Atsushi Yokotani

Growth of large KDP crystals for laser fusion experiments

The growth of huge KDP crystals which have a 40 cm × 40 cm cross section for a frequency converter for high power laser system for nuclear fusion experiments is described. The main factor limiting the growth rate is spontaneous nucleation. We could successfully grow a crystal by a three-vessel method (TVM) at the rate of 2.9 mm/day over half a year without nucleation by setting the temperature of the crystallizer lower than room temperature. The TVM proved to be superior to a conventional temperature reduction method. The bulk laser damage threshold was improved two to three times that of conventionally grown crystals by reducing organic impurities in the growth solution. These techniques are very important for the growth of large KDP crystals of good quality for high power lasers.

Efficient laser-diode-pumped neodymium-doped calcium-niobium-gallium-garnet laser.

Efficient lasing of Nd:Ca (Nb, Ga)(2-x) Ga(3)-garnet (CNGG) disordered crystal pumped by a laser diode was demonstrated. In the end-pumped cw lasing experiment with a single-stripe diode, a slope efficiency of 24.2% and a maximum optical conversion efficiency of 19.3% were obtained. It was shown that the dependence of cw output power on the pump wavelength is insensitive compared with Nd:YAG because of the broad pump absorption bandwidth of Nd:CNGG. From the diode-bar-pumped experiment, it was confirmed that Nd:CNGG is a promising material for diode-bar pumping to obtain high average power with good beam quality, because of its advantages of a broader pump absorption band compared with that of Nd:YAG and the higher thermal conductivity compared with Nd-doped glasses.

A single precursor photolitic chemical vapor deposition of silica film using a dielectric barier discharge xenon excimer lamp

Silica film has been produced at room temperature by a single precursor process of photolitic chemical vapor deposition using a newly developed Xe excimer lamp. Tetraethoxyorthosilicate (TEOS) has been used as a raw material. Transparent thin film of SiO2 was obtained on single crystalline Al2O3 substrates and its properties were evaluated by means of the reflection Fourier transformation‐infrared spectroscopy, the scanning electron microscopy, and ultraviolet‐visible spectrometry. Consequently, it was found that the main component of the film was SiO2 and very small amounts of residual organic materials were contained. It was also found that the film was very dense and the refractive indices were only 1.7% smaller than that of bulk silica glass.

Identification of defects associated with second-order optical nonlinearity in thermally poled high-purity silica glasses

A large second-order optical nonlinearity has been found to be generated in various kinds of silica glasses in which an even-order optical nonlinearity is inherently prohibited. Thermal poling is a typical procedure to generate such a second-order optical nonlinearity, but a mechanism behind the generation is not elucidated completely. It should be pointed out, however, that the nonlinearity was not added to high-purity silica glasses by poling. In this article, we show that the nonlinearity is generated in high-purity silica glasses irradiated by a KrF excimer laser before poling. We also show that the laser pulses erase the nonlinearity induced in the glasses. In addition to the laser irradiation effects on the generation and erasure, optical absorption and luminescence spectra in the glasses show that point defects of ≡Si–O− play a key role for the nonlinearity.

Second-order optical nonlinearity and change in refractive index in silica glasses by a combination of thermal poling and x-ray irradiation

Second-order optical nonlinearity was found to be generated in high-purity silica glasses when they were exposed to x-ray radiation and then thermally poled. Two kinds of second-order optical nonlinearity, near-surface nonlinearity localized in a thin layer near the sample surface and bulk nonlinearity spreading throughout the whole sample, were observed. The maximum χ33(2) values of near-surface and bulk nonlinearity were 0.20 and 0.43 pm/V, respectively. Further, a change in refractive index of Δn=4×10−3 was also observed in the sample. The χ33(2) value of the bulk nonlinearity and the change in refractive index increased with the x-ray intensity. On the other hand, the χ33(2) value of the near-surface nonlinearity had a tendency to saturate when the intensity was higher than 1.3 mW/cm2. Based on absorption spectra of the samples, both the bulk nonlinearity and the refractive index change were found to be associated with point defects such as the E′ center (≡Si⋅) and nonbridging oxygen ions (NBO−,≡Si–O−).

Development of dicing technique for thin semiconductor substrates with femtosecond laser ablation

Recently, the semiconductor substrates for integrated circuits have been required to become as thin as 50 micrometers , because the many electronics devices are strongly demanded to be miniaturized and light-weighted. Machining of such thin substrates with conventional dicing techniques is very difficult. Therefore, we have proposed to process them using femtosecond laser ablation, expecting advantage of efficient etching without undesirable mechanical and thermal damages such as cracking and melting is expected. In this work, we have investigated the influence of the laser conditions such as pulse duration and fluence on the cutting depth and diameter in order to develop a new photo-dicing technique for very thin ICs. Within the range of pulse energy used in the present experiments, the dependence of the pulse duration did not seem to be significant. It was also found that the lower energy of the laser pulses, the smaller and the deeper, i.e., the sharper holes were formed. The typical cutting depth and diameter for 0.20 mJ/pulse and 5 shots were 17 micrometers and 40 micrometers , respectively. These values are very promising for the practical dicing applications.

Estimation of Surface Tension of Molten Silicon Using a Dynamic Hanging Drop

A new method to determine the surface tension of high-temperature liquids was developed using the rotation of a hanging drop. The measurement of surface tension of silicon melt was performed by observing the oscillation of a silicon droplet hanging from a SiC-coated carbon rod. The oscillation of the liquid drop was induced by a sudden high rotation speed above 570 rpm. The surface tension of molten silicon was estimated as 0.819 N/m at the melting point of 1415° C and its temperature coefficient was -0.308×10-3 N/mK. We concluded that the dynamic hanging drop method could be used to measure the surface tension of high-temperature liquids.

SiO2 thin film preparation using dielectric barrier discharge-driven excimer lamps

Abstract By using a photochemical vapor deposition method with a Xe 2 excimer lamp (172 nm, 20 mW cm −2 output power), silica films have been prepared by means of a single precursor process from Tetraethoxyorthosilicate (TEOS) at room temperature. Transparent thin films of SiO 2 were obtained on sapphire and quartz single crystal substrates with a deposition rate of 0.9 nm min −1 . They were mainly composed of amorphous SiO 2 , although small amounts of residual organic materials were contained. The refractive index was 1.476 at 632.8 nm. The surface roughness decreased with the film thickness and reached 0.2 nm-rms. These findings indicate that the VUV excimer lamp CVD is a promising method for preparing smooth, dense and fine thickness-controllable films of SiO 2 at room temperature.

Silicon Nitride Film Deposition by Photochemical Vapor Deposition Using an Argon Excimer Lamp

In this paper, we report the deposition of silicon nitride (SiNx) films for the production of semiconductor devices and flat panel displays, by chemical vapor deposition with vacuum ultraviolet excimer lamps (VUV-CVD) using SiH4 and NH3 as raw materials. An Ar2* excimer lamp (λ=126 nm, hν=9.8 eV) with a high photon energy was used to directly excite and dissociate SiH4 through a photochemical reaction. SiNx films were successfully formed at a low temperature of 100 °C with the Ar2* excimer lamp. Although the Si-rich films were obtained using an Ar2* lamp, they showed a quality almost similar to that of films obtained by conventional plasma-CVD at 400 °C.

Generation and erasure of second-order optical nonlinearities in thermally poled silica glasses by control of point defects

Ultraviolet laser pulses were found to introduce and destroy point defects that play a key role in the generation of second-order optical nonlinearities by thermal poling in high-purity silica glasses. The characteristics of the generation process depended largely on not only ≡Si—OH,O2, and H2 content of the glasses but also the sequence of thermal poling and the pulse irradiation. There were two different kinds of nonlinearity: one localized in a thin layer near the sample surface (near-surface) and a bulk one spreading throughout the sample. The near-surface and bulk nonlinearities are associated with ≡Si—O- and ≡Si⋯Si≡, respectively.