Defects In Silica Glass Research Articles

Effect of sintering condition on the attenuation of silica glasses fabricated by additive manufacturing

Over the past few years, additive manufacturing (AM) of silica glasses and silica optical fibers has received great attention, mainly due to the high manufacturing flexibility and low manufacturing cost. However, compared with the traditional methods, the silica material made by AM technology is sintered at temperature below the melting point and has not gone through the long-time fusing or melting, so it has more defects, leading a higher attenuation. Due to the large amount of hydroxyl groups in the raw materials used in AM technology, it directly affects strong hydroxyl absorption. In this work, the influence of sintering pressure (SP) and temperature (ST) on the attenuation of printed and sintered silica glasses, including Rayleigh scattering, intrinsic point defects at characteristic wavelengths and hydroxyl absorption, was studied. The defects in silica glasses fabricated by AM technology were analyzed and discussed against sintering conditions. On one hand, we found that lower SP significantly reduced Rayleigh scattering and the intrinsic point defects due to moisture in the air and that the stress caused by densification in sintering process could further increase the intrinsic point defects. On the other hand, in the absence of crystallization, higher ST also reduced Rayleigh scattering, and the stress affected the inherent point defects. Hydroxyl absorption was determined by the diffusion of water and the amount of intrinsic point defects. The optimization of the SP and ST increased the glass transmittance at 600 nm by nearly 30% and 19% compared with non-vacuum and 1170 °C sintering conditions, respectively.

Recent Advances in Silica Glass Optical Fiber for Dosimetry Applications

In this paper, we review the highly promising silica glass, fabricated as doped and undoped optical fiber for intended use in radiation dosimetry. The dosimetry techniques reviewed here, underpinned by intrinsic and extrinsic defects in silica glass, focus on Thermoluminescence (TL), Optically Stimulated Luminescence (OSL) and Radioluminescence (RL), with occasional references to the much more established Radiation Induced Attenuation (RIA). The other focus in this review is on the various materials that have been reported earlier as dopants and modifiers used in silica glass optical fiber radiation dosimeters. This article also elaborates on recently reported optical fiber structures, namely, cylindrical fibers, photonic crystal fibers and flat fibers, as well as dimensions and shapes used for optimization of dosimeter performance. The various types of optical fiber radiation dosimeters are subsequently reviewed for various applications ranging from medical dosimetry such as in external beam radiotherapy, brachytherapy and diagnostic imaging, as well as in industrial processing and space dosimetry covering a dynamic dose range from μGy to kGy. Investigated dosimetric characteristics include reproducibility, fading, dose response, reciprocity between luminescence yield to dose-rate and energy dependence. The review is completed by a brief discussion on limitations and future developments in optical fiber radiation dosimetry.

Analysis of defects patterned by femtosecond pulses inside KBr and SiO2 glass

Colour centres in KBr and defects in silica glass were formed by focused femtosecond laser pulses. It is shown that under simple laser exposure, KBr develops a similar colouration as that achieved with electron and ion bombardment or high-energy X-ray irradiation. The three-dimensional (3D) character of direct laser writing in the volume of KBr allows a new level of control in the spatial arrangement of colour centres and defects. Five different colour centres were identified in KBr through the absorption spectrum; they have different charge and vacancy distribution configurations. The densities of the V- and F-centres were estimated to be 3.9 × 1019 and 3.4 × 1019 cm−3 using Smakula’s formula. In silica, a high density of paramagnetic E′ centres ~1.9 × 1020 cm−3 was determined by quantitative electron spin resonance spectroscopy. Birefringence due to colour centres and laser-induced defects was measured using Stokes polarimetry. In the case of colour centres in KBr, retardation in excess of 0.05 $$\pi$$ was determined throughout the visible spectrum from 400 to 800 nm. The use of polariscopy for analysis of high-pressure and high-temperature phase formation induced by 3D laser structuring is discussed.

Electronic structure and photoluminescence properties of Zn-ion implanted silica glass before and after thermal annealing

The results of XPS core-level and valence band measurements, photoluminescence spectra of a-SiO2 implanted by Zn-ions (E=30keV, D=1·1017cm−2) and Density Functional Theory calculations of electronic structure as well as formation energies of structural defects in silica glass induced by Zn-ion implantation are presented. Both theory and experiment show that it is energetically more favorable for implanted zinc ions to occupy the interstitial positions instead of cation substitution. As a result, the Zn-ions embedded to interstitials, form chemical bonds with the surrounding oxygen atoms, fabrication ZnO-like nanoparticles and oxygen-deficient SiOx matrix. The subsequent thermal annealing at 900°C (1h) strongly reduces the amount of ZnO nanoparticles and induces the formation of secondary α-Zn2SiO4 phase which markedly enhances the green emission.

Tailoring the size and distribution of Ag nanoparticles in silica glass by defects

The composites embedded with metallic nanoparticles show large nonlinear optical susceptibility and strong surface plasmon resonance absorption, which enable potential application in opto-electronics. Ion implantation has been proven to be a powerful technique of synthesis of metallic nanoparticles due to its versatility and compatibility. However, the synthesis of nanoparticles by ion implantation inevitably leads to a broad size distribution due to Ostwald ripening process. The broad size distribution has a negative effect on improving the figure of merits for nonlinear optics. In this paper, we tried to introduce defects in silica glass to act as pre-nucleation centers to mediate the size and distribution of Ag nanoparticles. In experiment, the silica glass samples were pre-irradiated by 200keV Ar ions to fluences of 0.8, 2.0 and 5.0×1016ions/cm2, and then 200keV Ag ions were implanted into the pre-irradiated samples to fluence of 2.0×1016ions/cm2. UV–VIS results show that the absorbance intensity of Ag SPR peak initially increases and then decreases with pre-irradiation fluence, which implies the change in size and density of Ag nanoparticles in samples. TEM results verify that Ag nanoparticles in the sample pre-irradiated to the fluence of 0.8×1016ions/cm2 grow bigger and distribute in a relatively narrow region comparing with that without pre-irradiation. With further increase of pre-irradiation fluence, the size of Ag nanoparticles shows a depth dependent distribution. A boundary can be clear seen at the depth of 110nm, larger Ag nanoparticles disperse in region shallower than 110nm, and smaller Ag nanoparticles disperse in the region deeper than 110nm. The average size of Ag nanoparticles initially increases and then decreases with pre-irradiation fluence. Therefore, the introduction of defects by pre-irradiation could be an effective way to tailor the size and distribution of metallic nanoparticles in matrix.

The transformation balance between two types of structural defects in silica glass in ion-irradiation processes

Structural modification in silica glass irradiated by Au ions was investigated by ultraviolet (UV), photoluminescence and Raman spectrum at the energies from 0.5 to 8 MeV with a fluence of 5 × 10 13 cm − 2 . In this transit energy region, both nuclear energy loss and electronic energy loss are not negligible. It was found that both the formation of irradiation-induced intrinsic defects and structural transformation from irregular large ring structure (LRS) to small three- and four-member ring structures (SRS) are dominated by the nuclear energy loss. Furthermore, unlike the case of irradiation with β, γ or proton, the concentration of non-bridging oxygen hole center is much enhanced followed with a distinct peak appearing at 5.05 eV in the UV absorption spectra that is attributed to divalent Si. The results suggest that the structural modification in silica glass in the transit energy region is dominated by nuclear energy loss. A mutual transformation balance model among irradiation-induced intrinsic defects, LRS and SRS is established to interpret the identical variation tendencies of intrinsic defects and structural transformation with ion energy.

Luminescence study of defects in silica glasses under near-UV excitation

In this paper, we report the results of photo-luminescence experiments on high purity silica glass. Two different types of defects located on optical pieces are investigated: indented sites and laser damage craters. Visible luminescence excited at the wavelength of 325 nm (3.81 eV) is observed using confocal microscopy. Each type of defect produces qualitatively different luminescence spectra. In the case of indented sites, the well known Oxygen Deficient Center (ODC) band peaking at 2.75 eV is clearly observed, while for laser damage areas, another classical luminescent object is found: the Non Bridging Oxygen Hole Center (NBOHC) which peaks at 1.9 eV. On both types of defects, the luminescence spectra yield a strong peak around 2.2 eV but its interpretation is still controversory.

Contribution of Spatially and Spectrally Resolved Cathodoluminescence to Study Crack-Tip Phenomena in Silica Glass

A current controversy over the nature of fracture in silica glass is revisited in the light of additional experimental evidence obtained by cathodoluminescence (CL) spectroscopy in the immediate crack-tip zone of two different types of silica glass. The Letter describes the dual experimental output obtained by monitoring optically active oxygen defects in silica glass, as follows: (i) analysis of local crack-tip stoichiometry (i.e., lattice defect population); and, (ii) spatially resolved stress analysis ahead of the crack tip based on a piezo-spectroscopic (PS) approach. CL experiments provided us with direct access to crack-tip stress fields, thus unfolding some missing detail about the complex mechanochemical interactions occurring at the crack tip in silica glass.

Annihilation of E′ center defects in silica glass by hydrogen treatment

It was found, using electron paramagnetic resonance (EPR), that the signal of E′ centers in silica glass totally disappeared following a 1 h heat-treatment at 1000 °C under hydrogen atmosphere. However, by subsequent heat-treatment at the same temperature under a dry nitrogen atmosphere, some of the E′ centers re-appeared.

Visualization of microscopic stress fields in silica glass in the scanning electron microscope

Quantitative measurements were made in silica glass of highly graded stress fields, as they developed: (i) in the K-dominated zone ahead of the tip of a median-type indentation micro-crack; and, (ii) at a silicon-silica interface of a metal-oxide semiconductor (MOS) device. Stress fields could be visualized on a microscopic scale using a field-emission scanning electron microscope (FE-SEM) equipped with a spectrally resolved cathodoluminescence (CL) device, according to piezo-spectroscopic (PS) assessments. The peculiarity of this newly proposed PS assessment resides in the fact that the performed CL/PS analysis exploited a peculiar luminescence emitted by optically active oxygen point defects in silica glass.

Conformational heterogeneity of the point defects in silica: The lifetime of the phosphorescence band at 2.7 eV

We have measured the excitation and emission energy dependence of the lifetimes of the 2.7eV photoluminescence band associated to oxygen deficient centers in silica glasses. The non-exponential behavior of this time decay is consistent with intrinsic conformational heterogeneity of these point defects in the amorphous matrix. Accordingly, we have analyzed the data in terms of a radiative rates distribution. Moreover, both surface and bulk typologies of these point defects have been studied. The mean value of the lifetime distribution of the surface defects increases from 12 to 15ms varying the excitation energy from 4.6 to 5.2eV, and it increases from 14 to 15ms in the emission energy interval between 2.6 and 3.0eV. As well similar variations of the lifetime are observed for interior defects, when measured at different excitation and emission energies. We can also estimate the width of the lifetime distribution of this ensemble of point defects in silica glass.

Improvement of Vacuum-Ultraviolet Transparency of Silica Glass by Modification of Point Defects(Review)

Silica glass is an important optical material in deep-ultraviolet (DUV, λ≤300 nm) and vacuum-ultraviolet (VUV, λ≤200 nm) spectral regions. However, DUV-VUV transparency of silica glass is significantly influenced by the presence of structural imperfections (point defects). This article describes point defects in silica glasses of low metallic impurity contents (“synthetic” silica glasses), and optical and chemical properties of these point defects. It is shown that controlled doping with specific point defects can improve the transparency and radiation hardness of silica glass in DUV-VUV spectral region. Such doped species include network modifiers that relax the glass structure and mobile interstitial hydrogen molecules. Utilization of these techniques in developing photomask substrates for F2 laser photolithography and DUV optical fibers for ArF laser transmission is presented.

Intense blue emission from tantalum-doped silicate glass

The authors report on an intense blue emission centered at 420nm under ultraviolet excitation from pentavalent tantalum ion-doped silicate glasses. The blue emission is distinctly different from emission due to defects in silica glass, which is also reflected in both absorption spectrum and remarkably enhanced emission. The authors suggest that localized Ta 5d0 energy level is responsible for the blue emission.

Piezo-spectroscopic assessment of nanoscopic residual stresses inEr3+-dopedoptical fibres

A quantitative characterization of residual stresses was performed with a nanometricspatial resolution in Er/Ge-doped silica glass fibres. Stresses were assessed by apiezo-spectroscopic technique (i.e., monitoring wavelength shifts), which used theelectro-stimulated luminescence of optically active defects in silica glass. Residual stressesof GPa magnitude were found near the core/cladding interface by using a nanometre-sizedelectron probe. Diffusion of Er/Ge ions from the core towards the cladding area was clearlyobserved in this optical device and the occurrence of this diffusive phenomenon permittedstress measurement in extended cladding regions beyond the core/cladding interface. Adiscussion is given of the impact of residual stresses on the optical performance of the fibredevice.

Formation of Radiation-Induced Defects in Silica Glasses at High Irradiation Temperatures

The mechanisms of formation of radiation-induced defects in silica glasses are investigated under both simultaneous and sequential exposure to γ-radiation and high temperatures. It is demonstrated that silica glasses contain E"(T) centers (T = Si and Al) in impurity-free regions andE"M (T) centers in the vicinity of alkali metal impurities. These centers can undergo transformations. The origin of a new center responsible for the absorption band at 360 nm is elucidated. This center is identified as a twofold-coordinated aluminum atom with a trapped electron.

Activation energy spectra for annealing of ion irradiation induced defects in silica glasses

In situ stress measurements were performed on alkali-borosilicate glass samples during and after 2 MeV Xe ion irradiation at several temperatures between 95 and 580 K. After switching off the ion beam, stress changes are observed that are related to the annealing of ion beam generated defects. The activation energy spectra for defect annealing are obtained from the data at each irradiation temperature. Defects are observed in the energy range from 0.26 to 1.85 eV. At each temperature the spectrum increases monotonously with activation energy. At each energy the defect density per unit energy is smaller at higher temperatures. This behavior can be explained using a binary collision model. The data are contrasted against the results obtained for 4 MeV Xe ion irradiation of thermally grown SiO 2 films, which can be explained using a thermal spike model. Measurements of the radiation induced viscosity support these ideas.

Defects and defect processes in silicon dioxide

Silicon dioxide has two outstanding characteristics: it is of great technological importance in several forms, and it involves physical phenomena of considerable scientific interest. In this paper we review several issues regarding defects and defect processes in SiO2. We consider issues associated with oxygen vacancies in different charge states in both α-quartz and silica glass, and ways that hydrogen can interact with vacancies. Interstitials are discussed, with particular focus on interstitial hydrogen. We reach two conclusions: first, that basic chemical notions of bonding seem to work well in describing processes in silicon dioxide, a relatively unconstrained material that can relax to yield chemically stable structures; and second, that considerable progress in understanding many of the questions that still surround defects in silica glass could be attained from a careful analysis of local structures in high-quality, many-atom structural models of silica.

Processing Induced Defects in Silica Glasses

The effects of the fiber drawing process on defects of silica fibers have been investigated. This study has focused on the oxygen deficient centers (ODCs) and E' centers induced by the fiber drawing process in low-OH oxygen deficient silica. The ODCs induced by the fiberization process have been investigated using the optical absorption band at 248 nm. In case of E' centers, optical absorption and electron spin resonance (ESR) have been used. The population of ODCs in the neck-down region has also been investigated. The concentration of ODCs are observed to increase dramatically within the neck-down region.

Positron annihilation and ESR study of irradiation-induced defects in silica glass

The angular correlation of positron annihilation radiation (ACAR), positron lifetime and electron spin resonance (ESR) have been measured on fused and synthetic silica glass samples before and after irradiation with fast neutrons up to a dose of 8.6 × 10 18 n/cm 2 at about 150°C or with 3 MeV electrons up to a dose of 1 × 10 18 e/cm 2 below 50°C. The ACAR curves are deconvoluted into two Gaussian components: a narrow and a broad component. The narrow component is due to self-annihilation of ortho-state of positronium (p-Ps) formed in “intrinsic” structural voids with radius of about 0.3 nm, while the broad component comes from pick-off annihilation of ortho-Ps and annihilation of positrons with valence electrons presumably associated with oxygen. Detailed post-irradiation experiments reveal that two kinds of positron trapping centers (defects) are introduced. Both kinds of the defects give almost the same broad component and markedly suppress Ps formation. This suggests that they are oxygen related centers. The first type defects with the positron lifetime of about 0.25 ns anneal out at 400–500°C, while the second type defects with lifetime of 0.47 ns recover after annealing at about 600°C. Electron spin resonance (ESR) measurements were also made on the same samples to detect irradiation-induced paramagnetic center (defects): E′ centers, peroxyradicals (POR) and non-bridging oxygen hole centers (NBOHC). The correspondence between the positron trapping centers and the ESR-active paramagnetic centers is not straightforward, partly because positrons can be trapped not only at the paramagnetic defects but also at diamagnetic defects. However, possible relations between these positron trapping centers and the paramagnetic defects are discussed.

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 absorption band peaking at 5.1 eV. This band is tentatively attributed to the optical absorption due to excitation from the inner shell of silicon.