Yoshihiro Miyauchi

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.

Selective observation of starch in a water plant using optical sum frequency microscopy

The photosynthesis, transfer, and storage of starch are the most important biogenic processes occurring in plants. In order to observe the colorless and transparent starch granules in a plant, a chemical pretreatment such as staining of the starch is currently required, which seriously damages the tissue cells in the plant. We demonstrate that nondestructive chemical analysis of starch granules in a plant can be performed by using optical second-harmonic and sum-frequency microscopy. These techniques for in vivo analysis will provide extremely useful information about saccharides in a plant and can be extended to the analysis of many other materials, from living tissue to semiconductors.

Optical second harmonic generation from Pt nanowires with boomerang-like cross-sectional shapes

We have fabricated Pt nanowires with boomerang-like cross-sectional shapes on the MgO(110) faceted template and observed their optical second-harmonic generation (SHG) response. In the TEM images the Pt nanowires on the MgO substrate had macroscopic C2v symmetry, however, their structure had microscopic imperfections. In the SHG response, as a function of the sample rotation angle around the substrate normal, we found contributions from the nonlinear susceptibility elements χ113, χ223, χ311, χ322, and χ333 originating from the broken symmetry in the 3; [110] direction of the MgO substrate. The indices 1 and 2 denote the [001] and [11¯0] directions, respectively. Under C2v symmetry no SHG is expected in the s-in/s-out polarization configuration, however, a finite SHG was observed in this polarization configuration. We suggest that the SHG in the forbidden configuration might originate from the imperfections in the nanowire structure.

Wetting effect on optical sum frequency generation (SFG) spectra of d-glucose, d-fructose, and sucrose

We report a sum frequency generation (SFG) spectroscopy study of d-glucose, d-fructose and sucrose in the CH stretching vibration regime. Wetting effect on the SFG spectra was investigated. The SFG spectrum of d-glucose changed from that of α-d-glucose into those of α-d-glucose monohydrate by wetting. The SFG spectra showed evidence of a small change of β-d-fructopyranose into other anomers by wetting. SFG spectra of sucrose did not change by wetting. Assignments of the vibrational peaks in the SFG spectra of the three sugars in the dry and wet states were performed in the CH stretching vibration region near 3000cm(-1).

Optical second harmonic generation at heterojunction interfaces of a molybdenum trioxide layer and an organic layer

We have observed optical second harmonic generation (SHG) from a space charge layer (SCL) in a stacked indium tin oxide (ITO)/molybdenum trioxide (MoO3)/N−N′-diphenyl-N−N′-bis(1-naphthly)-1,1′-biphenyl-4,4′-diamine (α-NPD) system. When the MoO3 thicknesses were increased, the SHG signals from this system decreased sharply at smaller MoO3 thicknesses, and were saturated at MoO3 thicknesses larger than 1 nm. These results prove the vital role of SCL in improvement of drive voltages of organic light-emitting diodes.

Optical second harmonic generation from V-shaped chromium nanohole arrays

contribution to the observed anisotropic SHG intensity patterns. Here, coordinate 1 is in the direction of the tip of V shapes in the substrate plane, and 3 indicates the direction perpendicular to the sample surface. The SHG intensity for the S-polarized output light was very weak, probably owing to the cancellation effect of the image dipoles generated at the metal-air boundary. The possible origin of the observed nonlinearity is discussed in terms of the susceptibility elements obtained.

Influence of the oxide thickness of a SiO2/Si(001) substrate on the optical second harmonic intensity of few-layer MoSe2

The nonlinear optical properties of few-layer MoSe2 on a SiO2/Si substrate were investigated with our optical second harmonic generation (SHG) microscope. Few-layer flakes were mechanically exfoliated from a single crystal onto a 90- or 270-nm-thick SiO2-coated Si(001) substrate. The polar plot of the second-harmonic (SH) intensity from a mono- or trilayer MoSe2 flake as a function of the rotation angle of incident polarization shows a threefold symmetry, indicating that the isolated few-layer flakes retain their single crystallographic orientation. SHG spectra were found to depend strongly on the oxide thickness of the substrate (90 or 270 nm), which was interpreted using the interference among the multiply reflected SH light beams in the system. By taking this interference into account, a resonant peak may be identified at a two-photon energy of equal to or less than 2.9 eV in an SHG spectrum. The spatial resolution of the SHG microscope was estimated as 0.53 µm.

Lateral Resolution of an Infrared Visible Optical Sum Frequency Generation Confocal Microscope

We have demonstrated experimentally that an infrared visible sum frequency generation (SFG) confocal microscope with an objective lens of numerical aperture 0.45 has a lateral resolution of 0.48±0.06 µm. As samples for the demonstration, we used a ZnS(100) wafer with a structure fabricated by a focused ion beam method and a ZnS polycrystalline pellet. The result was consistent with the theoretical resolution of a confocal microscope.

Modulation of Si-H vibrational mode as a function of the hydrogen coverage on a H-Si(111)1×1 surface

In this study, we have observed a SFG microscopic image of the H-Si(111)1×1 surface after pump IR light irradiation. As the results, resonant SFG signals attributed to Si-H stretching vibration disappeared at the irradiated areas, and non-resonant signals were generated at these areas. We also found that there were boundary areas between laser irradiated and non-irradiated areas. Both SFG signals were weak at the boundary areas. As the reason of these modulations of SFG signals, we assumed that the temperature did not sufficiently increase for hydrogen desorption at the boundary areas, since the areas were located at edges of spatial distribution of the pump laser intensity. In order to verify the assumption, we heated the Si surface at 711K, slightly above threshold temperature for activation of hydrogen desorption, and then observe SFG spectra of the Si surface as a function of hydrogen coverage at room temperature. As the results, the intensity of the peak at 2083.7cm-1 attributed to Si-H stretching vibration reduced as a function of hydrogen deficiency. Also, the peak position shifted toward red side. We simulated dipole-dipole interaction by using coherent potential approximation (CPA) method, and the simulated peak shift was qualitatively consistent with the experimental one. Thus, the peak shift corresponded to dipole-dipole interaction. On the other hand, the experimental peak width also broadened with the coverage reduction. However, the broadening was far wider than that of theoretical width. We suggest that the broadening was attributed to local structure defects and/or influence of neighbor dangling bonds.