Femtosecond double optical pulse experiments with nanosecond time delay were conducted in air. Terahertz pulses were generated from laser plasmas produced by the irradiation with the main pulses on shockwave excited by the pre-pulse illumination. Intensity of the terahertz pulses depends on the time difference Δt and relative position Δy between the pre- and main pulses. We found a characteristic dependence of the intensity of the terahertz pulses on Δy at a longer time delay, Δt=14.7 ns, which has two maxima while only one maximum was observed when delay times were at and shorter than 9.7 ns. The results are discussed taking into account air density distribution of the shockwave. The terahertz emission spectroscopy shown here can be a useful technique for investigating density structure and its time variation of shockwave with high temporal resolution.

This study investigated the dependence of ablation thresholds for Si on the incident angle of a P- or S- polarized XeCl excimer laser (0°, 15°, 30°, 45°, and 60°). As the incident angle increased, the ablation threshold decreased for the P-polarized laser and increased for the S-polarized laser. It was found that the dependence of the ablation threshold on laser incident angle was related to the incidence angle dependence of the reflectance for P-polarized or S-polarized laser light. Based on observed laser incident angle dependence of the ablation threshold for Si, periodic nanostructures of several hundreds of nanometers were formed over the entire surface of a pyramid structure when silicon solar cells were irradiated with linearly polarized laser pulses rotated 90° at a laser fluence of 0.5 J/cm².

In understanding the formation mechanism of laser-induced periodic surface structures (LIPSS), the dependence of the LIPSS formation threshold fluence F_// on the material properties is crucial. In this work, the LIPSS generated upon the irradiation of various semiconductor materials with a train of 11.4 µm femtosecond laser pulses were examined. The laser source was a Mid-infrared free-electron laser (MIR-FEL). Eight semiconductor substrates (i.e., Si, Ge, ZnO, SiC-4H, GaP, GaN, CdTe, and SiO₂) were used as the targets. On Si, Ge, SiC-4H, and GaN, the LIPSS were oriented parallel to the MIR-FEL polarization with interspaces of 1/8 to 1/4 of the MIR-FEL wavelength λ_FEL. These interspaces tended to match the wavelength of the second harmonic of the MIR-FEL light propagating in the substrates. The obtained F_// were relatively correlated with the bandgap energies of materials, as were those for near-infrared femtosecond lasers. Another type of LIPSS oriented perpendicular to the MIR-FEL polarization with interspaces of λ_FEL to λ_FEL/9 was also observed in the same spot on SiC-4H or GaN at higher fluences than only //-LIPSS was formed.

In this laboratory, we have been studying the decomposition of methylene blue contained in aqueous solutions containing organic matter (methylene blue) when the solutions are atomized (misted) and irradiated with ultraviolet LEDs. Irradiating a methylene blue aqueous solution with ultraviolet LED has a little decomposition effect. However, we report on the difference in the decomposition effect of misting methylene blue solution when irradiated with a deep ultraviolet LED. The results of this study suggest that misting increased the probability of contact between methylene blue and chemically active species that cause organic degradation.

In recent years, research on the application of plasma treated water to agriculture or medicine has been actively carried out. For further application of plasma-treated water, it is desirable to understand the reaction process and to prepare plasma-treated water of arbitrary rate and concentration. In this study, the plasma treated water prepared by spark discharges in capillary quartz tubes were investigated. The diameter of the capillary tube was changed from 1.0 mm to 3.5 mm, and the change over time of concentrations of NO₂^-, NO₃^-, H₂O₂, and pH were investigated. It was found that the rate of increase in NO₂^- was suppressed as the diameter of the capillary tube decreased. As a result, the treatment time at pH = 4.6, the treatment time when NO₃^- starts to increase, and the treatment time when H₂O₂ starts to decrease, were increased. The reason for the decrease in the rate of increase in NO₂^- was considered that the generation of NO₂ around the plasma was suppressed as the diameter of the capillary tube decreased, and NO₂ was less dissolved in water.

When a high electric field is applied to an insulating material for a long time, space charge may be injected into the insulating material. Space charge distorts the electric field in the insulating material and reduces the insulating performance. To elucidate the effects of space charge, one-dimensional simulations have been reported for homogeneous materials. However, space charge behavior in composite materials cannot be calculated using simulation techniques for homogeneous materials. Therefore, we have developed a two-dimensional simulation technique for space charge behavior in composite materials using the finite element method.