Tsuguo Sawada

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.

Pulsed UV Laser-Induced Stationary Capillary Vibration for Highly Sensitive and Direct Detection of Capillary Electrophoresis.

A stationary wave of the capillary vibration effect was successfully induced by a series of short laser pulses. This wave could be applied to highly sensitive detection of capillary electrophoresis as well as the already reported capillary vibration induced by an intensity-modulated CW laser (CVL effect). Generally, pulses with much shorter width than the period of the natural frequency of the vibrating system cannot induce a standing vibration. However, utilizing the time constant of CVL determined by heat dissipation time, we found conditions which could induce a stable stationary wave of the capillary by a series of nanosecond light pulses. We used the KrF excimer laser operated at 248 nm with a pulse width of 60 ns and output of ∼10 μJ/pulse as the CVL excitation source and applied it to highly sensitive detection of nonderivatized amino acids at the femtomole level. The sensitivity was at least 2 orders of magnitude superior to that of a commercially available UV absorbance detector. This technique extends the CVLs spectral regions. For example, in the UV region, where many biological materials have significant absorption bands, this technique will extend analytical applications in capillary electrophoresis by eliminating the need for a derivatization process.

Femtochemistry at Pt/aqueous solution interfaces as observed by transient reflectivity measurements

Abstract Photo-induced ultrafast responses of a polycrystalline Pt thin-film immersed in aqueous solutions were investigated by transient reflectivity measurements. Solvent dependence was observed to relate to the affinities solute anions had towards the metal surface. Numerical analysis showed a previously unknown relaxation component having a relaxation time of 20 fs. The experimental results were discussed in terms of an interface reaction between adsorbed solute anions and photoexcited electrons in the Pt film.

Hypersonic investigation of electrochemical interfaces

Abstract Evidence is presented that an electric double layer is observable with hypersonic waves. Potential-dependent behavior of hypersonic waves was investigated with the transient reflecting grating (TRG) method for a gold film (1 μm) electrode supported on a substrate in acidic solutions. Shapes of initial parts of the TRG-responses were dependent on the potential and composition of the solution. Theoretical calculation showed that longitudinal hypersonic echoes and heat diffusion dominate the initial part for the gold film. The acoustic wavelength of 22 nm is comparable to the thickness of the electric double layer whose structure is potential-dependent. Reflectivity of the hypersonic echoes depends on the potential.

Imagings of Picosecond-Photoexcited Carriers and Enhanced Auger Recombination Rate by Transient Reflecting Grating Measurements

Photoinduced dynamic processes at a silicon surface were investigated by time-resolved measurements of a transient reflecting grating with 532 nm excitation and detection. The signal caused by photoexcited carriers was separated from signals due to thermal and acoustic effects. The carrier signal was found to be more sensitive to ion induced damages than the thermal and acoustic effect signal. Use of the carrier signal provided an in-plane distribution image of near surface damage induced by helium ion implantation (energy, 200 keV; dose, 1015 atoms/cm2). The cause of the contrast formation was found to be the change of Auger recombination rate γ3. The obtained γ3 for intrinsic silicon was 4.0×10-29 cm6/s which was two orders of magnitude larger than the bulk value. The results indicated defects near the surface region ( ~100 nm) accelerated γ3.

Phase signal of optical beam deflection from single microparticles: Theory and experiment

The optical beam deflection (OBD) signal from a single microparticulate sample was theoretically derived for the photothermal response to an intensity‐modulated excitation in the transverse experimental configuration. The dependencies of phase signal on the normal and transverse offsets of the probe beam were calculated and then experimentally verified. The OBD phase signal was chosen as a means of inspecting the particle interior, since it contains information about the heat source depth. The results showed that the phase signal was independent of the excitation beam power and that the surface absorbing layer thickness could be estimated from the phase variation using the modulation frequency. The possibility of correcting beam offsets by the phase signal was also considered.

Ultrafast photothermal and photoacoustic measurement of solid/liquid interfaces

Ultrafast photothermal (PT) and photoacoustic (PA) investigation of a metal–solution interface was performed by using subpicosecond time‐resolved transient reflectivity (TR) measurement. A polycrystalline platinum film (thickness 30 nm) vapor‐deposited on a glass substrate and a polycrystalline platinum disk were investigated in air and in a variety of solutions with and without electrochemical potential control. Transient reflectivity change during and after pulsed laser illumination was measured with a high‐precision (<10−6) and high temporal resolution of the pulse width limit (pulse duration 240 fs FWHM). It was found that, in a picosecond time scale, the TR response showed electrolyte dependence as well as electrochemical potential dependence [A. Harata, T. Edo, and T. Sawada, Chem. Phys. Lett. 249, 112–116 (1996)]. These results suggest that interface structure in a molecular scale affects the ultrafast PT/PA phenomena occurring just at the interface. Also observed were ultrahigh‐frequency longitudi...

Ultrafast photothermal and photoacoustic phenomena and their applications

Photothermal techniques have been applied to investigate various physical and chemical phenomena occurring both in homogeneous systems like liquid solutions and in heterogeneous systems like surfaces and interfaces of opaque materials. It is expected that improvement of the temporal resolution in observing photothermal phenomena will provide a new investigative tool for a variety of material systems in condensed phases [A. Harata and T. Sawada, Trends Anal. Chem. 14, 504–515 (1995); A. Harata, T. Edo, and T. Sawada, Chem. Phys. Lett. 249, 112–116 (1996)]. These ultrafast photothermal methods may give one information about nonradiative relaxation, heat conduction, and acoustic propagation on a molecular scale. Recent results on ultrafast photothermal investigations will be presented.

GHz ultrasonic measurement using a transient reflecting grating method under a surface plasmon resonance condition

A transient holographic pattern, which is called a transient reflecting grating (TRG), is produced on a solid surface when two pulsed lights are simultaneously irradiated at the same spot of a small size. By using a probe light delayed relative to the above excitation pulses, a time profile of the TRG is observed by changing the delay time of the probe pulse. The signal waveform includes information about generation and relaxation of ultrasonics and heat. Using this technique, a few‐GHz ultrasonics were observed for a wide variation of material surfaces [A. Harata et al., Trends Anal. Chem. 14, 504–511 (1995)]. In this report highly sensitive detection of the TRGs utilizing surface plasmon resonance (SPR) is described. Energy deposition efficiency from light into heat was very effective under an SPR condition and the energy absorbed by a surface plasmon polariton (SPP) is concentrated on the solid surface. Utilizing an arrangement of total internal reflection for the SPP excitation, a TRG method about ten...