Near-field Two-photon Excitation Research Articles

Plasmon-Enhanced Fluorescence Near Single Gold Nanoplates Studied by Scanning Near-Field Two-Photon Excitation Microscopy

Plasmon-Enhanced Fluorescence Near Single Gold Nanoplates Studied by Scanning Near-Field Two-Photon Excitation Microscopy

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. The superposition of these modes was visualized in the near-field two-photon excitation image of the nanoplate. We performed time-resolved autocorrelation measurements on the nanoplate and found that the correlation width was broader than the excitation pulse due to the plasmon dephasing process. From the correlation width map of the nanoplate, we experimentally demonstrated that the out-of-plane plasmon mode exhibits a longer dephasing time than the in-plane plasmon mode. These findings indicate that the out-of-plane mode is desirable for improving the performance of plasmons in various applications.

Nanooptical Studies on Physical and Chemical Characteristics of Noble Metal Nanostructures

Abstract Studies on physical and chemical properties of noble metal nanostructures, mainly by near-field optical microscopy and spectroscopy, are described. Near-field optical microscopy provides optical observation methodology with a spatial resolution in nanometer regime beyond the diffraction limit of light. We developed near-field imaging systems equipped with various light sources including ultrashort pulsed lasers, which enables advanced nonlinear and ultrafast near-field measurements as well as conventional near-field imaging. In particular, near-field two-photon excitation imaging was shown to provide a convenient tool to visualize the enhanced optical fields in the vicinities of metal nanostructures. With these methods we demonstrated that nanoscale optical field structures in metal nanostructures can be directly visualized. For single noble metal nanoparticles, such as gold nanorods as typical examples, plasmon standing wave functions were visualized. Ultrafast imaging revealed that sub-picosecond relaxation was reflected on the plasmon wave function images through thermally induced dielectric function changes of the metal. In some cases optical field distribution features arising from the lightning rod effects were observed, depending on the resonance conditions of the incident wavelengths with the plasmon modes. We also found anomalous near-field transmission phenomenon for nanoapertures blocked by nanodisks near the plasmon resonance wavelengths, which arise from the efficient near-field to propagating-field conversion ability of the nanodisks. In assembled nanoparticles, enhanced optical fields at the gap sites between the particles were visualized, which elucidates experimentally the mechanism of surface-enhanced Raman scattering. The characteristic field distributions in many particle assemblies were also observed and analyzed. Through these studies, we established a valuable methodology to investigate optical and spectroscopic properties of metal nanostructures, and the information obtained is available only by optical measurements with high spatial resolution.

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 647.1 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.

Optical Field Imaging of Elongated Rectangular Nanovoids in Gold Thin Film

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 close 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 de...

Nanoscale Two-Photon Induced Polymerization of Diacetylene Langmuir−Blodgett Film by Near-Field Photoirradiation

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 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.

Spatial distribution of enhanced optical fields in monolayered assemblies of metal nanoparticles: Effects of interparticle coupling

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.

Properties of Photoluminescence from Single Gold Nanorods Induced by Near-Field Two-Photon Excitation

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.

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. 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.

Near-Field Optical Imaging of Nanoscale Optical Fields and Plasmon Waves

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.

Nanophotonics: Nanoscale Optical Interactions

Nanophotonics defined as nanoscale optical science and technology is a new frontier, which includes nanoscale confinement of radiation, nanoscale confinement of matter, and nanoscale photophysical or photochemical transformation. Selected examples of our research work in each of these areas are presented here. Nonlinear optical interactions involving nanoscale confinement of radiation is both theoretically and experimentally studied using a near-field geometry. The effort in nanoscale confinement of optical domains is focused to control excitation dynamics and energy transfer as well as to produce photon localization using nanostructured materials, rare-earth work on doped glasses and novel inorganic-oganic photorefractive nanocomposites are presented. Spatially localized photochemistry using a near-field two-photon excitation is being developed for nanofabrication and nanoscale memory.