Ken-ichi Muta

Simultaneous generation of the 7.6‐eV optical absorption band and F2 molecule in fluorine doped silica glass under annealing

We examined chemical state of fluorine doped silica glasses and its thermal behavior. Almost all of the fluorine atoms were found to have the ≡SiF structure with Raman spectroscopy. No optical absorption in the region of 3–9 eV was detected in the glass. When the glasses were annealed in a He atmosphere at 1000 °C, absorption bands peaking at 7.6 and 4.3 eV appeared. These two bands are attributed to the ≡SiSi≡ structure and to F2 molecules, respectively. We proposed a thermal decomposition reaction expressed as ≡SiF+FSi≡→≡SiSi≡+F2. The concentrations of the reaction products, ≡SiSi≡ and F2, estimated from the absorption cross sections were equal to each other within the errors of measurements. We also examined the radiation damage with γ ray. The concentration of E’ center was almost the same for the same dose in silica glasses having different concentrations of FSi≡ and ≡SiSi≡. We suggest that FSi≡ and ≡SiSi≡ were found to be stable for γ‐irradiation at room temperature.

Optical properties of oxygen‐deficient centers in silica glasses fabricated in H2 or vacuum ambient

Silica soots were sintered under a reducing ambient to study the generation mechanism of the oxygen‐deficient‐type defects in silica glasses. Glasses sintered under H2/He gas mixtures or vacuum were prepared with an intent to produce oxygen‐deficient centers. Two optical‐absorption bands were generated in the silica glasses sintered under the H2/He gas mixtures. One was 5.17‐eV absorption band and the other was an absorption tail near the Urbach tail of silica glasses. Both absorptions were attributed to the silicon lone pair center (SLPC). On the other hand, the other type of reduced silica glasses were prepared in the vacuum ambient. Three kinds of absorption bands could be detected clearly: one was peaking at 5.17 eV, another showed only its shoulder near the Urbach tail, (both were also attributed to the SLPC) and the third was peaking at 6.7 eV and was attributed to a SiSiSi structure. Fabricated in higher volatile ambient under vacuum, the color of the sample showed a smoky black and generated broad...

CHARACTERIZATION OF SILICA GLASSES SINTERED UNDER CL2 AMBIENTS

Chlorine incorporated into silica glasses by sintering porous soot rods under Cl2/He atmosphere or by fabricating with plasma methods were found to be mostly as ≡SiCl, which gives rise to an optical absorption tail above ≂7.5 eV. The cross section of the optical absorption at ≂7.77 eV (160 nm) was found to be 1.14×1020 cm2. This value was almost the same as that of ≡SiCl in SiCl4 molecule in the gas phase. A minor fraction (10−3∼10−1) was found to be present as Cl2 molecule, which gives the absorption band at 3.8 eV.

Preferred concentration enhancement of photobleachable defects responsible for 5 eV optical absorption band in SiO2:GeO2 glass preform by heating in a H2 atmosphere

Heat treatment of a 5GeO2‐95SiO2 glass preform in a H2 atmosphere at 500 °C for 70 h increased intensities of an absorption band centered at 5 eV by a factor of ∼3. Intensities of photobleachable component, which is the precursor of UV‐induced Ge E’ centers and assigned to a neutral oxygen monovacancy, of the 5‐eV band were enhanced by a factor of ∼8 by heating. This increment is ∼3 times as large as that of UV‐unbleachable component, which is assigned to Ge2+ centers coordinated by two oxygens (neutral oxygen divacancy).

Formation mechanism of hydrogen‐associated defect with an 11.9 mT doublet in electron spin resonance and red luminescence in 9SiO2:GeO2 fibers

The nature and the origin of the 11.9 mT hyperfine doublets in electron spin resonance spectrum observed in 9SiO2:GeO2 fibers were examined. When fibers were heated in hydrogen atmosphere at 100 °C, the intensity of the doublet increased. When gamma‐ray irradiated, the intensity drastically increased. The photoluminescence bands peaking at 1.91 eV (650 nm) and 1.83 eV (680 nm) excited with Ar ion laser operated at 488 nm were also observed in the treated fibers with the doublets. It should be noted that the relation between the photoluminescence and the doublets was found. The intensities of the photoluminescence peaking at 1.83 eV and the 11.9 mT ESR doublets have accumulation. The germyl radical was proposed as their structural model. The origin of the photoluminescence peaking at 1.91 eV was tentatively proposed as the small silicon particles with hydrogen impurities generated in the fibers.

Mechanisms of photo-bleaching of 5 eV optical absorption band in hydrogen loaded Ge-doped SiO2

Abstract Photo-induced effects in H 2 loaded 1GeO 2 :9SiO 2 glass were studied. The intensity of the absorption band at 5.14 eV was reduced with UV illumination. The neutral oxygen vacancy and also the Ge lone pair centers are reduced with the illumination. There is a one-by-one correlation between the reduction of Ge lone pair centers and the generation of GeE′ center. We propose that a Ge lone pair center reacts with H 2 molecules in glass to generate a GeE′ center.

Cotton-Mouton Effect of Alkyl- and Alkoxy-Cyanobiphenyls in Isotropic Phase

The magnetic field induced birefringence Δn has been measured for nCB (n=5–8) and nOCB (n=3, 5–9). Sufficiently above the clearing temperature Tc, all the materials investigated, except 7OCB, satisfy the relation Δn/H2=λKC/(T-Tc*) over a wide temperature range of 20°C or more. Here λ=633 nm is the wavelength used, Tc* a temperature slightly lower than Tc, and KC a constant. The anomaly observed in 7OCB is impurity sensitive. As the alkyl chain length increases, KC drops appreciably; 3OCB has the largest value, KC=2.98×10-10°C cm-1 G-2, while 7CB has the smallest value, KC=0.99×10-10. Near Tc, on the other hand, Δn/H2 becomes larger than the above relation predicts. The two constants Tc-Tc* and KC are not enough to characterize the materials; (Δn/H2)T=Tc is also an important factor. Among the materials investigated, 5OCB has the largest value of (Δn/H2)T=Tc, namely 2.50×10-14 G-2.

Structural imperfections in silica glasses with an optical absorption peak at 3.8 eV

Structural imperfections in silica glasses with an optical absorption peak at 3.8 eV were studied. The 3.8‐eV peak was assigned to Cl2 molecules trapped in the glasses. The peak absorption was increased with increasing partial pressure of chlorine for sintering. The peak and its full width at half maxima were the same as those of Cl2 molecules in the gas phase. ≡SiOH was generated in the glasses by annealing in H2 ambient. After irradiation with a KrF excimer laser (5 eV), an absorption peak at 4.8 eV and red emission were observed. The properties were the same as the photochemical reaction of O2 molecules in the gas phase. We proposed that O2 molecules were trapped in the glass network when silica glasses were sintered under Cl2 ambient. The reaction is expressed as ≡SiOH+HOSi≡+Cl2→≡SiCl+ClSi≡+H2O↑+1/2O2 (trapped).

Effects Of Sintering Atmosphere On Defects In SiO 2 :GeO 2 Vad Fiber

Effects of the sintering atmosphere for 90SiO 2 :10GeO 2 preform-rods produced by VAD process were examined. The sintering atmosphere was changed from reducing to oxidizing, and the type and relative concentration of defects were estimated by photoluminescence, UV absorption and ESR absorption. Reducing condition accelerated the formation of defects, because these centers were considered to be the intermediate products of the thermal decomposition reaction of GeO 2 which gives rise to reduction of Ge(IV) to Ge(II).

Unusual crystal orientation and its control in 4-(N, N-dimethylamino)-3-acetamidonitrobenzene (DAN) in glass capillary

Abstract We have studied organic crystal growth and its orientational control in glass capillaries using a modified Bridgman-Stockbarger method. In the course of this study, we obtained unusual crystal orientation in 4-(N, N-dimethylamino)-3-acetamidonitrobenzene (DAN) grown in a pre-baked capillary: the polarization axis ( Z axis) is parallel to the capillary axis and is different from the reported orientation; i.e., 54δ with respect to the capillary axis. The control of crystal orientation in glass capillaries was successfully achieved in DAN-cored fiber prepared using pre-baked capillaries, though it was impossible in untreated capillaries. These facts suggest the importance of the interaction between the inner wall of a glass capillary and organic molecules in organic crystal orientation in glass capillaries.