Akira Harata

Laser‐induced surface acoustic waves and photothermal surface gratings generated by crossing two pulsed laser beams

Gigahertz surface acoustic waves have been optically generated and detected on aluminum metallic films and silicon wafers using the laser‐induced grating technique. The acoustic velocities were in good agreement with those of Rayleigh waves. Photothermal surface gratings could also be detected as a nonpropagating component of the thermoelastic waves. The relaxation time of the grating was related to the thermal diffusivity of the material. A simplified theory for the generation of surface transient gratings containing acoustic waves is presented in addition to the experimental results.

Subpicosecond surface-restricted carrier and thermal dynamics by transient reflectivity measurements

Ultrafast measurements of photoexcited carrier dynamics within a 60 nm subsurface of a crystalline silicon wafer were carried out using subpicosecond transient reflectivity. A uv pump light was employed to restrict carrier generation to occur within the subsurface by direct interband transitions. Carrier diffusion was found to be suppressed in the subsurface region of the intrinsic silicon wafer. For ion-implanted silicon wafers, heat was generated within a few picoseconds after the laser irradiation. By scanning a partially ion-implanted silicon wafer, the two-dimensional image was obtained, which showed that time-resolved imaging can separately map photoexcited carrier density and transient temperature rise. The possibility of three-dimensional process monitoring was considered as well.

Quantitative depth profiling with photoacoustic spectroscopy using a new approximation method based on inversion of the Laplace transform

A new and simple approximation method for quantitative depth profiling using photoacoustic spectroscopy (PAS) is described. In the method, an approximate calculation of the inversion of the Laplace transform is adopted in order to simplify the calculation of the depth‐dependent optical‐absorption coefficient from photoacoustic frequency responses. Some results of simulations shown demonstrate the effectiveness of this method. The method has been applied to the depth profiling of the practical sample in which the optical‐absorption coefficient varies continuously with the depth from the surface. The result is qualitatively consistent with microscopic observations of the cross section of the specimen. This approximation could be useful not only for the depth profile with PAS but also for the general depth profile calculations based on the Laplace transformation.

Theory of Transient Reflecting Grating in Fluid/Metallic Thin Film/Substrate Systems for Thin Film Characterization and Electrochemical Investigation

A general theory was developed to calculate the diffraction signals from a transient reflecting grating (TRG) at metallic thin film/substrate interfaces and liquid/solid interfaces. The TRG signals for an Au film/soda lime glass system were experimentally measured and theoretically calculated to examine their agreement. It was found theoretically and experimentally that longitudinal acoustic pulses and echoes with ultrahigh-frequency components (>10 GHz) propagating normal to the sample surfaces are generated and can be detected at the surfaces of thin films, using the TRG technique, along with surface acoustic waves. By using the time intervals of two neighboring longitudinal acoustic echoes, thicknesses or longitudinal acoustic wave velocities for opaque thin films were determinable. Furthermore, it was shown that this method is applicable to the analysis of electrochemical interfaces in situ, through changes in the thermal and longitudinal acoustic properties of electric double layers with electrochemical potentials.

Study of the adsorptive behavior of water-soluble dye molecules (rhodamine 6G) at the air-water interface using confocal fluorescence microscope

A confocal fluorescence microscope was applied to directly study the characteristic behaviors of adsorbed molecules at the air-water interface for a water-soluble chromophore, rhodamine 6G (R6G), in its extremely low-concentration region (below 10(-10) M). Significant photon bursts were observed only from the surface, and their width, height, and frequency were found to depend on the bulk concentration, suggesting the inhomogeneous distribution of R6G molecules at the air-water interface. This property of the adsorbed molecules is different from that of the bulk one. The influence of the ionic strength on photon bursts from the interface was investigated. It was found that the addition of NaCl to the R6G solution caused a decrease of the fluorescence signal. A change in the size of the aggregate and in the fluorescence quantum yield of the adsorbed molecules was suggested to be responsible for this experimental result.

Ultrafast lensing effect of β-carotene in n-hexane solution

Abstract An ultrafast lensing effect of photo-induced beam focusing and defocusing was observed for dilute all-trans-β-carotene solutions in n-hexane. Sub-picosecond pump and probe pulses were used with a low power semi-resonant excitation at 390 nm and a non-resonant probe at 780 nm. High speed averaging with intensity-modulated mode-locked pulse trains successfully provided a double exponential response where the pre-exponential factors had different signs. The time constants were 0.2 and 10 ps, respectively corresponding to those of two excited states of β-carotene. Similarity in lensing intensity dependence between the ultrafast effect and a slower thermal effect was also examined.

Study of electrochemical interfaces by transient reflecting gratings

Abstract Picosecond photothermal/photoacoustic measurement of gold interfaces in some aqueous solutions of electrolytes was carried out using a transient reflecting grating (TRG) method. As the electric potential became more anodic, the TRG-induced diffracted-light intensity increased above that expected from the reflectivity change, and the transient waveform also changed, both in the electrical double-layer and Faradaic oxidation regions. These changes were related to the anions in the supporting electrolyte. These results are discussed with respect to the photothermal and photoacoustic effects at the nanometer-scale interface.

Ultrafast energy transfer of silver ultrafine particles in aqueous solutions as investigated by the ultrafast lensing effect technique

Abstract Ultrafast relaxation dynamics of silver ultrafine particles in aqueous solutions was measured by an ultrafast lensing effect (ULE) method using two types of samples with SDS (sodium dodecyl sulfate) and without SDS. The fit to the ULE time evolution required three exponential components: one for the increase and two others for the decrease. The time constants were estimated as 1, 3, 7 ps for the sample without SDS and as 1, 2, and 23 ps for the sample with SDS. The ultrafast relaxation dynamics was discussed in connection with the behavior of photoexcited electrons.

Analysis of the thermal and acoustic properties of ion-implanted diamondlike carbon films using the transient reflecting grating technique

The transient reflecting grating (TRG) technique was applied to evaluate both nitrogen‐ion‐implanted and nonimplanted thin‐film coatings (thickness: 40 nm) of diamondlike carbon (DLC) on metallic multilayered substrates. Different TRG responses corresponding to the different nitrogen ion doses were observed. Hypersonic surface wave velocities as well as the thermal diffusivities were obtained with a variety of grating spacings. Effective elastic moduli (Young’s moduli) for the DLC coatings were given by using a theoretical treatment with a model. Nitrogen ion implantation hardened the DLC coating.

Evaluation and imaging of materials using picosecond laser-induced ultrasonics

We have developed a novel microscopic method named laser-stimulated scattering microscopy for the nonde-structive evaluation of surface modification of materials. It is a kind of laser microscope based upon space- and time-resolved transient reflecting gratings.