γ-ray induced GeODC(II) centers in germanium doped α-quartz crystal

Main luminescence of α-quartz crystal doped with germanium results from the luminescence of a self-trapped exciton (STE) near germanium. In as grown Ge-doped α-quartz crystal, the luminescence associated with the twofold coordinated Ge center (GeODC) in amorphous silica glass doped with germanium, was never observed. In this work, we performed experiments to investigate if a GeODC like luminescence could appear after a γ-irradiation of a Ge-doped α-quartz crystal. The answer is positive: under excitation with pulsed light of an ArF laser (193 nm): a new luminescence with two bands — a blue one associated to a time constant of about 100 μs appears and another one with faster decay of ~ 1.5 ns appears in the ultraviolet part of the spectrum under the same excitation. This last emission has similar characteristics as the GeODC luminescence of silica glass. However, clear differences exist between the radiation-induced center associated with this luminescence and the GeODC. The excitation with a KrF laser does not provide emission with a decay time constant of about 100 μs but provides blue luminescence with a faster decay of about 4 μs. The pulses of the ArF laser also excite this component of decay for the blue band. We attribute this emission to various types of γ-ray created centers in radiation damaged areas of the Ge-doped crystal. Under excitation with an F₂ excimer laser (157 nm), the luminescence of STE near Ge remains in the irradiated sample.     

Absorption and luminescence in amorphous SixGe1-xO2 films fabricated by SPCVD

Optical absorption and photoluminescence of Ge-doped silica films fabricated by the surface-plasma chemical vapor deposition (SPCVD) are studied in the 2–8 eV spectral band. The deposited on silica substrate films of about 10 μm in thickness are composed as x·GeO₂-(1-x)·SiO₂ with x ranging from 0.02 to 1. It is found that all as‐deposited films do not luminesce under the excitation by a KrF (5 eV) excimer laser, thus indicating lack of oxygen deficient centers (ODCs) in them. After subsequent fusion of silicon containing (x < 1) films by a scanning focused CO₂ laser beam absorption band centered at 5 eV as well as two luminescence bands centered at blue (3.1 eV) and UV (4.3 eV) wavelengths arise, highlighting the formation of the ODCs. The excitation of unfused SPCVD films by an ArF (6.4 eV) excimer laser yields a luminescence spectrum with two bands typical for the ODCs, but with a faster decay kinetics. Intensities of these bands grow up with samples cooling down to a temperature of 80–60 K. Unfused films excited by the ArF laser also demonstrate luminescence due to recombination of a trapped charge resulted from the excitation of localized electron states of the glass network. In the unfused GeO₂ film luminescence related to a self-trapped exciton (STE) typical for GeO₂ crystals with α-quartz structure is observed. The observed STE luminescence can be indicative of the crystalline fraction availability in the film. Whereas GeO₂ crystals are known as not containing twofold coordinated germanium, a polycrystalline inclusion in the SPCVD GeO₂ film serves as a factor explaining the absence of any spectroscopic manifestation of this type of defects in it even after fusion. On the other hand, lack of STE luminescence in other unfused films with x < 1 testifies truly amorphous state of the matter in them.     

Vacuum ultraviolet excitation of the 2.7 eV emission band in neutron irradiated silica

The temperature dependence of the photoluminescence induced at 2.7 eV by ultraviolet (UV) and vacuum ultraviolet (VUV) excitation of neutron irradiated (10²¹ n/m² and 10²² n/m²) KU1 and KS-4V high purity silica, with different OH content, have been studied. Commercial silica Infrasil 301 has also been studied for comparison. At the highest neutron fluence and at the same temperature, the three irradiated silica grades show similar excitation spectra. Two close UV excitation bands, which show opposite temperature dependence, are observed at 4.8 and 5.1 eV. The 4.8 eV band, related to the triplet–singlet transition in SiODCs(II), decreases on decreasing temperature from 300 to 10 K and the band at 5.1 eV, probably related to SiODCs(I), is observed only at very low temperatures (～10 K). An important VUV excitation structure, observed at low temperature, could also be related to SiODCs(I). A shift of the irradiated bands is detected at low temperature.     

Anisotropic luminescence photo-excitation in H2-loaded Ge-doped silica exposed to polarized 193 nm laser light

We have investigated the polarization dependence of the photo-luminescence VUV excitation spectra in H₂-loaded Ge-doped SiO₂ glass exposed to polarized 193 nm laser light. As for non-H₂-loaded Ge-doped silica, we show that the β band photo-luminescence excited in the VUV spectral range (6–9 eV) remains positively polarized. In our experiments, the polarization degree P is quite high (P ≈ 0.4) due to the preferential bleaching of polarized UV-exposure. As a result, we observe a highly anisotropic luminescence photo-excitation since the luminescence is mainly polarized in the writing laser polarization direction.     

Thermal decomposition and new luminescence bands in wet,dry, and additional oxygen implanted silica layers

Wet and dry silica oxide layers have been treated thermally up to T_a = 1300 °C and were investigated by cathodoluminescence (CL) spectroscopy. Whereas the dry oxides after high temperature treatment show an increase of the yellow–red spectra region, contrary, in wet oxides the UV–blue region is enhanced. Even a new strong band in the near-UV region (NV) at 330 nm (3.76 eV) is found for wet oxides at liquid nitrogen temperature (LNT), but much broader and with lower intensity for room temperature (RT) in a triple band structure UV: 290 nm, NV: 330 nm, and V: 400 nm. These violet bands should be associated with a thermally decomposed and rapidly cooled-down silica network in presence of OH groups or even dissociated oxygen. Additional oxygen implantation into dry silica with high doses up to 10¹⁷ ions/cm² and high thermal treatment T > 1100 °C leads as well to enhanced UV–NV–V luminescence emission bands supporting the fact that oxygen and structural decomposition play a decisive role in formation of near-UV luminescent defects in silica.     

High-temperature treatment of hydrogen loaded GeO2:SiO2 glasses for photonic device fabrication

High-temperature treatment of hydrogen loaded silica- and germanium doped silica glass, also referred to as OH flooding, has been studied. The removal mechanism of hydroxyl groups in silica glass, during OH flooding, occurs by formation and diffusion of molecular hydrogen, while in germanium doped silica the main diffusion mechanism is attributed to diffusion of molecular water. UV exposure of OH flooded and non-treated germanium doped silica samples, from a ArF laser at 193 nm, show large changes in the asymmetric stretching vibration of Si–O–Si bridges, indicating compaction of the glass network. In addition, the thermal relaxation kinetics of the UV induced compaction are found to be similar for non-treated samples and OH flooded samples.     

VUV and IR absorption spectra induced in H2-loaded and UV hyper-sensitized standard germanosilicate preform plates through exposure to ArF laser light

H₂-loaded Ge-doped preform plates have been UV hyper-sensitized and subsequently post-exposed by means of UV ArF laser pulses at 193 nm. Both Fourier Transform Infrared (FTIR) and Vacuum Ultraviolet (VUV) absorption spectroscopy has been carried out at each step of the sensitization process with a view to get a better understanding of the UV hyper-sensitization process at 193 nm. Exposing the H₂-loaded samples firstly triggers a partial bleaching of the 5 eV absorption band, followed by a new growth of the band for longer exposure time. As it has already been reported for exposure at 248 nm, the exposure at 193 nm firstly yields an increase in the absorption ascribed to hydroxyl (≈3600 cm⁻¹), hydride species (≈2140–2185 cm⁻¹) and GeE′ defects (≈6.3 eV). The evolution of the absorption related to the GeH₂ species is not monotonous but rather the absorption reaches a peak (N_pre = 2000) and then slightly decreases for further exposure time. Post-exposing the hyper-sensitized samples (N_pre = 2000) lead to a partial bleaching of the absorption ascribed to hydride species and to increases in the intensity of the GeE′ defect-related absorption and in the absorption ascribed to hydroxyl species. The accurate determination of the species concentration from the absorption spectra proved to be rather tricky due to the problem of accurately measuring the depth of the UV beam penetration at each time of the exposure. Nevertheless, a correlation could be established between the growth of the UV-induced UV excess loss ascribed to GeE′ species and the UV-induced decrease in the intensity of the IR bands related to hydride species. These observations are discussed within the frame-work of the two step model.     

Crucial dependence of excimer laser toughness of “wet” silica on excess oxygen

Creation of point defects by ArF (6.4 eV) and F₂ laser (7.9 eV) irradiation in synthetic “wet” silica glass thermally loaded with interstitial O₂ molecules was studied by optical absorption, electron paramagnetic resonance and infrared absorption. The presence of excess oxygen caused a significant increase of laser-induced ultraviolet (UV) absorption, which was 4 times (7.9 eV-irradiation) and > 20 times stronger (ArF irradiation) as compared to O₂-free samples. The spectral shape of photoinduced absorption nearly completely coincided with the spectral shape of oxygen dangling bonds (NBOHC) in 3 to 6.5 eV regions. The contribution of Si dangling bonds (E' centers) was less than few % and was not dependent on oxygen content. Peroxy radical defects were not detected. The photoinduced NBOHCs thermally decayed at 400...500 C. However, a subsequent brief 7.9 eV irradiation restored their concentration up to 70%. This sensitization can be in part attributed to generation of interstitial Cl₂ and HCl. These data show that oxygen stoichiometry is an important factor for maximizing laser toughness of wet silica.     

Photoluminescence characterization of aged and regenerated mesoporous silica

The investigation of the photoluminescence features of aged and regenerated mesoporous silica is reported. The emission spectrum of aged samples displays a blue band peaked at about 450 nm at room temperature with excitation channels at 250 and 200 nm. No UV emission band is detected. The regenerated samples recover the optical transparency of the samples, the vibrational properties in the 3000–3800 cm^? 1 but for the isolated silanols band and the UV-blue emission features. Indeed beside the blue band peaked around 450 nm a UV band centred at 340 nm is reported whose relative contribution depends on the excitation wavelength. The aging and regeneration processes are discussed in view of the attribution of the observed emission features to specific surface defects.     

Photoinduced changes in UV absorption spectra of nitrogen-doped silica caused by exposure to ArF excimer laser

Absorption spectra of high-purity silica, in which 1.2% or 3.0% oxygen atoms are replaced by nitrogen, are measured in the spectral interval of 3.0–6.5 eV. Photoinduced changes of these spectra when exposed to 193 nm wavelength excimer laser radiation, pulse intensity being ～50 mJ/cm², are examined. Absorption spectra relaxation under subsequent annealing as well as changes brought about by saturation of glass with molecular hydrogen are studied. Parts of graded-index fiber ～1 mm in length and 200 μm thick transverse slices of a fiber preform served as samples. It is found that exposure of fiber samples to laser irradiation brings about a significant decrease of initially more intense absorption band of Si-ODC in the region of 5.0 eV and an increase of initially less intense band centered at 5.8 eV at a time. In bulk samples correlation of these bands is opposite, photoinduced changes are less expressed against structureless absorption tail increase in the spectral interval of 3.0–6.5 eV. It is shown that subsequent 20 min 700 °С annealing leads to the relaxation of photoinduced changes in absorption spectra in bulk and fiber samples. By placing irradiated samples into molecular hydrogen atmosphere at room temperature absorption bands are suppressed and transparency at shorter wavelengths of UV region is increased. Data obtained is discussed in the context of photosensitivity of nitrogen-doped silica-core fibers, which serve a technological platform for thermoresistant in-fiber Bragg gratings fabrication.     

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 7.6 eV. 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 7.6 eV absorption band was deliberately increased by treatment in hydrogen at 1200 C for 100 h. 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 7.6 eV 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 7.6 eV 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 7.6 eV. 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 luminescence properties and the surface structures of submicron silica particles

We investigated the correlation between the luminescence properties and the surface structures of submicron silica particles prepared by the Stöber method. After annealing in a non-oxidizing atmosphere, the submicron-sized silica particles show a broad photoluminescence (PL) band at 500–540 nm by excitation at an ultraviolet wavelengths (254 and 365 nm), and the one at the 600 nm by excitation an Ar⁺ laser (488 nm). The PL appeared to result from the removal of impurities and subsequent formation of several luminescent structures on the internal surface of the primary particles by thermal annealing.     

Rapid lifetime testing of fused silica for DUV laser applications

Lifetime testing of fused silica's absorption degradation upon 193 nm is shortened by enhancing the two-photon absorption (TPA) induced generation of E′ and NBOH defect centers per laser pulse. Increasing the irradiation fluence from typical marathon test values H < 1 mJ/cm² to H = 10 mJ/cm² gives a more efficient TPA process. In addition, the sample's temperature is lowered to ?180 °C during irradiation yielding an increased breaking efficiency of weak Si–O bonds per pulse. A small sample length of 10 mm combined with the laser induced deflection (LID) technique for direct absorption measurements prevents microchannel (MC) formation, a common break-down criterion in marathon tests.For a UV grade fused silica sample (type III) the end of absorption degradation is found after about 1.2 * 10⁷ laser pulses. Absorption measurements between 3 and 25 mJ/cm² before and after lifetime testing reveal that the laser induced absorption change decreases with decreasing fluence. The experimental results are in good agreement with a real marathon test at a fixed fluence.     

Effects induced by 4.7 eV UV laser irradiation on pure silica core multimode optical fibers investigated by in situ optical absorption measurements

We investigated by in situ optical absorption measurements the effects induced by 4.7 eV UV laser irradiation on pure silica core optical fibers. Laser irradiation with 100 MW cm^? 2 laser intensity generates in the fiber E′ centers which partially decay after irradiation due to their reaction with diffusing H₂. An absorption band peaked at 5.3 eV is observed to grow in the post-irradiation stage with a kinetics anti-correlated to the decay of the 5.8 eV band of the E′ centers. The defect absorbing at 5.3 eV is proposed to be formed by trapping on pre-existing precursors of hydrogen atoms made available by breaking of H₂ on E′. We also show by repeated irradiation experiments that the 5.3 eV-absorbing center is photochemically destroyed by 4.7 eV laser light, and we estimate the cross section of this process. Possible structural models for this defect are discussed.     

Role of diffusing molecular hydrogen on relaxation processes in Ge-doped glass

Temperature dependencies of steady-state and time-resolved photoluminescence (PL) from triplet state at 3.1 eV and singlet state at 4.2 eV ascribed to the twofold-coordinated Ge have been measured in unloaded and H₂-loaded Ge-doped silica samples under 5.0 eV excitation in the 10–310 K range. Experimental evidences indicate that diffusing molecular hydrogen (H₂) depopulates by a collisional mechanism the triplet state, decreasing both its lifetime of about 14% and the associated triplet PL intensity, whereas those of the singlet are insensitive to the presence of H₂.     

Photosensitivity of SiO2–Al and SiO2–Na glasses under ArF (193 nm) laser

Photosensitivity of SiO₂–Al and SiO₂–Na glass samples was probed by means of the induced optical absorption and luminescence as well as by electron spin-resonance (ESR) after irradiation with excimer-laser photons (ArF, 193 nm). Permanent visible darkening in the case of SiO₂–Al and transient, life time about one hour, visible darkening in the case of SiO₂–Na was found under irradiation at 290 K. No darkening was observed at 80 K for either kind of material. This investigation is dedicated to revealing the electronic processes responsible for photosensitivity at 290 and 80 K. The photosensitivity of both materials is related to impurity defects excited directly in the case of SiO₂–Na and/or by recapture of self-trapped holes, which become mobile at high temperature in the case of SiO₂–Al. Electrons remain trapped on the localized states formed by oxygen deficient defects.     

Luminescence properties of nonbridging oxygen hole centers at the silica surface

Two variants of the surface-nonbridging oxygen hole center, (Si–O)₃Si–O^? and (Si–O)₂(H–O)Si–O^?, stabilized in porous films of silica nano-particles were investigated by time resolved luminescence excited in the visible and UV spectral range by a tunable laser system. Both defects emit a photoluminescence around 2.0 eV with an excitation spectrum evidencing two maxima at 2.0 and 4.8 eV, this emission decreases by a factor ～2 on increasing the temperature from 8 up to 290 K. However, the different local structure influences the emission lineshape, the quantum yield and the decay lifetime. Such peculiarities are discussed on the basis of the symmetry properties of these defects.     

Influence of Ce codoping and H2 pre-loading on Er/Yb-doped fiber: Radiation response characterized by Confocal Micro-Luminescence

Confocal microscopy luminescence measurements were applied to study the X-ray radiation response of Er/Yb-doped optical fibers in connection with H₂ pre-loading and with the addition of another lanthanide element (Cerium) in the core composition. Laser excitations at 488 nm and 325 nm allow deriving the emission and absorption pattern of Er³⁺, the latter derived from the dips appearing in a wide luminescence band related to defects in silica. We found that the luminescence spectrum of the X-irradiated Er/Yb-doped core fiber evidences an increase in the emission intensity around 520 and 660 nm; in contrast, no changes are induced by radiation neither after H₂ pre-loading nor when the Cerium is added to the core composition. Both treatments reduce the generation of defects in the Er-doped fibers thus providing hardness in the radiative environment.