Coefficients of stationary ArF laser pulse absorption in fused silica (type III)

Weak stationary bulk absorption coefficients α^stat(Φ, f) in fused silica are measured directly in dependence on the ArF laser fluence Φ and repetition rate f by the laser induced deflection technique (LID). The absorption is quantified using a modified Beer’s law. The investigated samples show non-linear increases α^stat(Φ, f = const.) for 3 ⩽ Φ ⩽ 25 mJ cm⁻² pulse⁻¹ and α^stat(Φ = const., f) for 50 ⩽ f ⩽ 400 Hz. The experimental data are fitted by a rate equation model consisting of an equilibrium between E′ center generation and annealing. Generation occurs photolytically from SiH and strained SiO bonds via 1- and 2-photon absorption, respectively. Annealing is due to the reaction with atomic hydrogen and other reaction channels, e.g. ODC generation.     

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.     

Stability of E′ centers induced by 4.7 eV laser radiation in SiO2

The kinetics of E′ centers ( Si) induced by 4.7 eV pulsed laser irradiation in dry fused silica was investigated by in situ optical absorption spectroscopy. The stability of the defects, conditioned by reaction with mobile hydrogen of radiolytic origin, is discussed and compared to results of similar experiments performed on wet fused silica. A portion of E′ centers and hydrogen are most likely generated by laser-induced breaking of Si–H precursors, while an additional fraction of the paramagnetic centers arise from another formation mechanism. Both typologies of E′ centers participate in the reaction with H₂ leading to the post-irradiation decay of the defects. This annealing process is slowed down on decreasing temperature and is frozen at T = 200 K, consistently with the diffusion properties of H₂ in silica.     

Temperature dependence of the generation and decay of E′ centers induced in silica by 4.7 eV laser radiation

We report a study of the generation of silicon dangling bonds (E′ centers) induced in fused silica by 4.7 eV laser irradiation in the 10 < T < 475 K temperature range, carried out by in situ optical absorption spectroscopy. The generation of the defects, occurring by transformation of pre-existing precursors, results to be a thermally activated process, quenched below 150 K and with a 0.044 eV activation energy. At T > 200 K the induced defects undergo a post-irradiation decay due to their reaction with mobile H₂. The interplay between generation and annealing gives rise to a bell-shaped temperature dependence of the concentration of induced E′ centers, peaking at 250 K.     

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.     

The shoving model for the glass-former LiCl·6H2O: A molecular dynamics simulation study

Molecular dynamics (MD) simulations of LiCl·6H₂O showed that the diffusion coefficient D, and also the structural relaxation time <τ>, follow a power law at high temperatures, D^?1 ∝ (T ? T_o)^?μ, with the same experimental parameters for viscosity (T_o = 207 K, μ = 2.08). Decoupling between D and <τ> occurs at T_x ～ 1.1T_o. High frequency acoustic excitations for the LiCl·6H₂O model were obtained by the calculation of time correlation functions of mass current fluctuations. The temperature dependence of the instantaneous shear modulus, G_∞(T), was considered in the shoving model for supercooled liquids [J.C. Dyre, T. Christensen, N.B. Olsen, J. Non-Cryst. Solids 352 (2006) 4635] resulting in a linear relationship log (D^?1) vs. G_∞/T.     

Thulium environment in a silica doped optical fibre

Thulium-doped optical fibre amplifiers (TDFA) are developed to extend the optical telecommunication wavelength division multiplexing (WDM) bandwidth in the so-called S-band (1460–1530 nm). The radiative transition at 1.47 μm (³H₄ → ³F₄) competes with a non-radiative multi-phonon de-excitation (³H₄ → ³H₅). The quantum efficiency of the transition of interest is then highly affected by the phonon energy (E_p) of the material. For reliability reasons, oxide glasses are preferred but suffer from high phonon energy. In the case of silica glass, E_p is around 1100 cm^?1 and quantum efficiency is as low as 2%. To improve it, phonon energy in the thulium environment must be lowered. For that reason, aluminium is added and we explore three different core compositions: pure silica, and silica slightly modified with germanium or phosphorus. The role of aluminium is studied through fluorescence decay curves, fitted according to the continuous function decay analysis. From this analysis, modification of the thulium local environment due to aluminium is evidenced.     

Melting enthalpy ΔHm for describing glass forming ability of bulk metallic glasses

A new melting enthalpy ΔH_m criterion for the prediction of glass forming ability (GFA) of alloys is proposed and five Zr–Al–Ni–Cu bulk metallic glasses (BMG) with critical dimension Z_max up to ? 7.5 mm are also developed by us in the light of the optimum ΔH_m of Zr–Al–Ni–Cu alloy system. And then, we researched the relationships between ΔH_m and two GFA parameters (critical cooling rate R_c and Z_max) of five bulk metallic glass (BMG) systems, such as Mg–Ni–Nd, Pd–Cu–Si, La–Al–Ni–Cu, Zr–Al–Ni–Cu and Zr–Ti–Ni–Cu–Be, respectively. The results show that the relationships between ΔH_m and R_c are all concave upward parabolas, and the optimum ΔH_ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.3960 kJ mol^?1, 21.2202 kJ mol^?1, 19.7146 kJ mol^?1, 18.1455 kJ mol^?1 and 13.1558 kJ mol^?1, respectively. On the contrary, the relationships between ΔH_m and Z_max are all concave downward parabolas, and the optimum ΔH_ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.5530 kJ mol^?1, 21.0830 kJ mol^?1, 19.6603 kJ mol^?1, 19.7231 kJ mol^?1 and 13.1173 kJ mol^?1, respectively. Furthermore, other BMGs’ R_cs or Z_maxs predicted by above-mentioned relationships satisfactorily agree with the tested results, which indicates that these relationships are reliable. However, the predicted results are reliable only if the main components are similar with the fitted BMGs or the additive is sparkle enough that the alloy’s character does not change. On the whole, the ΔH_m can act as a criterion for quickly predicting the alloy’s GFA and be helpful for the development of new BMGs.     

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.     

A calibration method for accurate prediction of amorphous to nanocrystalline transition from line intensities of optical emission spectrum

To be able to use the simple technique of optical emission spectroscopy (OES) for the prediction of the transition of growth from a-Si to nc-Si via the H_α/Si^? emission ratio, a regime-dependent correction factor is required to relate the measured H_α/Si^? emission ratio to the true flux (to the substrate) ratio of atomic hydrogen to deposited silicon radicals. Through an in-depth study in a very high frequency plasma enhanced chemical vapor deposition process, we obtained that the flux ratio of atomic hydrogen and deposited silicon radicals to the growing surface,Γ^H/Γ^Si, is related to the emission ratio of H_α and Si^?, I_rad^H_α/I_rad^Si*, by the relation, $R_{\text{rad}}\frac{I_{\text{rad}}^{{\text{H}}_{\alpha }}}{I_{\text{rad}}^{{\text{Si}}^{\text{*}}}}/\frac{{\Gamma }^{\text{H}}}{{\Gamma }^{\text{Si}}}=a\left\{{\left(p{d}_{}\right)}^2/k{T}_{\text{gas}}\right\}+b$, where the parameters p (pressure), d (inter-electrode distance) and T_gas (gas temperature) are experimentally obtained quantities and R_rad is the ratio of the rate coefficients for radiation of Si^? and H_α. We obtained the calibration parameters a and b to be 1.9·10^? 21 ± 2·10^? 22 Pa m^? 1 and 5.5 ± 1.9 respectively which is valid in a broad range of power and pressure settings. With these parameters, it is easy to estimate the flux ratio of atomic hydrogen and silicon species at any deposition condition using the OES data and this will allow accurate prediction of the phase transition. According to simulations in the linear low-pressure regime, the amorphous to nanocrystalline phase transformation occurs at the flux ratio Γ^H/Γ^Si = 12, which translates, using the factors a and b, to the required emission ratio.     

Alkaline earth zinc borate glasses doped with Cu2+ ions studied by EPR,optical and IR techniques

Copper ions incorporated into alkaline earth zinc borate glasses 10RO + 30ZnO + 60B₂O₃ (R = Mg, Ca and Sr) and 10SrO + (30 ? x)ZnO + 60B₂O₃ + xCuO (x = 0, 0.1, 0.3, 0.5, and 0.7 wt.%) were characterized by electron paramagnetic resonance (EPR), optical absorption and FTIR techniques. The EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in alkaline earth zinc borate glasses were present in octahedral sites with tetragonal distortion. The spin concentration (N) participating in resonance was calculated as a function of temperature for strontium zinc borate (SrZB) glass sample containing 0.7 wt.% of Cu²⁺ ions and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility (χ) was calculated at different temperatures and the Curie constant was evaluated from the 1/χ-T graph. The optical absorption spectra of these samples show only one absorption band. The optical band gap energies (E_g) and Urbach energy (ΔE) are calculated from their ultraviolet edges. The FTIR studies show different stretching and bending vibrations of alkaline earth zinc borate glasses.     

Improved method for preparation of anhydrous silica by microwave irradiation with spectroscopic characterization and toxicity assay

Amorphous anhydrous silica SiO_{2 (mw)} (99.99%) is successfully synthesized through microwave irradiation technique and time of reaction is reduced up to 1 h. The dehydration phase study of Si–water, Si–OH, Si–O–Si networking, elemental analysis and surface morphology was carried out by FTIR, FTNIR, SEM and EDAX spectroscopic techniques. The broad absorption stretching and bending of Si–OH and H₂O at 3695.38–2832.96 cm^? 1, 1638 cm^? 1 and 1191.20–1017.14 cm^? 1 completely disappeared and appearance of new bands at 946.93 and 797.63 cm^? 1 confirmed the amorphous anhydrous silica with Si–O–Si networking. The SEM images of SiO_{2 (mwc)} described the smooth and fine particle texture and confirmed 99.99% Si–O–Si networking of anhydrous silica. The 99.99% purity was verified by EDAX spectra which exhibited sharp signals only for oxygen and silicon. Toxicity against Monomorium minimum and Tribolium castaneum with 100% mortality and LT₅₀ 91 min and 7.5 h respectively is being reported. It can be used for long storage of grains in the future.     

Origins of optical absorption between 4.8 and 4.9 eV in silica implanted with Si and with O ions

We have determined some of the sources of the optical absorption bands between 4.8 and 4.9 eV in Si and O-implanted silica using several ion energies to produce layers of implanted ions with constant concentrations. The concentrations of implanted ions in the implanted layers ranged from >0.015 at.% to <3 at.%. Optical absorption was measured from 2.0 to 6.5 eV. Electron paramagnetic resonance measurements were made at ∼20.3 and 33 GHz for sample temperatures ranging from 77 to 100 K for most measurements. The components identified in the spectra, based on comparison with reported parameters, were, in the Si case, due to E′ centers and peroxy radicals. In the O case they were due to E′ centers, non-bridging oxygen hole centers, peroxy radical centers, and a newly appearing state which we labeled the OS center. By comparing the changes in the absorption at 4.83 eV with the changes in the concentrations of the various electron paramagnetic resonance components and with the reports in the literature, we conclude that there are at least four oxygen related centers and one Si related center absorbing between 4.8 and 4.9 eV.     

Growth of CdSiP2 single crystals by self-seeding vertical Bridgman method

Using a high purity CdSiP₂ polycrystalline charge synthesized in a single-temperature zone furnace, a CdSiP₂ single crystal with dimensions of 8 mm in diameter and 40 mm in length was successfully grown by the vertical Bridgman method. The quality of the crystal was characterized by high resolution X-ray diffraction and the full width at half maximum (FWHM) of the rocking curve for the (200) face is 33″. Thermal property measurements show that: the mean specific heat of CdSiP₂ between 300 and 773 K is 0.476 J g^?1 K^?1; the thermal conductivity of the crystal along the a- and c-axes is 13.6 W m^?1 K^?1 and 13.7 W m^?1 K^?1 at 295 K, respectively; and the thermal expansion coefficient measured along the a- and c-axes is 8.4×10^?6 K^?1 and ?2.4×10^?6 K^?1, respectively. The optical transparency range of the crystal is 578–10,000 nm, and there is no absorption loss in the spectrum from 0.7 to 2.5 μm, as often exists with ZnGeP₂ crystals grown from the melt.     

Kerr studies of several tellurite glasses

Estimates of Kerr electrooptical sensitivity of several tellurite glasses are presented. The highest value of Kerr coefficient B ～ 190 × 10^?16 m V^?2 is registered for 0.6TeO₂–0.3TlO_0.5–0.1ZnO glass. This evidences the prospects of thallium–tellurite glass system for electrooptical applications. A gradual decrease of B from 41 × 10^?16 to 26 × 10^?16 m V^?2 in (1 ? x) TeO₂ – xNbO_2.5 system is revealed for x increasing from 0.1 to 0.15. No crystalline phase was found in that system, thus allowing attributing its Kerr sensitivity to the intrinsic properties of the glass matrix. The Kerr coefficient variation from 66 to 81 × 10^?16 m V^?2 was observed for 0.85TeO₂–0.15WO₃ glasses co-doped with small amounts of silver and cerium. The analysis of optical absorption spectra of several silver-containing tellurium–tungsten oxide glasses makes it possible to think that introducing cerium provokes formation of new mid-range orderings.     

Smooth titania nanotubes: Self-organization and stabilization of anatase phase

We present transmission electron microscopy and Raman scattering measurements showing that niobium inhibits the processes of nucleation and growth of anatase crystallites in the initial amorphous titania nanotubes and thus shifts the temperature of the complete amorphous-to-anatase phase transition to higher values up to 550 °C. Niobium dopants stabilize the anatase phase in titania nanotubes up to 650 °C. The size of anatase crystallites can reach 30–50 nm. Excess niobium atoms which are pulled off from the volume of anatase crystallite form polymeric or monomeric Ti–O–NbO groups at the interface area. Slight shift and broadening of E_g (144 cm^?1), A_1g (515 cm^?1) and E_g (630 cm^?1) modes in Raman spectra can be explained by niobium insertion into the anatase structure.     

Preparation of super hydrophobic silica aerogel and study on its fractal structure

The experimental results on silica aerogels with super hydrophobic property are reported. Silica alcogels were prepared via a two-step acid/base process by keeping the molar ratio of tetraethyoxysiliane (TEOS), ethanol (EtOH), water (H₂O), hydrochloric acid (HCl) and ammonia (NH₄OH) constant at 1:6:8:1.0 × 10^?3:1.1 × 10^?2, respectively, and varying the molar ratio of N,N-dimethylformamide (DMF)/TEOS (G) from 0 to 1.2. After two aging treatment steps, they were modified by isopropyl alcohol (IPA)/trimethylchlorosilane (TMCS)/n-hexane solution at 60 °C. It was found that G value at 0.8 resulted in low density (～0.2 g cm^?3) and the minimum volume shrinkage (～6%), with the total water adsorption ratio ～5.1% when exposed to water for 3 months and the contact angle θ ≈ 178°. Besides, the aerogels (G = 0.8) had higher volume fractal dimension (～1.8), which indicted that it possessed better connectivity and more uniform particle sizes.     

Effects of hydration,annealing, and melting on heat transport properties of fused quartz and fused silica from laser-flash analysis

Thermal diffusivity (D) at high temperature (T) was measured from 15 samples of commercial SiO₂ glasses (types I, II, and III with varying hydroxyl contents) using laser-flash analysis (LFA) to isolate vibrational transport, in order to determine effects of impurities, annealing, and melting. As T increases, D_glass decreases, approaching a constant (~ 0.69 mm²s^? 1) above ~ 700 K. From ~ 1000 K to the glass transition, the slope of D is small but variable. Increases of D with T of up to 6% correlate with either low water and/or low fictive temperature and are attributed to removal of strain and defects during annealing. Upon crossing the glass transition, D substantially decreases to 0.46 mm²s^? 1 for the anhydrous melt. Hydration decreases D_glass, makes the glass transition occur over wider temperature intervals and at lower T, and promotes nucleation of cristobalite from supercooled melt. Due to the importance of thermal history, a spread in D of about 5% occurs for any given chemical type. Combining prior steady-state, cryogenic data with our average results on type I glass provides thermal conductivity (k_lat = ρC_PD) for type I: k_lat increases from ~ 0 K, becoming nearly constant above 1500 K, and drops by ~ 30% at T_g. We find that D^? 1(T) correlates with thermal expansivity times temperature from ~ 0 K to melting due to both properties arising from anharmonicity.     

Pronounced variation in the collective dynamics of highly correlated lightest liquid alkali metal: Lithium

Recently measured inelastic X-ray spectra (IXS) of detailed coherent dynamical structure factor S(κ, ω) and hence the equilibrium collective dynamics, of the lightest liquid alkali metal, lithium at 475 K, have been successfully explained using the modified microscopic theory of the collective dynamics of a simple liquid, in a huge wave-vector, κ, range: 1.4 nm^?1 ≤ κ ≤ 110.0 nm^?1, ?κ is the linear momentum transfer. The role of single particle motion in the collective dynamics of the liquid changes from diffusive for smaller values of wave-vector, κ < 21 nm^?1 to that of a free particle for higher κ-values, 21 nm^?1 ≤ κ ≤ 110 nm^?1. The quantum correction due to detailed balance condition in S(κ, ω) for liquid Li, whose dynamics, unlike that of quantum liquid ⁴He, is essentially classical, yields results in better agreement with the corresponding experimental S(κ, ω) and the quantum correction becomes significant for higher values of κ and ω. The wave-vector dependent variation of longitudinal viscosity, η_l, is in good agreement with the corresponding results obtained from memory function approach. The wave-vector dependent variation of single characteristic relaxation time lies in between the variation of two relaxation times of memory function approach.     

Electrooptical Kerr phenomenon and Raman spectroscopy of one lithium–niobium–silicate glass-forming system

Raman spectra and electrooptical Kerr coefficients of glasses belonging to one lithium–niobate–silicate glass-forming system xNb₂O₅ · (66 ? x)SiO₂ · 19Li₂O · 11K₂O · 2B₂O₃ · 2CdO are studied. It has been found that these glasses demonstrate a record value of electrooptical Kerr coefficient; the glass with x = 35 showed electrooptical Kerr coefficient equal to 266 × 10^?16 m/V². Using Raman spectroscopy combined with the concept of Constant Stoichiometric Groupings, a correlation of electrooptical Kerr coefficients of these glasses with the content of Li₂O · Nb₂O₅ (or 2LiNbO₃) groupings has been demonstrated. The hypothesis that electrooptical Kerr sensitivity of glasses is related to the ordered regions with composition and symmetry corresponding to some of known electrooptical crystals has been verified. These regions, which the authors called ‘Crystal Motifs’, are identified with the groupings found in studying Raman spectra of the glasses.