Mai Takase

Raman Enhancement via Polariton States Produced by Strong Coupling between a Localized Surface Plasmon and Dye Excitons at Metal Nanogaps.

Polarized Raman scattering measurement was carried out using a hybridized system of Ag nanodimer structures and organic dye molecules. Tuning of the localized surface plasmon resonance energy leads to modulation of the hybridized polariton energy. The anticrossing behavior of the polariton energy implies a strong coupling regime with maximum Rabi splitting energy of 0.39 eV. The observation proves the effective Raman enhancement via the excitation of the upper and the lower branches of the hybridized states at the gap of the metal dimer. Maximum Raman enhancement was obtained at an optimized resonant energy between the hybrid states and Raman excitation.

Hyper-Raman scattering enhanced by anisotropic dimer plasmons on artificial nanostructures.

Silver nanodimers with a small gap of a few nanometers aligned on glass substrates were used to enhance hyper-Raman scattering of crystal violet dye molecules. When localized surface plasmon of the dimer array was resonantly excited along the interparticle axis, hyper-Raman intensity was significantly enhanced. Moreover, the spectral appearance was slightly different between the two excitation polarizations, suggesting a possibility of two resonance contributions at one-photon and two-photon energies. Since the plasmonic property of dimer arrays can be controlled by the dimer geometry, the dimer arrays are expected to be well-defined substrates for surface-enhanced hyper-Raman spectroscopy.

Plasmonically nanoconfined light probing invisible phonon modes in defect-free graphene.

We present a simple plasmonic method that enables tuning of accessibility to the dipole-forbidden transition states of matter. This technique is realized by well-controlled plasmonic dimers, which can confine optical fields on the order of molecular dimensions. As an example, the approach is applied to activate invisible noncenter phonon modes of defect-free graphene in resonance Raman spectra. The relative intensity of the normally forbidden modes with respect to the dipole allowed modes progressively increases as the degree of field confinement increases. This opens up a novel avenue for both photochemical excitation of molecular systems and nanoscale characterization of materials.

A fingerprint of metal-oxide powders: energy-resolved distribution of electron traps.

Here we propose a method for the identification of metal-oxide powders with the energy-resolved distribution of electron traps and conduction-band bottom position reflecting a surface structure and a bulk structure, respectively, as a fingerprint, based on the degree of coincidence for a given pair of samples, measured using newly developed reversed double-beam photoacoustic spectroscopy.

Plasmon-Based Optical Trapping of Polymer Nano-Spheres as Explored by Confocal Fluorescence Microspectroscopy: A Possible Mechanism of a Resonant Excitation Effect

In optical trapping using photon force much enhanced by localized surface plasmon (LSP) in solution, we found that a resonant excitation effect can further enhance photon force. In this LSP-based optical trapping under a resonant excitation condition, an incident laser beam excites both LSP and electronic resonant transition of a target object simultaneously. Fluorescence microspectroscopy clearly showed that nanospheres under the resonant condition were much more efficiently trapped as compared to that under a non-resonant condition. The resonant LSP-based trapping mechanism was further reinforced by theoretical calculations taking the resonant excitation effect into account. Such resonant LSP-based trapping methodology will provide a novel approach for efficient trapping of small molecules.

Production of Single- and Few-Layer Graphene from Graphite

Intensive research has been carried out over the past few years to find industrial-scale methods for the preparation of monolayer or few-layer graphene. However, large-scale, economical production of graphene with a low level of defects remains challenging. In this chapter, we review the research on several techniques for production of single- and few-layer graphene, particularly concerning mechanical exfoliation of high-quality graphene. We report our production scheme for graphite nanosheets from natural graphite. Crystalline graphite nanosheets were successfully produced from natural graphite powder by solution-phase synthesis of graphite intercalation compounds, following wet planetary-ball milling. We emphasize the high potential of graphene as a conductive composite film. Some composite films derived from phenolic resin and graphite nanosheets displayed much higher electrical conductivities than those of films from natural graphite particles. We also show that the stage structure of synthetic graphite intercalation compounds affected film conductivity.