ArF laser induced pulse absorption in fused silica (type III): Modeling the fluence and repetition rate dependence

At 193 nm, weak stationary bulk absorption coefficients α^stat in standard and experimental grade fused silica (type III) are measured in dependence on the laser fluence H and repetition rate f. The samples show non-linear increases α^stat(H) for 0.2 ? H ? 5 mJ cm^?2 pulse^?1 (f = const.) and α^stat(f) for 100 ? f ? 1000 Hz (H = const.). An absorption model, focussing on ArF laser induced E′ center generation and annealing, and the associated rate equations are applied to simulate the experimental data quantitatively. From the simulations, material parameters like the 2-photon absorption (TPA) coefficient, the E′ center absorption cross section σ_E′ and the hydrogen related E′ annealing rate are calculated. TPA coefficients values of 9.7 · 10^?9 cm/W (standard grade material) and 1.4 · 10^?8 cm/W (experimental grade material), E′ center cross sections of 4.5 · 10^?18 and 3.6 · 10^?18 cm² and hydrogen annealing rates of 1.5 s^?1 (standard grade) and 3.4 s^?1 (experimental grade) are found.     

Generation and annealing of defects in virgin fused silica (type III) upon ArF laser irradiation: Transmission measurements and kinetic model

The transmission of ArF laser pulses in virgin fused silica (type III) samples changes during N = 10⁶ pulses at an incoming fluence H_in = 5 mJ cm⁻² pulse⁻¹. The related absorption is determined by the pulse energy absorption coefficient α(N, H_in) using a modified Beer’s law, yielding initial values α_ini around 0.005 cm⁻¹, a maximum α_max ? 0.02 cm⁻¹ at N = 10³-10⁴ and stationary values 0.0045 cm⁻¹ ? α_end ? 0.0094 cm⁻¹ after N ≈ 6 × 10⁵ pulses. The development α(N, H_in = const.) is simulated by a rate equation model assuming a pulse number dependent E′ center density E′(N). E′(N) is determined by a dynamic equilibrium between E′ center generation and annealing. Generation occurs photolytically from the precursors ODC II and unstable SiH structures upon single photon absorption and from strained SiO bonds via two-photon excitation. Annealing in the dark periods between the laser pulses is dominated by the reaction of E′ with H₂ present in the SiO₂ network. The development α(N, H_in = const.) is observed for the very first sample irradiation only (virgin state). The values α_end are not accessible by simple spectrophotometer measurements.     

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.     

Laser-induced defects in fused silica by UV laser irradiation

The structure of fused silica with irradiation of the third harmonic of the Nd:YAG laser was investigated by Fourier transform infrared, Raman and Photoluminescence spectroscopy. Some variation in the Si/O stoichiometry of silica in the ablated spot was induced. The primary defect species are oxygen deficient centers and oxygen interstitials. The frequency shift of the Si–O–Si vibration proves that the central force constant between oxygen and silicon atoms, and the band angle of Si–O–Si increases in the UV-laser ablation spot. Small silicon clusters within SiO_x appear to be a possible explanation for the 564 nm Fluorescence peak, and the 181 cm⁻¹ Raman peak.     

Use of EELS to study the absorption edge of fused silica

Electron energy loss spectroscopy (EELS) was used to study the absorption edge of various types of fused silica. The relationships between transmission (%T), fictive temperature (T_f), band gap energy (E₀), and slope of the Urbach tail (K) were investigated. The trends are in agreement with other published studies and provide initial validation of the new technique. The measured band gap energies, E₀, varied very little among the different silicas, and there was no obvious relationship between E₀ and either %T or T_f. The slopes of the Urbach tail, K, varied significantly and were related to both %T and structural disorder (influenced by both T_f and F concentration). The observed trends were an increase in transmission with an increase in K, and an increase in K with a decrease in structural disorder. The results of this work support the idea that structural disorder affects transmission near the absorption edge.     

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.     

High refractive index contrast in fused silica waveguides by tightly focused,high-repetition rate femtosecond laser

A new domain of optical waveguide writing with record high refractive index contrast (0.022) is reported in fused silica by strong focusing of a 522 nm wavelength, 500 kHz repetition rate femtosecond laser with oil-immersion optics. The strongly confining waveguide supports a mode of only 7 μm mode field diameter at 1550 nm wavelength, opening the door for higher density integration in photonic circuits formed by femtosecond lasers. It is found that green and fundamental wavelengths have similar absorption in femtosecond laser waveguide writing in fused silica and that the advantage of the second harmonic is simply from an increased fluence through a smaller focal volume.     

Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses

The morphology and microstructure induced in high quality fused silica by UV (355 nm) laser pulses at high fluence (10-45 J/cm²) have been investigated using a suite of microscopic and spectroscopic tools. The laser beam has a near-Gaussian profile with a 1/e² diameter of ∼0.98 mm at the sample plane and a pulse length FWHM (full width at half maximum) of 7.5 ns. The damage craters consist of a molten core region (thermal explosion), surrounded by a near concentric region of fractured material. The latter arises from propagation of lateral cracks induced by the laser-generated shock waves, which also compact the crater wall, ∼10 μm thick and ∼20% higher in density. The size of the damage crater varies with laser fluence, number of pulses, and laser irradiation history. In the compaction layer, there is no detectable change in the Si/O stoichiometry to within ±1.6% and no crystalline nano-particles of Si were observed. Micro- (1-10 μm) and nano- (20-200 nm) cracks are found, however. A lower valence Si³⁺ species on the top 2-3 nm of the compaction layer is evident from the Si 2p XPS. The results are used to construct a physical model of the damage crater and to gain critical insight into laser damage process.     

Molecular dynamics study of UV-laser-induced densification of fused silica. II. Effects of laser pulse duration,pressure, and temperature,and comparison with pressure-induced densification

Numerical simulations on UV-laser-induced densification of fused silica have been performed using classical molecular dynamics. The effect of laser irradiation is modeled by the energy transfer from the absorbed laser photons to the bonded silicon and oxygen atoms, i.e., the thermal effects of laser-irradiation on silica glass. Results from simulations at various laser pulse duration, pressure, and temperature conditions show that longer laser pulse duration, higher pressure, and higher temperature cause larger densification. We have also compared the microstructural and elastic properties of fused silica densified by UV-laser and hydrostatic pressure, respectively. Similar changes are observed in both cases; several notable differences are noticed, too, and include Si–O bond length change, number of over-coordinated atoms, and ring distributions.     

The thermodynamics of water and hydrogen solubility in fused silica

A statistical thermodynamic model of the solubility of water vapor in glass is developed as an extension of previous models of monatomic and diatomic gas solubility in glass. The model predicts the $\text{p}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence of water vapor solubility in fused silica and indicates the binding energies of dissociated H and OH species to be about ?63 and ?99 kcal/mol, respectively. The OH binding energy is found to be slightly temperature dependent, namely E_OH(0) = ?81.3?0.0163T. The tendency of chemical solubility of H₂ to predominate over physical solubility above 500°C correlates with the model equations for H₂ solubility.     

Emission enhancement of Eu(III) and/or Tb(III) ions entrapped in silica xerogels with ZnO nanoparticles by energy transfer

Luminescent materials consisting of Eu(III) and/or Tb(III) ions and quantum dots of the ZnO semiconductor were immobilized in oxide xerogels by the impregnation method. The material’s excitation spectra showed the characteristic absorption bands of lanthanide(III) ions and sharp bands at low wavelengths attributed to excitons in the semiconductor nanoparticles. The luminescence emission of Ln(III) ions (Ln = Tb and/or Eu) showed very low intensity when typical excitation wavelengths for both ions were used (λ_exc = 394 for Eu(III), 350 nm for Tb(III)). In three-component materials (ZnO quantum dots and Ln(III) ions in oxide xerogels) only the Tb(III) emission could be improved by using an excitation wavelength corresponding to the position of one of the exciton absorption bands. The energy transfer enhancing the Eu(III) emission was possible in the four-component system (ZnO quantum dots plus Tb(III) and Eu(III) incorporated into the oxide matrix). The best results of the Eu(III) emission intensity were obtained when the silica xerogel served as the matrix of the four-component material thermally treated at 80 °C.     

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.     

Luminescent materials consisting of Eu(III) ions complexed in heteropolyoxometalates incorporated into silica xerogels

Luminescent materials based on the energy transfer effect consist of heteropolyoxometalates of the type K₁₃[Eu(SiMo_xW_11−xO₃₉)₂] incorporated into xerogel matrices. Results of our experiments suggest that not only the composition parameter x (where x = 1, 3 and 5) of the salts with Eu(III) complex but also the type of the matrix influence the luminescent properties of these materials. The luminescent samples were characterized by such luminescence parameters as emission intensity, luminescence lifetime and quantum yield. The highest emission intensity of Eu(III) ions was exhibited by the salt with x = 1 incorporated into a silicate modified with 3-glycidoxypropyl groups. The longest lifetime was found in the material with a methylated silicate matrix with the same salt. For the complexed Eu(III) ions in these materials there is a correlation between emission intensity changes of the ⁵D₀ → ⁷F₂ band and the quantum yield. The materials with a high organic content in matrices such as the silicates with 3-glycidoxypropyl groups (either with closed or opened epoxy cycles) are more thermally unstable and they undergo larger photochemical degradation during exposure on UV radiation than the systems with limited organic content.     

The fatigue of high-strength fused silica optical fibers in low humidity

The strength and dynamic fatigue of UV-acrylate coated silica optical fibers were measured as a function of relative humidity in the range ∼0.025-13% at 25 °C. The degradation kinetics of silica in low humidities was investigated and it was found that the reaction order was approximately first-order with respect to humidity. In our previous work, a second-order reaction was found in the humidity range 20-95% RH at 25 °C and the process for obtaining this reaction order was found to be independent of the mathematical form of the kinetic models used. The change in reaction order observed here verifies some earlier results based on the power law which implied a change in the reaction order from ∼2 (15-100% RH) to ∼1 at low humidity (<0.01%).     

Dynamic condensation of water at crack tips in fused silica glass

Water molecules play a fundamental role in the physics of slow crack propagation in glasses. It is commonly understood that, during stress-corrosion, water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and, thus, support crack propagation. Yet the details of the environmental condition at the crack tip in moist air are not well determined. In a previous work, we reported direct evidence of the presence of a 100 nm long liquid condensate at the crack tip in fused silica glass during very slow crack propagation (10^?9–10^?10 m/s). These observations are based on in situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Here, we discuss the physical origin of the AFM phase contrast between the liquid condensate and the glass surface in relation to tip-sample adhesion induced by capillary bridges. We then report new experimental data on the water condensation length increase with relative humidity in the atmosphere. The measured condensation lengths were much larger than what predicted using the Kelvin equation and expected geometry of the crack tip.     

Quantum Size Effect and very localized random laser in ZnO@mesoporous silica nanocomposite following a two-photon absorption process

ZnO@mesoporous silica nanocomposite was prepared by the impregnation of a CMI-1 material in a Zn(NO₃)₂ solution followed by calcination under O₂. Intensive characterization was carried out by N₂ adsorption–desorption measurements, scanning and transmission electron microscopy. The optical properties of the ZnO@mesoporous silica nanocomposite were studied by photoluminescence spectroscopy. Quantum Size Effect was firstly demonstrated by subjecting the sample to a 254 nm excitation light, and was further confirmed by using a 680 nm excitation laser beam, which implies a two-photon absorption process. By focusing the 680 nm laser beam on different places in the sample, a very localized random laser effect, also induced by a two-photon absorption process, was detected.     

An X-ray absorption spectroscopy study of FeCo alloy nanoparticles embedded in ordered cubic mesoporous silica (SBA-16)

Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered cubic mesoporous silica (SBA-16) matrix were prepared using two different synthetic methods, co-precipitation and impregnation. Extended X-ray Absorption Spectroscopy (EXAFS) technique at both Fe and Co K-edges was used to investigate the structure of FeCo nanoparticles and the presence of additional disordered oxide phases. EXAFS technique gives evidence of differences in the oxidation degree of the FeCo nanoparticles depending on the synthetic method used.     

Changes in the refractive index of fused silica due to implantation of transition-metal ions

The changes in the refractive index of high purity silica glass produced by implantation of Ti⁺, Cr⁺, Mn⁺, Fe⁺ and Cu⁺ ions at 160 keV were measured using ellipsometry at a wavelength of 633 nm. Implantation doses ranged from 10¹⁵ to 6 × 10¹⁶ ions cm⁻². At the highest doses, a relative increase in refractive index of approximately 15% for implanted Cu ions and an increase of about half that for implanted Cr, Mn and Fe ions are observed. These variations in index probably result from colloid formation. For implanted Ti, virtually no change in the measured index of refraction was observed even at the highest doses.