Oxygen-deficient Silica Glass Research Articles

The radiation resistance nature of single-mode optical fiber with an oxygen-deficient silica glass core

The radiation resistance of single-mode fluorosilicate optical fiber with oxygen deficiency in a silica glass core was investigated. Fiber has been made by MCVD method. At γ-irradiation by a dose of 0.12 MGy optical fibers have shown two processes of change of signal attenuation different in velocity both at radiation processing and after its end. The difference in attenuation of optical fiber at wavelengths of 1.31 µm and 1.55 µm can be caused by higher radiation resistance of the fiber core material in comparison with the fiber fluorosilicate cladding. Our hypothesis is that this is due to the increased oxygen content in the cladding compared to the core, which leads to more prominent destruction of the glass matrix of the cladding and change in its optical properties.

Promoting vitreous silica devitrification by placement on a NaCl grain at 800 °C–1150 °C

This study investigated devitrification of a silica glass substrate put on a NaCl grain heated at temperatures of 800 °C–1150 °C. The devitrified areas are concentric double circles with respective diameters of ca. 1.5 and 1 cm. Devitrification was especially eminent in the area within the inner circle. No change in diameter of outer and inner circles was observed, at least for heating times of 10–480 min. Only the depth of the region within the inner circle increased concomitantly with increasing heating time. No marked difference was found among silica glass species with different amounts of OH and oxygen-deficient silica glasses. The heating temperature depth-dependence characteristics of the center and inner circular region at a heating time of 8 h were compared. The depth of the center of the circular region increased linearly at temperatures greater than about 1000 °C. No temperature dependence was observed at temperatures less than about 900 °C. Devitrification did not proceed under vacuum. The circular region showed alkalinity. Based on these findings, we proposed a model to elucidate the devitrification mechanism.

Radiation-resistant optical fiber with oxygen-deficient silica glass core

Oxygen deficiency in a silica glass core of fluorosilicate optical fiber manufactured by the modified chemical vapor deposition method provides extra high level of its radiation resistance. Gamma-radiation-induced attenuation (RIA) in such a fiber increases with decreasing both the test temperature (from + 25 to minus 60 °C) and the wavelength (from 1.55 to 1.31 μm). After exposure to radiation, RIA completely disappears at the room temperature. At the temperature of minus 60 °C, the optical losses restore much worse. RIA increases due to the superposition of Rayleigh scattering and absorption, with the latter decreasing with increasing wavelength.

Luminescence of polymorphous SiO2

The luminescence of self-trapped exciton (STE) was found and systematically studied in tetrahedron structured silica crystals (α-quartz, coesite, cristobalite) and glass. In octahedron structured stishovite only host material defect luminescence was observed. It strongly resembles luminescence of oxygen deficient silica glass and γ or neutron irradiated α-quartz. The energetic yield of STE luminescence for α-quartz and coesite is about 20% of absorbed energy and about 5(7)% for cristobalite. Two types of STE were found in α-quartz. Two overlapping bands of STEs are located at 2.5–2.7 eV. The model of STE is proposed as Si–O bond rupture, relaxation of created non-bridging oxygen (NBO) with foundation of a bond with bridging oxygen (BO) on opposite side of c or x,y channel. The strength of this bond is responsible for thermal stability of STE. Similar model of STE was ascribed for coesite and cristobalite with difference related to different structure. STE of Silica glass is strongly affected by disordered structure.

Luminescence of dense, octahedral structured crystalline silicon dioxide (stishovite)

It is obtained that, as grown, non-irradiated stishovite single crystals possess a luminescence center. Three excimer pulsed lasers (KrF, 248 nm; ArF, 193 nm; F 2, 157 nm) were used for photoluminescence (PL) excitation. Two PL bands were observed. One, in UV range with the maximum at 4.7±0.1 eV with FWHM equal to 0.95±0.1 eV, mainly is seen under ArF laser. Another, in blue range with the maximum at 3±0.2 eV with FWHM equal to 0.8±0.2 eV, is seen under all three lasers. The UV band main fast component of decay is with time constant τ=1.2±0.1 ns for the range of temperatures 16–150 K. The blue band decay possesses fast and slow components. The fast component of the blue band decay is about 1.2 ns. The slow component of the blue band well corresponds to exponent with time constant equal to 17±1 μs within the temperature range 16–200 K. deviations from exponential decay were observed as well and explained by influence of nearest interstitial OH groups on the luminescence center. The UV band was not detected for F 2 laser excitation. For the case of KrF laser only a structure less tail up to 4.6 eV was detected. Both the UV and the blue bands were also found in recombination process with two components having characteristic time about 1 and 60 μs. For blue band recombination luminescence decay is lasting to ms range of time with power law decay ∼ t −1. For the case of X-ray excitation the luminescence intensity exhibits strong drop down above 100 K. such an effect does not take place in the case of photoexcitation with lasers. The activation energies for both cases are different as well. Average value of that is 0.03±0.01 eV for the case of X-ray luminescence and it is 0.15±0.05 eV for the case of PL. So, the processes of thermal quenching are different for these kinds of excitation and, probably, are related to interaction of the luminescence center with OH groups. Stishovite crystal irradiated with pulses of electron beam (270 kV, 200 A, 10 ns) demonstrates a decrease of luminescence intensity excited with X-ray. So, irradiation with electron beam shows on destruction of luminescent defects. The nature of luminescence excited in the transparency range of stishovite is ascribed to a defect existing in the crystal after growth. Similarity of the stishovite luminescence with that of oxygen deficient silica glass and induced by radiation luminescence of α-quartz crystal presumes similar nature of centers in those materials.

Luminescence of silicon Dioxide — silica glass, α-quartz and stishovite

Abstract This paper compares the luminescence of different modifications of silicon dioxide — silica glass, α-quartz crystal and dense octahedron structured stishovite crystal. Under x-ray irradiation of pure silica glass and pure α-quartz crystal, only the luminescence of self-trapped exciton (STE) is detected, excitable only in the range of intrinsic absorption. No STE luminescence was detected in stishovite since, even though its luminescence is excitable below the optical gap, it could not be ascribed to a self-trapped exciton. Under ArF laser excitation of pure α-quartz crystal, luminescence of a self-trapped exciton was detected under two-photon excitation. In silica glass and stishovite mono crystal, we spectrally detected mutually similar luminescences under single-photon excitation of ArF laser. In silica glass, the luminescence of an oxygen deficient center is presented by the so-called twofold coordinated silicon center (L.N. Skuja et al., Solid State Commun. 50, 1069 (1984)). This center is modified with an unknown surrounding or localized states of silica glass (A.N. Trukhin et al., J. Non-Cryst. Solids 248, 40 (1999)). In stishovite, that same luminescence was ascribed to some defect existing after crystal growth. For α-quartz crystal, similar to silica and stishovite, luminescence could be obtained only by irradiation with a lattice damaging source such as a dense electron beam at a temperature below 80 K, as well as by neutron or -irradiation at 290 K. In spite of a similarity in the luminescence of these three materials (silica glass, stishovite mono crystal and irradiated α-quartz crystal), there are differences that can be explained by the specific characteristics of these materials. In particular, the nature of luminescence excited in the transparency range of stishovite is ascribed to a defect existing in the crystal after-growth. A similarity between stishovite luminescence and that of oxygen-deficient silica glass and radiation induced luminescence of α-quartz crystal presumes a similar nature of the centers in those materials.

Crystallization of silica glass upon heating by contact with a NaCl crystal grain

Silica glass might be crystallized from the surface under existence of contaminants such as alkali metal compounds at high temperatures.1) The crystallization of glass from the surface upon heating is called devitrification. Devitrification of silica glass depends closely on the temperature, the holding time, impurities in the silica glass, and contaminants2) from the ambient materials. Wagstaff et al. reported dynamics of devitrification of silica glasses.3)­6) They measured the crystallization rate of stoichiometric and oxygendeficient silica glasses. Under a humidified atmosphere, the activation energy of the crystallization becomes approximately half that in a dry atmosphere. In general, vitreous silica is crystallized at temperatures higher than about 1100°C without contaminants.7) Therefore, the studies reported in the literature are limited to temperatures higher than approximately 1200°C. Devitrification is promoted when in contact with alkali metal compounds; amorphous silica is devitrified at temperatures as low as 1000°C when in contact with alkali metal compounds.8)­11) Bassett12) and Higuchi13) reported crystallization of silica glasses conducted with alkali metal compounds of various types. These studies specifically addressed the phase transitions of the silica crystal, and did not address the dynamics of the crystallization. In these studies, silica glass powder was used for quantitative analyses to assess the dynamics of the devitrification; plate-type samples must be used, as in the case of the study of Wagstaff and colleagues described above. Therefore, in this study, devitrification of the silica glass occurred when a grain of NaCl crystal contacted the silica glass plate. Four commercially available silica glasses provided by Tosoh Corp.®ES, ED-B, HR, and N®were used for samples. The ES and ED-B are synthetic fused silica containing ca. 1000 and <1 wt. ppm of OH, respectively. The N and HR are produced respectively by melting quartz powder containing ca. 200 and 1 wt. ppm of OH. These samples were cut to 20 © 20 © 1 mm and optically polished on two facing surfaces. The NaCl crystal grain was placed on the center of the silica glass substrate. Then the substrate was heated in a tubular electric furnace (type ARF-50K; Asahi Rika Co. Ltd.). The soaking area length («5°C) was 62 mm. The temperatures can thereby be controlled to within «2°C. The heating time was counted after the furnace temperature reached the target temperature. Heat treatments are repeated after measuring the morphology and depth. The heat-treated sample was cooled to room temperature without temperature control before measurements. The devitrification depth was determined from the distance from the backside of the substrate to the bottom of the devitrification region using a digital optical microscope (VH7000; Keyence Co.) with correction of the effect of the refractive index. Figure 1 portrays a photograph of the devitrification of samples that had been heat-treated at 1150°C for 10 min. The devitrified region consisted of double concentric circles around the NaCl grain (0.14 « 0.01 mg). Cross-section profiles of the devitrified region of ED-B heat-treated for 8 h at various temperatures are presented in Fig. 2. The mass of the NaCl in this measurement was 0.12 « 0.01 mg. At 800°C, the devitrification profile is flat throughout the devitrified region. At temperatures higher than 1000°C, the

Photoluminescence excited by ArF and KrF lasers and optical absorption of stishovitemono-crystal

Two photoluminescence bands were found in a stishovite (silicon dioxide) mono-crystalsample under ArF (193 nm) and KrF (248 nm) excitation. The blue band is situated at3.17 ± 0.02 eV in the caseof ArF and at 3 ± 0.2 in the case of KrF. The UV band is at4.55 ± 0.05 eV in the caseof ArF and at 4.5 ± 0.05 eV in the case of KrF. The position of the UV emission band correlates with that excited by x rays. This positionis 4.6 ± 0.05 eV withFWHM 0.8 ± 0.05 eV (Truhins et al 2003 Solid State Commun. 127 415). Theblue band possesses slow decay kinetics with time constant16 ± 2 µs and the UV band isfast on the level of 2 ± 0.5 ns, similarly for both lasers. Thermal quenching of both bands begins forT higher than 150 K. The activation energies are similar for intensity and time constant, and are equal to0.23 ± 0.01 eVand 0.13 ± 0.01 eV for blue and UV bands, respectively, with equal values of frequency factor,2 × 1011 s−1. Optical absorption contains bands at 4.5, 5.5, and 7 eV and a strong band starting from7 eV adjacent to the intrinsic absorption threshold above 8.75 eV. Excitation at 7.86 eV(F2 laser) does not provide luminescence. The nature of the luminescence excited in thetransparency range of stishovite is ascribed to a defect, presumably created byprevious irradiation of the crystal. Similarity of the stishovite luminescence tothat of oxygen-deficient silica glass and also to that induced by irradiation ofα-quartz crystals allows us to conclude similar natures for the defect centers in thesedissimilar materials.

Radiation processes in oxygen-deficient silica glasses: Is ODC(I) a precursor of E′-center?

The accumulation of radiation-induced defects under non-destructive X-ray and destructive cathodoexcitation was studied in pure silica KS-4V glasses possessing an absorption band at 7.6eV. The correspondence between the existence of this band and the creation of the E′-center by radiation was checked. Detection of induced defects was accomplished by measurement of the luminescence during irradiation, post irradiation afterglow or phosphorescence, induced optical absorption, and thermally stimulated luminescence. In all samples, these observed phenomena associated with charge trapping and recombination on the oxygen-deficient luminescence center. Others centers of luminescence were not significant contributors. In some samples, the intensity of the 7.6eV absorption band was deliberately increased by treatment in hydrogen at 1200C for 100h. The intensity of luminescence in hydrogen-treated samples was smaller because of the known quenching effect of hydrogen on the luminescence of oxygen-deficient centers. The optical absorption method does not reveal an induced absorption band for the E′-center in the hydrogen-free samples with different levels of oxygen deficiency. Therefore, we did not detect the transformation of the defect responsible for the 7.6eV absorption band or the ODC(I) defect into the E′-center. In the hydrogen-treated sample, the absorption of the E′-center was detected. The E′-centers creation in the hydrogen-treated sample was associated with precursors created by hydrogen treatment (≡Si–O–H and ≡Si–H) in the glass network. The destructive e-beam irradiation reveals an increase with dose of the ODC luminescence intensity in the sample exhibiting a small 7.6eV band. That means that the corresponding luminescence centers are created. Optical absorption measurements in that case reveal the presence of E′-centers and a broad band at 7.6eV. A compaction of the irradiated volume was detected. Therefore, we conclude that the E′-center is produced by heavy damage to the glass network or by the presence of precursors.

Correlation between the 2.7 eV and 4.3 eV photoluminescence bands in silica glass

Our previous studies have reported the excitation energy dependence of the 2.7 and 4.3eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (∼20K). An oxygen vacancy (O3SiSiO3) was thought to be the origin of the two PL bands. In order to verify the origin of the 2.7 and 4.3eV PL bands in silica glass, we measured the PL band of various thermally heat treated silica glasses. In the sample after heat treatment, we did not observe the 4.3eV PL band, though we did observe the 2.7eV PL band. These results suggest that these two PL bands do not have a common origin.

Photoluminescence band near 2.2 eV in γ-irradiated oxygen-deficient silica glass

Our previous studies have reported different radial distributions of the 2.25 eV photoluminescence (PL) band (full width at half maximum (FWHM): ∼0.4 eV, decay time (τ): ∼25 ns) under 3.8 and 4.66 eV excitation in γ-irradiated oxygen-deficient silica glass. As a result of our measurements, we believe that the origin of this PL band is different for 3.8 and 4.66 eV excitations. In order to understand this difference better, we measured the time-resolved PL spectra and decay time of the near 2.2 eV band under 3.49 and 4.66 eV excitation in γ-irradiated oxygen-deficient silica glass. We have found two PL bands at approximately 2.2 eV in the γ-irradiated oxygen-deficient silica glass. One band was observed at 2.25 eV (described above) under 4.66 eV excitation; and the other, at 2.15 eV (FWHM: ∼0.5 eV, τ: ∼150 ns) under 3.49 eV excitation. Two PL bands, one at 2.15 eV and the other at 2.25 eV coexist with excitation at 3.49, 3.8 and 4.66 eV. From our results, we conclude that the difference in the radial distributions of the 2.25 eV PL under 3.8 and 4.66 eV excitation is due to the 2.15 eV PL band. The main component of the observed PL intensity under 3.8 eV excitation was the 2.15 eV PL component, and the main component of the observed PL intensity under 4.66 eV excitation was the 2.25 eV PL component.

Excitation energy dependence of the photoluminescence band at 2.7 and 4.3 eV in silica glass at low temperature

In this paper, we report the excitation energy dependence of the 2.7 and 4.3 eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (∼20 K). The increase or decrease of the PL intensity at low temperatures is different for different exciting light wavelengths. The PL intensity tended to decrease with low temperatures when the excitation was near the upper and lower end of the excited level. The peak energy of the excitation spectrum increases with cooling. These results indicate that the change in excitation level with cooling is associated with the low-temperature dependence of light emission. Thermal motion is suppressed, when the sample temperature is lowered, and the energy-width of the excited level decreases, i.e., the light emission probability decreases (the emission intensity decreases), when near the upper and lower end of the excitation level. These phenomena were observed in the low-temperature dependence of the 4.3 eV emission intensity.

The 3.1 eV photoluminescence band in oxygen-deficient silica glass

Previous studies have reported the existence of a photoluminescence (PL) band near 3.1 eV in silica glass when exposed to ultraviolet (UV) light. The O 3SiOOH group, the twofold coordinated silicon lone-pair center ( O S ̈ i O ) and a germanium impurity were thought to be the origin of this PL band. In this paper, we describe the characteristics of a PL band near 3.1 eV in undoped oxygen-deficient silica glass (low-OH) that has been synthesized with chemical vapor deposition (CVD) soot remelting. At room temperature, we have found two types of PL bands at approximately 3.1 eV in the low-OH glass. One band is observed at 3.08 eV (lifetime, τ∼100 μ s); and the other, at 3.15 eV ( τ∼10 μ s).

Green photoluminescence band in γ-irradiated oxygen-surplus silica glass

Previous studies have reported the existence of a green photoluminescence (PL) band in oxygen-deficient silica glass when exposed to ultraviolet (UV) light. Oxygen deficient defects (Eδ′) were thought to be the origin of this PL band. In this article, we describe the characteristics of a green PL band in oxygen-surplus silica glass (excited by visible and UV light). At room temperature, we observed the full width at half maximum and lifetime of this PL band to be about 0.2 eV and 300 ns, which differed from the previously reported values of 0.4 eV and 20–30 ns, respectively. We propose that the peroxy radical (O3≡Si–O–O↑,↑: unpaired electron) or other oxygen surplus defects are the cause of this green PL band rather than oxygen deficient defects.

Investigation of optical and radiation properties of oxygen deficient silica glasses

The deficiency of oxygen in pure silica manifests an absorption band at 5 eV as well as an absorption band of higher intensity at 7.6 eV. The band at 5 eV is associated with lone twofold-coordinated silicon centers. The nature of the main band at 7.6 eV has been studied using silica samples with different levels of oxygen deficiency. The excitation via the 7.6 eV band produces a photoelectric response as well as inner center and recombination type luminescence. Two main luminescence bands of the twofold-coordinated silicon center appear: a blue band (2.7 eV) and a UV band (4.4 eV). Induced absorption with several bands as well as thermally stimulated luminescence with complex peak structure were observed. Analyzing these data, the nature of the 7.6 eV band cannot be ascribed to a lone point defect, rather, it can be ascribed to the localized states of the disordered structure of silica modified by an oxygen deficit.

Blue Photoluminescence from Si-Doped Amorphous Silica Films by RF Sputtering

Si-doped amorphous silica films were prepared by rf magnetron sputtering. Blue photoluminescence (PL), with a peak wavelength of about 450 nm, was observed from films with relatively low Si content after thermal annealing. PL intensity showed the dependence on annealing temperature. The peak wavelength of this PL was very similar to that of oxygen-deficient silica glass. However, PL excitation spectra and the temperature dependence of blue PL intensity showed that the origin of this blue PL was different from that of blue PL for oxygen-deficient silica glass.

Direct observation of ultraviolet laser induced photocurrent in oxygen deficient silica and germanosilicate glasses

UV laser induced inonization of silicon oxygen deficient centers (SODC) in silica glasses and germanium oxygen deficient centers (GODC) in germanosilicate glasses has been studied by direct displacement photocurrent measurements. For both SODC and GODC the two-photon (two-step) nature of photoionization has been established. The cross sections of transitions from first excited to upper (ionizable) states were estimated as σ12=6×10−18 cm2 for SODC and (0.5–1.0)×10−19 cm2 for GODC. The effect of pulse-to-pulse degradation of the photocurrent signal caused by the screening effect is applied to estimate the lifetimes and ranges of the free carriers. The conclusion was drawn, that ionized SODC and GODC play the key role for free charge trapping.

Reaction kinetics of oxygen-deficient centers with diffusing oxygen molecules in silica glass

The decay kinetics of the 7.6 eV optical absorption band of hydroxy-free silica glasses during heating in oxygen gas flow was determined. It was found that annihilation of oxygen-deficient centers, ODC(I)s, agrees with the tarnishing reaction model. The absorption cross-section of that center, σ, was determined to be 7.8 ± 0.5 × 10 −17 cm 2. This value is close to 7.5 × 10 −17 cm 2 which was previously determined spectroscopically by hydrogen-gas treatment of oxygen-deficient silica glass.

Effects of nitrogen and carbon ion implantation on devitrification of silica glasses

The effects of nitrogen and carbon ion implantation on devitrification of stoichiometric and oxygen deficient silica glasses were investigated. Linear crystal growth was observed in stoichiometric silica glass and was not affected by ion implantation. Parabolic crystal growth was observed in oxygen-deficient glasses, which was retarded by ion implantation, with the rate of crystal growth retardation being greater for nitrogen implantation than for carbon implantation. Activation energy for parabolic rate constant for crystal growth was about ∼ 105 kcal/mol. When the oxygen-deficient silica glasses were hydrated, their crystal growth became linear and the activation energies were smaller. The reduction in activation energy by hydration was approximately equal to the activation energy for water diffusion in silica. It is suggested that nitrogen ion implantation decreased the crystal growth rate in the oxygen-deficient glasses by forming an oxynitride layer, which is an effective water diffusion barrier.