Atomistic mechanism of structural and volume relaxation below glass transition temperature in a soda-lime silicate glass revealed by Raman spectroscopy and its DFT calculations.

To elucidate the atomistic origin of volume relaxation in soda-lime silicate glass annealed below the glass transition temperature (Tg), the experimental and calculated Raman spectra were compared. By decomposing the calculated Raman spectra into specific groups of atoms, the Raman peaks at 800, 950, 1050, 1100, and 1150 cm-1 were attributed to oxygen and silicon in Si-O-Si, non-bridging oxygen in the Q2 unit, bridging oxygen in low-angle Si-O-Si, non-bridging oxygen in the Q4 unit, and bridging oxygen in high-angle Si-O-Si, respectively. Based on these attributions, we found that by decreasing the fictive temperature by annealing below Tg - 70 K, a homogenization reaction Q2 + Q4 → 2Q3 and an increase in average Si-O-Si angle occurred simultaneously. By molecular dynamics simulation, we clarified how the experimentally demonstrated increase in average Si-O-Si angle contributes to volume shrinkage; increasing Si-O-Si angles can expand the space inside the rings, and Na can be inserted into the ring center.

Ultra-high temperature Soret effect in a silicate melt: SiO2 migration to cold side.

The Soret effect, temperature gradient driven diffusion, in silicate melts has been investigated intensively in the earth sciences from the 1980s. The SiO2 component is generally concentrated in the hotter region of silicate melts under a temperature gradient. Here, we report that at ultra-high temperatures above ∼3000 K, SiO2 becomes concentrated in the colder region of the silicate melts under a temperature gradient. The interior of an aluminosilicate glass [63.3SiO2-16.3Al2O3-20.4CaO (mol. %)] was irradiated with a 250 kHz femtosecond laser pulse for local heating. SiO2 migrated to the colder region during irradiation with an 800 pulse (3.2 ms irradiation). The temperature analysis indicated that migration to the colder region occurred above 3060 K. In the non-equilibrium molecular dynamics (NEMD) simulation, SiO2 migrated to the colder region under a temperature gradient, which had an average temperature of 4000 K; this result supports the experimental result. On the other hand, SiO2 exhibited a tendency to migrate to the hotter region at 2400 K in both the NEMD and experimental study. The molar volume calculated by molecular dynamics simulation without a temperature gradient indicates two bends at 1650 and 3250 K under 500 MPa. Therefore, the discontinuous (first order) transition with coexistence of two phases of different composition could be related to the migration of SiO2 to colder region. However, the detailed mechanism has not been elucidated.

Role of mixing thermodynamic properties on the Soret effect.

We demonstrate that the modified Kempers model, a recently developed theoretical model for the Soret effect in oxide melts, is applicable for predicting the composition dependence of the Soret coefficient in three binary molecular liquids with negative enthalpies of mixing. We compared the theoretical and experimental values for water/ethanol, water/methanol, water/ethylene glycol, water/acetone, and benzene/n-heptane mixtures. In water/ethanol, water/methanol, and water/ethylene glycol, which have negative enthalpies of mixing across the entire mole fraction range, the modified Kempers model successfully predicts the sign change of the Soret coefficient with high accuracy, whereas, in water/acetone and benzene/n-heptane, which have composition ranges with positive enthalpies of mixing, it cannot predict the sign change of the Soret coefficient. These results suggest that the model is applicable in composition ranges with negative enthalpies of mixing and provides a framework for predicting and understanding the Soret effect from the equilibrium thermodynamic properties of mixing, such as the partial molar volume, partial molar enthalpy of mixing, and chemical potential.

Composition-dependent sign inversion of the Soret coefficient of SiO2 in binary borosilicate melts.

Using a laser-induced local-heating experiment combined with temperature analysis, we observed the composition-dependent sign inversion of the Soret coefficient of SiO2 in binary silicate melts, which was successfully explained by a modified Kempers model used for describing the Soret effect in oxide melts. In particular, the diffusion of SiO2 to the cold side under a temperature gradient, which is an anomaly in silicate melts, was observed in the SiO2-poor compositions. The theoretical model indicates that the thermodynamic mixing properties of oxides, partial molar enthalpy of mixing, and partial molar volume are the dominant factors for determining the migration direction of the SiO2 component under a temperature gradient.

Speciation of tin ions in oxide glass containing iron oxide through solvent extraction and inductively coupled plasma atomic emission spectrometry after the decomposition utilizing ascorbic acid.

Determining the concentrations of different Sn ions in glass containing iron oxide by wet chemical analysis is a challenge because a redox reaction occurs between Sn2+ and Fe3+. A chemical analysis method for determining the concentrations of Sn2+ and Sn4+ in soda lime glass containing iron oxide was proposed. A mixture of ascorbic acid, hydrochloric acid, and hydrofluoric acid was used to decompose the sample in a vessel with nitrogen flow. Ascorbic acid functioned as a reductant for Fe3+. Subsequently, the Sn2+ were separated as a diethyldithiocarbamate complex. Furthermore, inductively coupled plasma atomic emission spectroscopy was used to determine the concentrations of Sn4+ and total Sn, from which the concentration of Sn2+ can be calculated. The results were validated by comparing ratios of Sn2+ to total Sn to results obtained using Mössbauer spectroscopy. The results were in agreement, thereby validating the use of the proposed approach.

Structural origin of thermal shrinkage in soda-lime silicate glass below the glass transition temperature: A theoretical investigation by microsecond timescale molecular dynamics simulations.

Microscopic dynamical features in the relaxation of glass structures are one of the most important unsolved problems in condensed matter physics. Although the structural relaxation processes in the vicinity of glass transition temperature are phenomenologically expressed by the Kohlrausch-Williams-Watts function and the relaxation time can be successfully interpreted by Adam-Gibbs theory and/or Narayanaswamy's model, the atomic rearrangement, which is the origin of the volume change, and its driving force have not been elucidated. Using the microsecond time-scale molecular dynamics simulations, this study provides insights to quantitatively determine the origin of the thermal shrinkage below Tg in a soda-lime silicate glass. We found that during annealing below Tg, Na ions penetrate into the six-membered silicate rings, which remedies the acute O-O-O angles of the energetically unstable rings. The ring structure change makes the space to possess the cation inside the rings, but the ring volume is eventually reduced, which results in thermal shrinkage of the soda-lime silica glass. In conclusion, the dynamical structural relaxation due to the cation displacement evokes the overall volume relaxation at low temperature in the glassy material.

Determination of thermodynamic and microscopic origins of the Soret effect in sodium silicate melts: Prediction of sign change of the Soret coefficient.

The Soret effect in silicate melts has attracted attention in earth and material sciences, particularly in glass science and engineering, because a compositional change caused by the Soret effect modifies the material properties of silicate melts. We investigated the Soret effect in an Na2O-SiO2 system, which is the most common representative of silicate melts. Our theoretical approach based on the modified Kempers model and non-equilibrium molecular dynamics simulation was validated for 30Na2O-70SiO2(mol. %). The sign and order of the absolute values of the calculated Soret coefficients were consistent with the experimental values. The positive Soret coefficient of SiO2 in the SiO2-poor composition range was accurately predicted. Previous experimental studies have focused on SiO2-rich compositions, and only the negative sign, indicating SiO2 migration to the hot side, has been observed. In the SiO2-poor composition range, the Q0 structure was dominant and had four Si-O-Na bonds around an SiO4 unit. The Si-O-Na bond had high enthalpic stability and contributed to the large negative enthalpy of SiO2 mixing. According to our model, components with a large negative partial molar enthalpy of mixing will concentrate in the cold region. The microscopic and thermodynamic origins of the sign change in the Soret effect were determined.

Role of partial molar enthalpy of oxides on Soret effect in high-temperature CaO-SiO2 melts.

The Soret effect or thermodiffusion is the temperature-gradient driven diffusion in a multicomponent system. Two important conclusions have been obtained for the Soret effect in multicomponent silicate melts: first, the SiO2 component concentrates in the hot region; and second, heavier isotopes concentrate in the cold region more than lighter isotopes. For the second point, the isotope fractionation can be explained by the classical mechanical collisions between pairs of particles. However, as for the first point, no physical model has been reported to answer why the SiO2 component concentrates in the hot region. We try to address this issue by simulating the composition dependence of the Soret effect in CaO-SiO2 melts with nonequilibrium molecular dynamics and determining through a comparison of the results with those calculated from the Kempers model that partial molar enthalpy is one of the dominant factors in this phenomenon.

Polarization imaging camera with a waveplate array fabricated with a femtosecond laser inside silica glass.

In this study, we demonstrate a polarization imaging camera with a waveplate array of a silica glass fabricated by femtosecond (fs) laser direct writing. To use a waveplate array of silica glass for polarization imaging, non-uniformity of the transmittance and retardance in the waveplates must be considered. Therefore, we used a general method of polarization analysis with system matrices determined experimentally for all the units in the waveplate array. We found that a figure of merit based on the determinant of the system matrix could be applied to improve the accuracy of analysis and the robustness to the retardance dispersion for both the simulated and the fabricated waveplate array.

Diamond photonics platform enabled by femtosecond laser writing.

Diamond is a promising platform for sensing and quantum processing owing to the remarkable properties of the nitrogen-vacancy (NV) impurity. The electrons of the NV center, largely localized at the vacancy site, combine to form a spin triplet, which can be polarized with 532 nm laser light, even at room temperature. The NV's states are isolated from environmental perturbations making their spin coherence comparable to trapped ions. An important breakthrough would be in connecting, using waveguides, multiple diamond NVs together optically. However, still lacking is an efficient photonic fabrication method for diamond akin to the photolithographic methods that have revolutionized silicon photonics. Here, we report the first demonstration of three dimensional buried optical waveguides in diamond, inscribed by focused femtosecond high repetition rate laser pulses. Within the waveguides, high quality NV properties are observed, making them promising for integrated magnetometer or quantum information systems on a diamond chip.

Direct laser-writing of ferroelectric single-crystal waveguide architectures in glass for 3D integrated optics.

Direct three-dimensional laser writing of amorphous waveguides inside glass has been studied intensely as an attractive route for fabricating photonic integrated circuits. However, achieving essential nonlinear-optic functionality in such devices will also require the ability to create high-quality single-crystal waveguides. Femtosecond laser irradiation is capable of crystallizing glass in 3D, but producing optical-quality single-crystal structures suitable for waveguiding poses unique challenges that are unprecedented in the field of crystal growth. In this work, we use a high angular-resolution electron diffraction method to obtain the first conclusive confirmation that uniform single crystals can be grown inside glass by femtosecond laser writing under optimized conditions. We confirm waveguiding capability and present the first quantitative measurement of power transmission through a laser-written crystal-in-glass waveguide, yielding loss of 2.64 dB/cm at 1530 nm. We demonstrate uniformity of the crystal cross-section down the length of the waveguide and quantify its birefringence. Finally, as a proof-of-concept for patterning more complex device geometries, we demonstrate the use of dynamic phase modulation to grow symmetric crystal junctions with single-pass writing.

Condensation of Si-rich region inside soda-lime glass by parallel femtosecond laser irradiation.

Local melting and modulation of elemental distributions can be induced inside a glass by focusing femtosecond (fs) laser pulses at high repetition rate (>100 kHz). Using only a single beam of fs laser pulses, the shape of the molten region is ellipsoidal, so the induced elemental distributions are often circular and elongate in the laser propagation direction. In this study, we show that the elongation of the fs laser-induced elemental distributions inside a soda-lime glass could be suppressed by parallel fsing of 250 kHz and 1 kHz fs laser pulses. The thickness of a Si-rich region became about twice thinner than that of a single 250 kHz laser irradiation. Interestingly, the position of the Si-rich region depended on the relative positions between 1 kHz and 250 kHz photoexcited regions. The observation of glass melt during laser exposure showed that the vortex flow of glass melt occurred and it induced the formation of a Si-rich region. Based on the simulation of the transient temperature and viscosity distributions during laser exposure, we temporally interpreted the origin of the vortex flow of glass melt and the mechanism of the formation of the Si-rich region.

Modulation of laser induced-cracks inside a LiF single crystal by fs laser irradiation at multiple points.

Crack formations inside a LiF single crystal after femtosecond laser irradiation at multiple points were investigated. In the case of sequential laser irradiation at three points, the propagations of some cracks were prevented by the dislocation bands generated by the previous laser irradiation. On the other hand, in the case of simultaneous laser irradiation at three points with a spatial light modulator, cracks in all the <100> directions from the photoexcited regions were generated clearly, but the length of one crack depended on the distribution of laser irradiation positions. The simulation of elastic dynamics after fs laser irradiation at three points elucidated that the interference of laser induced stress waves depended on the distributions of the irradiation positions. We found that the constructive interference of stress waves at a crack tip should have prevented the crack from propagating further and the tensile stress by destructive interference of stress waves along a crack should have facilitated the propagation of the crack.

Shape control of elemental distributions inside a glass by simultaneous femtosecond laser irradiation at multiple spots.

The spatial distributions of elements in a glass can be modulated by irradiation with high repetition rate femtosecond laser pulses. However, the shape of the distribution is restricted to being axially symmetric about the laser beam axis due to the isotropic diffusion of photo-thermal energy. In this study, we describe a method to control the shape of the elemental distribution more flexibly by simultaneous irradiation at multiple spots using a spatial light modulator. The accumulation of thermal energy was induced by focusing 250 kHz fs laser pulses at a single spot inside an alumino-borosilicate glass, and the transient temperature distribution was modulated by focusing 1 kHz laser pulses at four spots in the same glass. The resulting modification was square-shaped. A simulation of the mean diffusion length of molten glass demonstrated that the transient diffusion of elements under heat accumulation and repeated temperature elevation at multiple spots caused the square shape of the distribution.

White emission of Yb²âº:fluoride glasses efficiently excited with near-UV light.

Various Yb²âº-containing fluoride glasses melting under a reductive atmosphere were prepared. The brightest white light emission was observed for an AlF3-based fluoride glass not containing Hf or Zr. The largest full width at half maximum of the white emission spectra was 202 nm. In addition, incorporation of chloride into the AlF3-based glass enabled efficient excitation with near-ultraviolet light corresponding to a GaN bandgap of 3.4 eV and the maximum internal quantum efficiency of Yb²âº: AlF3-based fluoride glass was 42%.

Photoinduced luminescent carbon nanostructures with ultra-broadly tailored size ranges.

Carbon nanoparticles (CNPs), hollow CNPs, nanodiamonds, and hybrid graphene spheres (HGSPs) are produced by using fs laser ablation in solution. These carbon nanostructures emit tunable photoluminescence and two-photon luminescence. The photoinduced layer-by-layer assembly of graphene nanosheets is observed to form HGSPs with tailored broadly-ranged sizes for the first time.

Plasmonically enhanced Faraday effect in metal and ferrite nanoparticles composite precipitated inside glass.

Using femtosecond laser irradiation and subsequent annealing, nanocomposite structures composed of spinel-type ferrimagnetic nanoparticles (NPs) and plasmonic metallic NPs have been formed space-selectively within glass doped with both α-Fe(2)O(3) and Al. The Faraday rotation spectra exhibit a distinct negative peak at around 400 nm, suggesting that the ferrimagnetic Faraday response is enhanced by the localized surface plasmon resonance (LSPR) due to metallic Al NPs. At the interfaces in the nanocomposites, the ferrimagnetism of magnetite NPs is directly coupled with the plasmon in the Al NPs. The control of the resonance wavelength of the magneto-optical peaks, namely, the size of plasmonic NPs has been demonstrated by changing the irradiation or annealing conditions.

Localized control of light-matter interactions by using nanoscale asymmetric TiO2.

This paper reports an asymmetry structure-mediated route for highly localized control of light-matter interactions by using tapered TiO(2). We demonstrate for the first time that the growth habit of Ag nanostructures on tapered TiO(2) can be tuned by controllable photolysis. Site-selective anchoring of Ag nanoparticles or nanowires on tapered TiO(2) can be achieved by simply changing the external light. We further show that the obtained tapered TiO(2)-Ag hetero-nanostructures present excellent light-trapping ability over a wide range of wavelengths which is considered to originate from the unique synergistic effects of graded waveguiding and plasmonic light trapping. This improved photon-management capability renders the prepared substrate a very promising candidate for optical sensing application. For this purpose, an enhanced sensitivity for trace detection is confirmed. These findings open up promising avenues for tailoring of light-matter interactions which are of special interest for studying controllable photolysis activation processes and diverse applications such as nanostructure growth, trace detection, photocatalysis and solar cells.

Three-dimensional temperature distribution and modification mechanism in glass during ultrafast laser irradiation at high repetition rates.

We experimentally determined the three-dimensional temperature distribution and modification mechanism in a soda-lime-silicate glass under irradiation of ultrafast laser pulses at high repetition rates by analyzing the relationship between the morphology of the modification and ambient temperature. In contrast to previous studies, we consider the temperature dependence of thermophysical properties and the nonlinear effect on the absorbed energy distribution along the beam propagation axis in carrying out analyses. The optical absorptivity evaluated with the temperature distribution is approximately 80% and at most 3.5% smaller than that evaluated by the transmission loss measurement. The temperature distribution and the strain distribution indicate that visco-elastic deformation and material flow play important roles in the laser-induced modification inside a glass.

Photosensitivity control of an isotropic medium through polarization of light pulses with tilted intensity front.

We present the first experimental evidence of anisotropic photosensitivity of an isotropic homogeneous medium under uniform illumination. Our experiments reveal fundamentally new type of light induced anisotropy originated from the hidden asymmetry of pulsed light beam with a finite tilt of intensity front. We anticipate that the observed phenomenon, which enables employing mutual orientation of a light polarization plane and pulse front tilt to control interaction of matter with ultrashort light pulses, will open new opportunities in material processing.