Tamitake Itoh

Femtosecond light scattering spectroscopy of single gold nanoparticles

We have developed an ultrafast, light-scattering spectroscopic system combining a conventional microscope with a pump–probe setup. We successfully measured the surface plasmon resonance band for the individual gold particles with a mean radius of 40 nm. The results on the pump–probe experiment demonstrate that both the electron–phonon and the phonon–phonon coupling processes in the individual gold particles take place with the lifetimes of 4 ps and >25u200aps, respectively.

Direct Demonstration of Environment-Sensitive Surface Plasmon Resonance Band in Single Gold Nanoparticles

We investigated single gold nanoparticles embedded in poly(vinyl alcohol) (PVA) films with different thicknesses using Rayleigh light-scattering microspectroscopy. First, gold nanoparticles with a mean radius of 40 nm were dispersed in a thin polymer film. Then, the film was covered with multiple layers of PVA to the desired thickness. As a result, we confirmed that the surface plasmon (SP) resonance band of single particles demonstrated redshift with the film thickness increase. In a thick film, the SP band peak position was well explained in terms of the size of the gold nanoparticles. In contrast, the spectral variance observed in a thin film did not follow the size-dependence trend. Since nanoparticles in a thin film are partially covered with PVA layers and thus are partially exposed to air, our results confirm that SP resonance strongly depends not only on a particles size but also on its surrounding dielectric conditions.

Time-resolved surface scattering imaging of organic liquids under femtosecond KrF laser pulse excitation

Time-resolved surface scattering imaging was performed for liquid benzyl chloride and liquid toluene under femtosecond KrF laser ablation conditions. No scattering image was obtained until 1 ns, while scattering started from 2 ns when the laser fluence exceeded 25u2009mJ/cm2, and its intensity increased with the passage of time. The higher the laser fluence was, the steeper the increasing slope was. The scattering is due to surface roughness, which is the initial stage of macroscopic morphological changes. Root-mean-square surface roughness was estimated from the scattering intensity by using frosted fused-silica plates as reference samples. The induced surface roughness increases to a few hundred nm in 10 ns.

Time-resolved ultraviolet–visible absorption spectroscopic study on femtosecond KrF laser ablation of liquid benzyl chloride

Abstract A time-resolved absorption spectroscopy system with an intense femtosecond KrF laser has been constructed and applied to liquid benzyl chloride under conditions of laser ablation. It has been directly confirmed over a wide fluence range that benzyl radical was formed immediately after excitation. In addition to the decay of benzyl radical, scattering of a probe light pulse was observed after ∼1–2 ns at higher fluences, which was ascribed to morphological changes; the threshold was determined to be 30 mJ cm −2 . The femtosecond KrF laser ablation molecular dynamics of liquid benzyl chloride is well explained in terms of a photochemical mechanism.

All-Solid-State Mirror-Dispersion-Controlled Sub-10 fs Ti:Sapphire Laser

We report what is to our knowledge the first use of a frequency-doubled cw diode-pumped solid-state laser to pump a mirror-dispersion-controlled Ti:sapphire laser. Stable, sub-10 fs duration pulses, with output powers of ~230 mW, were obtained for a pump power of ~4 W. The system (including the pump laser) fits on a 1.2×0.6 m optical breadboard.

Evaluation of probes for tip-enhanced Raman scattering by darkfield microspectroscopy and calculation

Tip-enhanced Raman scattering (TERS) can be observed highly sensitive spectral image with high spatial resolution. However, it shows low reproducibility due to difference and change in optical properties of the metallic tips. For surfaceenhanced Raman scattering (SERS), the spectra can be reproduced by the scattering spectra due to localized surface plasmon resonance (LSPR) of the individual metallic nanostructures, which observed with a dark field illumination, and the calculated electromagnetic field around the nanostructures. In the present study, we tried to relate TERS spectra with the LSPR spectra and the calculation, in a similar way of SERS. By conventional dark field illumination, LSPR scattering spectra at the apex of the tip were measured and were compared with the corresponding TERS spectra. By excitation using polarization parallel to the tip, the polarized LSPR peak was stronger than that by perpendicular polarization. Also in the case of TERS, the similar trend was observed. It was confirmed whether the vertical polarization to the sample plane (Z-polarization) is effective or not by the polarized LSPR and TERS spectra. By excitation at different wavelengths, moreover, TERS enhancement factors were compared. In the calculation for TERS, the nanostructure like a monopole antenna was adopted, because the EM field is enhanced not at both sides, but at only apex. The dependence on taper and curvature of the tip were compared with the calculated results for the nanostructure like a conventional dipole antenna.

Chapter 3 Ultrafast diffuse reflectance and Rayleigh scattering spectroscopies and their applications to TiO2 and gold nanoparticles

Publisher Summary This chapter describes the recent advances of time-resolved diffuse reflectance and scattering spectroscopies, which are indispensable for nanoparticle studies. The excited and intermediate states and transient chemical species have been identified, their relaxation or reaction rates have been determined directly, and dynamics and mechanism have been elucidated in electronic and molecular levels. Ultrafast decay relaxation processes can be followed directly by the diffuse reflectance spectroscopy. The pump pulse creates transient states during its propagation in the scattering medium, but excited states decay quickly and their homogeneous distribution is never achieved when their lifetime is very short. The time resolution of the present diffuse reflectance spectroscopy is in fs time scale. Titanium dioxide (TiO2) is considered one of the most useful photocatalysts and its photocatalytic reactivity strongly depends on the nature of the particles, such as crystal structures, particle sizes, defects, and impurities. As fluorescence spectroscopy is limited to fluorescent materials, absorption spectroscopy is another important method for general identification and the characterization of materials.