The progress in femtosecond laser has enabled us to observe coherent motions of lattice and molecular vibrations in solids. Nevertheless the study of coherent phonons in Si, which is an important material for modern devise application, has not been made so far. We report the dynamics of transient interaction of electrons with phonons, followed by generation of coherent optical phonon, in which the pump-probe technique with 400 nm, 10 fs laser is used for the observation. A comparison of the results is made with other semiconductors.
Infrared-visible sum-frequency generation (SFG) spectroscopy is a modern surface vibrational spectroscopy that uses non-linear optical processes with ultra-short laser pulses. An advantage of the SFG spectroscopy is its ultra-short time resolution, which enables the observation of vibrational spectra on the picosecond time scale. Irradiation with near-infrared (NIR) pulses causes a temporary temperature jump at metal surfaces, with the surface temperature jumping by several hundred degrees, before recovering in the time of sub-nanosecond. The surface species is thermally excited by the irradiation and sometimes changes its chemical structure, without desorbing from the surface. This technique is regarded as a powerful tool for the identification of unstable states of surface molecules involved in thermal reactions. This article describes the dynamic behavior of surface molecules under the irradiation with NIR pulse for the CO/Ni(111), cyclohexane/Ni(111), and D2O/CO/Pt(111) systems.
In this paper, we focus on our recent study on vibrational coherence and coherent phonon of Cs or K adsorbed on Pt(111) by using femtosecond time-resolved second harmonic generation. The dynamics of vibrational coherence of alkali-Pt stretching mode is observed in time-domain and the mechanism of vibrational dephasing is discussed. We also study an effect of pulse shaping on a mode-selective coherence excitation.
A metallic probe having a nano-scaled tip strongly enhances the optical field in the local vicinity of the tip apex. Using this effect, which is analogous to the surface enhanced Raman scattering, we can excite Raman scattering in a nanometric region, and consequently realize nano-identification, nano-analysis and nano-imaging of molecules and crystals. In this article, we introduce the mechanism and recent results of the tip-enhanced near-field Raman scattering spectroscopy. In particular, unique spectral shapes observed in the tip-enhanced Raman scattering spectra are discussed via a quantum chemical calculation. We also demonstrate that tip-enhanced spectroscopy has been applied to nonlinear Raman scattering and has achieved molecular nano-imaging with a high resolution and a high sensitivity.
A scanning tunneling microscopy study, combined with high-sensitive detection of desorbed neutral species, has revealed that laser-light excitation induces electronic bond breaking of constituent atoms at intrinsic sites on covalent semiconductor surfaces. The bond breaking forms vacancies at individual surface sites, and leads to desorption of atoms in the electronic ground state with translational energies characteristic of electronic process. The bond-breaking rate shows prominent site- and species-dependent effects, and depends super-linearly on the excitation density. The excited species responsible for the electronic process has been identified to be valence holes, and the characteristics experimentally observed are explained reasonably in terms of two-hole localization.
The intense electromagnetic field produced by a focused laser beam attracts microparticles, dispersed in liquid, into the focal spot. This is known as optical trapping and used to manipulate microparticles dispersed in liquid. If the size of the particles is much smaller than that of the focal spot, i.e. the order is of wavelength, a number of particles are trapped in the focal spot. This means that the focal spot of the laser beam acts as a microcage for nanoparticles and the confined nanoparticles form a microassembly. Nanoparticles are attractive materials for their characteristic chemical, physical, and optical properties. We have studied “optical assembly” of nanoparticles and found the following unique properties. (1) By a single particle counting measurement, it was revealed that the assembling process of polystyrene nanoparticles is dependent on laser power and concentration. (2) It was demonstrated that the assembly structure and optical properties of gold-nanoparticles can be controlled by optical force.
Surface structuring of optically transparent materials toward nanometer-scale resolution for the fabrication of optics and micro-chemical devices has been performed by UV and Vacuum UV laser Ablation. Recently, we have investigated a one-step method to fabricate a microstructure on the transparent materials such as silica glass, sapphire, and fluoride crystals by using a conventional UV laser under the atmospheric pressure. Laser-Induced Backside Wet Etching (LIBWE) fabricates microstructures with well-defined micro-patterns on the surfaces. Our idea of LIBWE is based on an indirect excitation onto the surface of materials by using ablation of organic solutions such as dye solution or toluene. Upon UV laser irradiation of the solution, shock wave and vapor bubble were formed and propagated. These transient phenomena are possible for the surface microstructuring of the materials.
Self-organized formation of ordered micro- and nano-structures of metals and semiconductors at solid surfaces has been attracting keen attention in view of nanotechnology. The self-organization method has an advantage over the photolithography and surface probe method in that it meets both the conditions of atomic-scale fabrication and the adaptability of mass production. Recent studies on non-equilibrium, nonlinear chemical dynamics have proved a large possibility of self-organized formation of a variety of ordered structures such as stripes, dot arrays, and target and spiral patterns. However, the patterns thus far reported are limited to those of two dimensions (2-D) lying parallel to the substrate surface. The formation of organized “vertical” structures and further organized 3-D structures will need novel strategy. From this point of view, oscillatory electrodeposition is an interesting target because it has an ability to produce ordered electrodeposits by recording ever-changing self-organized spatiotemporal patterns during oscillations. Here we review studies on the structurization by oscillatory electrodeposition, with a focus placed on the formation of layered structures.
For large area coating of architectural glass work with excellent quality and performance of self-cleaning, the sputtering method is suitable for the thickness uniformity and the appearance. Usage of a seed layer is illustrated to form the anatase structure of TiO2. The coating shows the good photo-activity and the excellent self-cleaning performance. This seed layer effectively makes the incomparable lattice matching to the anatase TiO2. Finally, it was found that the double side sputtering with high performance LowE coating is real process when the seed layer is utilized.