Akihiro Kameyama

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−).

X-ray emission spectroscopic studies of silicon precipitation in surface layer of SiO2 induced by argon excimer laser irradiation

Abstract The ultra-soft X-ray emission spectra were taken from surfaces of bulk silica glass and silica glass films exposed to an argon excimer laser ( λ =126 nm) and compared with the spectra taken from the virgin surfaces. The precipitation of crystalline silicon is found to take place in thin surface layers of the irradiated bulk silica glass and 15 nm film. An estimation of concentration of crystalline silicon precipitation with the depth is given on the basis of the measurements of Si L 2,3 X-ray emission spectra obtained at different accelerating voltages of the electron beam on the X-ray tube. Based upon the precipitation conditions for these samples, we discuss the crystalline silicon precipitation mechanism: the electronic excitation induces the bond-breaking between Si and O atoms, although there is a critical density of photons for the bond-breaking and temperature rise enhances the crystalline silicon precipitation.

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.

X-ray radiation effects on second-harmonic generation in thermally poled silica glass

We observed a strong second-order optical nonlinearity in a fused silica glass poled under the condition of a static electric field of 4 kV/mm at 260 °C. The nonlinearity layer was localized in the surface region contacted on the positive electrode during poling in a thickness comparable to or thinner than the interaction length of 22 µm. The second-order nonlinearity was not observed in synthetic silica glass under the same poling condition. However, when the synthetic glass was first exposed to x-ray radiation, the poling induced a nonlinearity of almost the same value as that in the fused silica glass, which we attribute to the x-ray formation of defects. The values were found to depend on water contents in the synthetic silica glass.

Erasing Process of Thermally Poled Optical Nonlinearities in Silica Glasses with KrF Excimer Laser Pulses

We observed a second-order optical nonlinearily of χ33(2)=Q.30 pm/V in a fused silica glass poled under the condition of static electric field of 4 kV/mm at 260°C. The nonlinearity layer localized in the surface region contacted on the positive electrode during poling in a thickness comparable with or thinner than the interaction length of 22 µm. Even though a nonlinearity phase formation was not observed in synthetic silica glass under the same condition, the poling induced a nonlinearity of 0.24 pm/V by exposing the sample to X-ray radiation before poling. We also observed that 248-nm pulses as low as 10 mJ/cm2 from a KrF excimer laser erased the nonlinearity.

Crystallized hydroxyapatite coatings deposited by PLD with targets of different densities

We successfully achieved the poly-crystallized coatings of bio-active hydroxyapatite on titanium plates. We used several ceramic HAp targets sintered at a temperature of 500°C, 700°C, 900°C and 1100°C, and irradiation by KrF excimer laser at a fluence of about 4J/cm2. The depositions were performed under 1Torr H2O atmosphere at room temperature. In this condition, a poly-crystallized HAp layer was formed only using a target sintered at 900°C. We estimated the charged fragments from these targets by a simple ion-probe collection and found that the energy distribution of charged fragments depended on the densities of the targets.

Ultrasoft X-Ray Emission Spectroscopic Analysis for Effects of Vacuum Ultraviolet Rare Gas Excimer Laser Irradiation on Silicon Nitride Films

The results of measurements of Si-L2,3 X-ray emission spectra of Si3N4 films deposited on GaAs substrates after irradiation by Kr2 and Ar2 excimer lasers are presented. By using the excitation of X-ray emission spectra with focused electron beam, local precipitation of crystalline silicon is found.

Influence of the cladding diameter of optical fiber on the simultaneous measurement of the refractive index and temperature of liquids using tilted fiber Bragg grating

We considered the influence of the core and cladding diameters on the simultaneous measurement of the refractive index and temperature of liquids using a tilted fiber Bragg grating (TFBG). When the cladding diameter of a TFBG was only reduced from 125 to 123 µm, the calculated refractive index differed by 0.0068 from the expected value. When the core diameter was reduced from 9.6 to 7.8 µm, the change was less than 0.0005. We found that equalization of cladding diameter is required to improve the precision of the simultaneous measurement of the refractive index and temperature of liquids.

Photochemical reactions in silicon nitride with ArF excimer laser

Exposure of silicon nitride to above-bandgap 6.5-eV photons from an ArF excimer laser drives both the dissociation of silicon-nitrogen bonds and the desorption of nitrogen atoms and/or molecules over a wide fluence range. Crystalline silicon precipitates are also generated on laser exposed surfaces, however, only for fluences exceeding 0.2 J/cm2. The rates of nitrogen desorption and the concentration of silicon precipitation were found to depend strongly on laser fluence, rising rapidly above 0.2 J/cm2, and saturating at approximately 0.5 J/cm2. This saturation was also observed in the thickness of the silicon precipitate layer, which peaked at 35 nm depth for fluences greater than 0.5 J/cm2. Such saturation phenomena can be explained by the onset of laser ablation at approximately 0.5 J/cm2 fluence which removes material in the laser affected zone. The formation of silicon precipitates is discussed in the context of photochemical reactions that follow band-to-band electronic transitions.