl dimer can also produce HCl+(A) ions in the reaction with Ne(3P) just like its corresponding monomer reaction. The internal energy distribution of HCl+(A) in the dimer reaction is cooler than that in the monomer reaction, reflecting the third-body effect due to the other HCl in the dimer.

A supersonic cluster beam containing isomers of Al(C6H6) complexes was generated by laser evaporation, and non-destructively selected using a 2 m long electrostatic hexapole. The focusing curve shows clear evidence that there are two kinds of Al(C6H6) isomers which are slightly different from each other in geometry; one is an asymmetric 1,2-complex and the other nearly C-6v symmetric 1,4-complex. The electric dipole moments of the two isomers are found to be 1.5 +/- 0.1 and 1.4 +/- 0.1 D, respectively. We carried out computation using density functional theory in order to estimate their structures. We found that the 1,2-complex is more stable than the 1,4-complex. The present work confirms that the electrostatic hexapole technique is useful for non-destructive selection of geometrical isomers in a beam.

The supersonic beams of the (1-1) metal-ligand complexes of Al-CH3CN and Al-NH3 were produced by a laser evaporation method. Nondestructive structure selection of the complexes and the dipole moment determination were performed by using a 2-m electrostatic hexapole field. The experimentally determined permanent dipole moments are 1.2 ± 0.1 D for Al-CH3CN and 2.7 ± 0.2 D for Al-NH3. We find that the dipole moment of Al-NH3 becomes larger than that of neat NH3, while the formation of the Al-CH3CN complex produces a smaller dipole moment than that of neat CH3CN on the other hand. We performed the ab initio calculations to draw out plausible complex structures and to clarify the bonding character after formation of the complex, and we made comparisons with the computational results done by several groups. The Mulliken population analysis suggests the Al→CH3CN charge flow, but on the other hand the Natural population analysis indicates very little charge flow. For the Al-NH3 complex, the polarization effect of NH3 and the N→Al σ donation would enhance the dipole moment strength. However, there still remains a controversial disagreement between the theoretical predictions and the experimental results. Further experimental determination using the hexapole method for various metal-ligand complexes and clusters could reveal the basic nature of interaction in the complex systems in general, and this method would complement theoretical calculations.

The tunneling motion in (HCl)(2) hydrogen bonded dimer and its deuterate was probed by a 2 m long electrostatic hexapole field. The focusing curves of the dimers confirmed the existence of homo and heterodimers in the cluster beam. The homodimer, either (HCl)-Cl-35-(HCl)-Cl-35 Or (HCl)-Cl-37-(HCl)-Cl-37, undergoes a fast tunneling motion for the two hydrogen atoms in the dimer. The heterodimer, namely (HCl)-Cl-35-(HCl)-Cl-37, On the other hand, does not show such fast tunneling motion in the time scale of the experiment. The electric dipole moments for both (DCl)(2) isotopomers were determined to be 1.5 +/- 0.2 D, which is the same value for (HCl)(2). The observed ratio of homo to heterodimer was estimated to be 30 +/- 10, and this value differs largely from the natural abundance for the chlorine isotope.
An experimental scheme to discern homo and heterodimers is proposed here. By looking at fragments in the (HCl)(2) dimer photodissociation using a Doppler-selected time-of-flight (TOF) technique, internal energy distribution of the [ClHCl] fragment was measured in 121.6 nm photodissociation. The TOF spectrum consists of fast and slow velocity components for the dissociated H atoms. It is found that the slow H component that arises from the hydrogen escapes after many collisions. The fast H component that arises from the direct H escape without any collision, thus this component reflects an internal and/or electronic state of the counter part fragment, i.e. [ClHCl]. The vibrational structure of [ClHCl] was observed for the fast H component of the TOF spectrum. (C) 2000 Elsevier Science B.V. All rights reserved.

The reaction of O(D-1) with water and water clusters was re-examined. We monitored the nascent product state distributions in the reaction photo-initiated by the dissociation of N2O at 193 and 212.8 nm, and the corresponding photo-initiated intracluster reaction. The study at two different dissociation wavelengths and the use of D2O allowed us to obtain direct information on the effect of initial collision kinetic energy on the energy distribution in the product. Based on the new results obtained we conclude that the reaction of O(D-1) with water occurs through abstraction mechanism with a relatively short lived collision complex. In the case of the intracluster reaction, we have indication that more internal energy is deposited in the N-2 moiety, compared to the dissociation of an isolated N2O. In addition the results indicate that the reaction between the oxygen atom and the water in the complex involved the formation of a short lived collision complex, with a lifetime of probably only few rotations of OH. (C) 1999 American Institute of Physics. [S0021-9606(99)01732-8].

The focusing of HCl and DCl dimers was observed using a 2-m-long electrostatic hexapole field. The results indicate the existence of two types of species. The first is the homodimers, either the (HCl)-Cl-35-(HCl)-Cl-35 or the (DCl)-Cl-35-(DCl)-Cl-35, for which the data indicate a fast tunneling motion. The second is the heterodimers, (HCl)-Cl-35-(HCl)-Cl-37 or (DCl)-Cl-35-(DCl)-Cl-37, that do not show evidence for significant tunneling motion on the time scale of the experiment. In the case of HCl dimers, even at relatively high fields, only one species could be focused, the heterodimer. The electric dipole moments for both (DCl)(2) isotopomers were determined to be 1.5+/-0.2 D, which is the same value as observed for (HCl)(2). (C) 1999 American Institute of Physics. [S0021-9606(98)02048-0].

An active control for changing the relative population of the 3P0 fine-state of Ar* in a supersonic beam was investigated in a glow discharge by adding a small amount of foreign gases to pure Ar gas. The Ar* mean velocity was determined to be constant at major positions of the time profile, except for a rise and the decay of the beam for all gas mixtures, and also pure Ar. The relative population of 3P0 decreased with the discharge potential for the addition of ethylene, cyclohexane, and benzene. However, the addition of N2 and CH4 and pure Ar, did not show such a 3P0 decrease. These dissimilar behaviors of the foreign mixed gases could be interpreted based on a difference in the ionization potential of the molecules. A useful application of this population control is discussed.

The formation ratio of the two spin-orbit states, P-3(0)/P-3(2), of the metastable argon, Ar*, was investigated by the glow discharge of a pulsed Ar supersonic beam. The P-3(0)/P-3(2) ratio decreased with the increase of the glow discharge potential V-g, by which the electron bombardment initiates the discharge. The observed V-g-dependence clearly showed an inverse behavior against V-g as compared with that in the conventional electron impact by low beam densities.