Kou Kurosawa

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.

SiO bond breaking in SiO2 by vacuum ultraviolet laser radiation

Abstract Serious optical damage is observed in the surface layer of synthetic quartz glass (SiO2) when it has been used for a high power argon excimer laser operating at 126 nm as a cavity mirror. Microscopic surface observations, XUV reflectivity measurements and XPS analysis show that the damage is mainly ascribed to a photochemical effect. The photochemical effect results in the SiO bond breaking and subsequent crystalline Si isolation in the surface layer.

A VACUUM ULTRAVIOLET FLASH LAMP WITH EXTREMELY BROADENED EMISSION SPECTRA

We have developed a vacuum ultraviolet (VUV) flash lamp using a binary rare gas mixture excited by a pulsed silent discharge. In a Kr/Xe silent discharge, the VUV emission spectral width was extended up to 26 nm full width at half‐maximum (FWHM) at a center wavelength of 162 nm. According to kinetic analyses, such a spectral extension in mixed rare gases is attributed to the simultaneous emissions from heteronuclear rare gas excimers (KrXe*) and homonuclear rare gas excimers (Kr2* and Xe2*).

Processing applications with the 157-nm fluorine excimer laser

Excimer-laser processing techniques can be extended to a broader and more diverse range of materials by moving to vacuum-ultraviolet laser source such as the molecular fluorine laser. The 157-nm output wavelength takes advantage of the high opacity in most materials and a short pulse duration to minimize thermal loading of target surfaces. The laser readily drives photochemical interactions and affords patterning of approximately 0.1-micrometers features. In this paper, we summarize the recent progress in our laboratory on applying these principles to the development of F2 laser applications. Examples include micromachining of high- bandgap optical materials, fabricating rib waveguides, growing debris-free silica films, driving photosensitivity responses in optical fibers, photochemical processing of III-V semiconductors, and writing fine-feature holographic structures.

Laser beam profiler in the vacuum ultraviolet spectral range using photostimulable phosphor

We propose and demonstrate the use of a photostimulable phosphor material, BaFBr:Eu2+, for recording two‐dimensional intensity distributions within output beams of vacuum ultraviolet lasers. The sensitivity characteristics were measured not only for nanosecond pulsed radiation from KrF, ArF, and Ar2 excimer lasers but also for quasi‐continuous‐wave synchrotron radiation. We designed and constructed a laser beam profiler to record, read, and erase images in vacuum. The intensity distribution in the output beams from an Ar2 excimer laser was measured with it, and then the threshold gas pressure for obtaining the well collimated beams was found to be 22 kg/cm2. The intensity is not uniform in the beams but stronger in the peripheral part than in the central one.

Super-polished silicon carbide mirror for high-power operation of excimer lasers in a vacuum ultraviolet spectral range

We demonstrate the usefulness of a high‐quality SiC mirror for the high‐power operation of vacuum ultraviolet lasers. A void free SiC film of 400‐μm thickness was deposited onto a high‐purity graphite substrate by chemical vapor deposition. The super‐polished SiC mirror, whose surface roughness was 0.2 nm rms, was used as a cavity reflector of a high‐power Ar excimer laser, which operates at 126 nm. The highest output energy of 400 mJ/cm2 per pulse, which corresponds to the energy density of 1 J/cm2 on the SiC mirror surface, has been obtained without any damage to the SiC mirrors.

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.

Surface alterations of SiO2 optics by 9.8 and 8.5 eV laser photons

Abstract Alterations of SiO 2 mirror surfaces, measured by means of surface profiler, reflectance and transmission spectroscopy, and X-ray photoelectron spectroscopy, are reported. The alterations were formed when the mirrors were used as a cavity reflector for Ar and Kr excimer lasers. It is found that Si is enriched in the surface layers which were exposed to 9.8 eV photons from the Ar excimer laser. The Kr excimer laser photons, the energy of which is 8.5 eV, do not induce such a phenomenon. The Ar excimer laser photons, surmounting the fundamental band gap of SiO 2 , 9 eV, are assumed to create a high density of excitons that induce SiO breaking, resulting in silicon enrichment and oxygen depletion. The Si enrichment is observed for both glassy and crystalline SiO 2 .

Resonant Tunneling in Double-Barrier Structures with Trapezoidal Potential Profiles

Transmission coefficients and current-voltage characteristics have been calculated for double-barrier diodes with trapezoidal potential profiles. The potential profiles have been taken up as a model for structures in which the flat interfaces have been disturbed through roughened surfaces and interdiffusion of the constituent atoms. The time-independent one-dimensional Schrodingers equation has been numerically solved using Airy functions. Studying the effects of the slope widths on the diode characteristics for a particular case of the AlAs/GaAs/AlAs double-barrier structure having the barrier width of 23 A and the well width of 50 A, we have shown that a slope region less than 6 A does not significantly change the characteristics.

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−).