Three-Dimensional Visualization of Chiral Nano-Optical Field around Gold Nanoplates via Scanning Near-Field Optical Microscopy.

In this study, we examine the three-dimensional chiral optical field in the vicinity of a gold nanoplate using aperture-type scanning near-field optical microscopy. Near-field imaging indicates that the chiral optical field shows a unique spatial distribution and depends on the incident polarization. We also evaluate the modal dependence of chiral optical fields, which reveals that the plasmon mode with E symmetry contributes substantially to the chiral optical field while that with A symmetry contributes little because of the high spatial symmetry.

Plasmon dephasing time and optical field enhancement in a plasmonic nanobowl substrate studied by scanning near-field optical microscopy.

Plasmonic substrates have been extensively investigated due to their potential applications in fluorescence microscopy, chemical sensing, and photochemical reactions. The optical properties of the substrate depend on the spatial and temporal features of the plasmon excited. Hence, the ability to directly visualize plasmon dynamics is crucial.

Nanoantenna effect dependent on the center structure of Bull's eye-type plasmonic chip.

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).

Observation of the plasmon mode transition from triangular to hexagonal nanoplates.

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.

Photoluminescence from Carbon Dot-Gold Nanoparticle Composites Enhanced by Photonic and Plasmonic Double-Resonant Effects.

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.

Near-field transmission and reflection spectroscopy for revealing absorption and scattering characteristics of single silver nanoplates.

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.

Spatial characteristics of optical fields near a gold nanorod revealed by three-dimensional scanning near-field optical microscopy.

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.

Space-Selective Fabrication of Light-Emitting Carbon Dots in Polymer Films Using Electron-Beam-Induced Chemical Reactions.

Nanocarbon-based materials have excellent properties, including high electrical conductivity as well as charity-dependent optical absorption and luminescence; therefore, the materials are promising in applications for nanoelectric devices, nanophotonics, and so on. Carbon dots (CDs) are one of the carbon materials recently fabricated. Optical properties of CDs have been reported to be similar to those of polycyclic aromatic hydrocarbons (PAHs).

Characterization of Overlapped Plasmon Modes in a Gold Hexagonal Plate Revealed by Three-Dimensional Near-Field Optical Microscopy.

A detailed characterization of plasmon modes is important not only for a deeper understanding of plasmons but also for their practical applications. In this study, we investigated the three-dimensional near-field characteristics of high-order plasmon modes excited in a gold hexagonal nanoplate. From the near-field spectroscopic images, we found that both in-plane and out-of-plane plasmon modes observed near 900 nm were spectrally and spatially overlapped.

Static and Dynamic Near-Field Measurements of High-Order Plasmon Modes Induced in a Gold Triangular Nanoplate.

Precise understanding of the spatiotemporal characteristics of plasmons is essential for the development of applications of plasmonic nanoparticles. In this study, we investigated the spatiotemporal properties of high-order plasmon modes induced in a gold triangular nanoplate by static and dynamic near-field measurements. The near-field transmission measurements revealed that in-plane and out-of-plane polarized plasmon modes were simultaneously excited and these modes spectroscopically and spatially overlapped.

Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance.

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.

Near-field spectroscopic properties of complementary gold nanostructures: applicability of Babinet's principle in the optical region.

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.

Multiple Resonances Induced by Plasmonic Coupling between Gold Nanoparticle Trimers and Hexagonal Assembly of Gold-Coated Polystyrene Microspheres.

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.

Observation of Plasmon Wave Packet Motions via Femtosecond Time-Resolved Near-Field Imaging Techniques.

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.

Local optical responses of plasmon resonances visualised by near-field optical imaging.

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.

Optical control of plasmonic fields by phase-modulated pulse excitations.

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.

Spatial distribution of enhanced optical fields in one-dimensional linear arrays of gold nanoparticles studied by scanning near-field optical microscopy.

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.

Strong optical coupling between mutually orthogonal plasmon oscillations in a silver nanosphere-nanowire joined system.

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.

Anomalous light transmission from plasmonic-capped nanoapertures.

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.

Two-photon imaging of localized optical fields in the vicinity of silver nanowires using a scanning near-field optical microscope.

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.

Near-field Raman imaging and electromagnetic field confinement in the self-assembled monolayer array of gold nanoparticles.

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.

Visualization of localized intense optical fields in single gold-nanoparticle assemblies and ultrasensitive Raman active sites.

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.

Morphological and spectroscopic properties of thin films of self-assembling amphiphilic porphyrins on a hydrophilic surface as revealed by scanning near-field optical microscopy.

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.

Near-field two-photon-induced photoluminescence from single gold nanorods and imaging of plasmon modes.

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.

Reciprocity in scanning near-field optical microscopy: illumination and collection modes of transmission measurements.

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.