Goro Mizutani

Detection of starch granules in a living plant by optical second harmonic microscopy

We demonstrate a new method for detecting starch granules in a plant, based on optical second harmonic microscopy. In this method a plant is irradiated with infrared laser pulses and the image formed by scattered light at the second harmonic frequency is observed. A starch granule consists of well-ordered arrays of chiral glucose molecules and shows an intense optical second harmonic response. Thus they are selectively seen in the SH intensity image.

Sum Frequency Generation Microscopy Study of Cellulose Fibers

Sum frequency generation (SFG) microscopy images of cotton cellulose fibers were observed at the infrared wavenumber of ∼ 2945 cm−1 and with a spatial resolution of 2 μm. Domains of different cellulose microfibril bunches were observed and they showed different second-order nonlinear responses. The intensity of the peak of the asymmetric CH2 stretching mode at 2945 cm−1 depended strongly on the orientation of the electric fields of the incident visible and infrared light with respect to the cellulose fiber axis. The second-order nonlinear susceptibility arising from the chirality in the cellulose structure was found to be dominant. The SFG of the cross section of the cellulose fiber was relatively weak and showed a different spectrum from that measured from the side of the fiber axis.

Discovery of deep and shallow trap states from step structures of rutile TiO2 vicinal surfaces by second harmonic and sum frequency generation spectroscopy.

In this report, local electronic structures of steps and terraces on rutile TiO(2) single crystal faces were studied by second harmonic and sum frequency generation (SHG∕SFG) spectroscopy. We attained selective measurement of the local electronic states of the step bunches formed on the vicinal (17 18 1) and (15 13 0) surfaces using a recently developed step-selective probing technique. The electronic structures of the flat (110)-(1×1) (the terrace face of the vicinal surfaces) and (011)-(2×1) surfaces were also discussed. The SHG∕SFG spectra showed that step structures are mainly responsible for the formation of trap states, since significant resonances from the trap states were observed only from the vicinal surfaces. We detected deep hole trap (DHT) states and shallow electron trap (SET) states selectively from the step bunches on the vicinal surfaces. Detailed analysis of the SHG∕SFG spectra showed that the DHT and SET states are more likely to be induced at the top edges of the step bunches than on their hillsides. Unlike the SET states, the DHT states were observed only at the step bunches parallel to [1 1 1] [equivalent to the step bunches formed on the (17 18 1) surface]. Photocatalytic activity for each TiO(2) sample was also measured through methylene blue photodegradation reactions and was found to follow the sequence: (110) < (17 18 1) < (15 13 0) < (011), indicating that steps along [0 0 1] are more reactive than steps along [1 1 1]. This result implies that the presence of the DHT states observed from the step bunches parallel to [1 1 1] did not effectively contribute to the methylene blue photodegradation reactions.

Optical second-harmonic spectroscopy of Au nanowires

We have measured the optical second-harmonic (SH) intensity spectra from an array of Au nanowires of average width 60 nm on a faceted NaCl(110) template. For the s-polarized input and p-polarized output polarization combination, the SH intensity with the incident field parallel to the nanowire axes was much stronger than that with the incident field perpendicular to the axes. The anisotropy is attributed to the anisotropic depolarization field in the Au nanowires.

Surface optical second harmonic generation from rutile TiO2(110) in air

We have measured the azimuthal angle dependence of the SH intensity from the (110) face of TiO2 (rutile) in air at the fundamental photon energy of 2.33 eV. The pattern of the SH intensity shows two-fold symmetry for the p -in/ p -out polarization configuration, four-fold symmetry for the p -in/ s -out polarization configuration, and two-fold symmetry for the s -in/ p -out polarization configuration. From the results of the experimental and theoretical analyses we have concluded that the observed SH light is generated from the surface electric dipole polarization.

Normal mode analysis of surface adsorbed and coordinated pyridine molecule

We have carried out a normal mode analysis of the molecular vibrations of pyridine adsorbed on silver surfaces or coordinated to various molecules and ions. There are two types of bonding that result in distinct shift patterns of the normal mode frequencies upon adsorption or coordination. ‘‘Metal ion‐type bonding’’ is characterized by a strong pyridine–substrate bond which results in large frequency shifts, while ‘‘halogen‐type bonding’’ causes comparatively smaller shifts due to weaker bonding. Adsorption on silver surfaces corresponds to weak halogen‐type bonding, and the major cause of the observed frequency shift is the electron transfer (back donation) into the aromatic ring upon adsorption.

SHG observation of rutile TiO2(110)/H2O interface under UV light illumination

We have measured the SH intensity from the TiO 2 (110)/H 2 O interface under UV-light illumination as a function of the azimuthal angle of sample rotation about the surface normal. The SH intensity increases during UV-light illumination and then decreases in the dark. The SH intensity pattern as a function of the azimuthal angle remains unchanged during this change in absolute SH intensity. We suggest that some interaction between the UV generated surface defects and the interface Ti-O-Ti-O- chain enhances the optical transition in the chain without modifying the shapes of the wavefunctions.

Anomalous electro-migration of oxygen vacancies in reduced TiO2

Abstract We have found that the reduced rutile TiO 2 shows anomalous electrical property under a DC electric field of several tens of V/cm. When a DC electric field was applied to the sample of reduced TiO 2 , a single pulse of electric current was observed together with a constant background current. This pulsed electric current occurs only once when the polarity of the electric field was kept unchanged. However, when we changed the polarity of the electric field we again observed a pulsed electric current. This pulsed current is attributed to migration of oxygen vacancies in reduced TiO 2 . We have also found that the migration of oxygen vacancies is visually observable under a DC electric field. Namely, when a DC electric field was applied to the sample after repeated change of its polarity, the color of the sample around the negative electrode changed into dark blue due to higher density of oxygen vacancies, while the region around the positive electrode lost the color.

Azimuthal angle dependence of optical second harmonic intensity from a vicinal GaAs(001) wafer

Abstract We demonstrate that the tilt angle of a zinc blende type single crystal (001) wafer can be measured by optical second harmonic generation. The SH intensity patterns were analyzed for all four combinations of p - and s -polarized incidence and output, considering both the bulk and surface optical nonlinearities in the electric dipole approximation. We found that the measurement using s -incident polarization is particularly useful in determining the tilt angle of the crystal axes. The parameters determined by the present method agree well with those obtained by X-ray diffraction measurements. The [110] and [1 1 0] directions can be distinguished through the analysis of the p -incident and p -output SH intensity patterns.

Absolute Raman scattering cross sections of surface adsorbed Cu-phthalocyanine molecules

We have measured the angle and polarization dependence of the absolute Raman scattering intensities of the 1340 and 1530 cm1 lines of Cu-phthalocyanine (CuPc) molecules adsorbed on the surfaces of an evaporated Ag film and a glass substrate with the incident laser line at 514.5 nm. The thickness of the CuPc layer was about a monolayer on the Ag substrate and five monolayers on the glass substrate. The scattering dipoles were found to lie parallel to the surface in both cases. The absolute molecular Raman cross sections were determined by including corrections for the effect of the substrate on the local incident field at the molecule and on the propagation of the scattered light. The Raman cross section of CuPc on the Ag substrate is about 150 times greater than that on the glass substrate. The relative enhancement of the Raman cross section on Ag is due to a resonant Raman effect arising from the electronic transitions that also cause absorption of the light at 514.5 nm.