John Schroeder

Non-exponential structural relaxation, anomalous light scattering and nanoscale inhomogeneities in glass-forming liquids

Abstract Light scattering from glass-forming liquids exhibits an anomalous time dependence in the glass transition region, e.g. maxima in the scattering intensity versus temperature curves during heating. It is shown that this behavior is consistent with the presence of nanoscale inhomogeneities (density fluctuations) which relax at different rates. It is suggested that this could be the source of non-exponential structural relaxation kinetics. An expression relating the size of these regions to structural relaxation kinetic parameters has been developed and predicts sizes in excellent agreement with those determined by other methods.

Carrier‐induced strain effect in Si and GaAs nanocrystals

New experimental results on Raman scattering from porous silicon and silicon and gallium arsenide nanocrystals are reported. In all of these systems, almost all vibrational modes become Raman active and are remarkably soft. A carrier‐induced strain model is proposed to explain the optical properties of these nanocrystal systems. According to this carrier‐induced strain model, the selection rule of crystal momentum conservation for Raman scattering is greatly relaxed in Si and GaAs nanocrystals due to the dilatation strain caused by coupling of excited free carriers with the particle lattice and the optical properties of such systems are dominated by multiphonon assisted free‐electron transition processes.

NIRATAM-NATO infrared air target model

NIRATAM (the NATO Infrared Air Target Model) was developed by the NATO AC 243, Panel IV, Research Study Group 6 (RSG-6). RSG-6 is composed of representatives from Denmark, France, Germany, Italy, the Netherlands, the United Kingdom, the United States of America, and Canada (as an observer). NIRATAM is based on theoretical studies, field measurements, and infrared data analysis performed over many years. The model encompasses all the major signature components required to simulate the infrared signature of an aircraft and the atmosphere. The vehicle fuselage, facet, model includes radiation due to aerodynamic heating, internal heat sources, reflected sky, earth, and solar radiation. Plume combustion gas emissions are calculated for H2O, CO2, CO, and other gases as well as solid particles. Lowtran 7 is used for the atmospheric transmission and radiance. The software generates graphical outputs of the target wireframe, plume flowfield, atmospheric transmission, total signature, and plume signature. Imagery data can be used for system development and evaluation. NIRATAM can be used for many applications such as measurement planning, data analysis, systems design, and aircraft development. Ontar has agreed to assist the RSG-6 by being the NIRATAM distribution center in the United States for users approved by the national representatives. Arrangements have also been made to distribute a user-friendly NIRATAM interface. This paper describes the model, presents results, makes comparisons with measured field data, and describes the availability and procedure for obtaining the software.

Brillouin and raman scattering from glasses under high pressure

Abstract Brillouin and Raman Scattering Spectra in SiO2 and GeO2 glasses have been measured in a diamond anvil cell up to pressures of 14 GPa. The elastic properties and equation of state for each glass type were obtained from the Brillouin scattering measurements with respect to pressure. Both elastic constants and compressibility of SiO2 and GeO2 showed anomalous behavior with respect to pressure. This anomalous behavior is reconciled with a model based on the pressure dependent bending of the oxygen angles in both glass types. The Raman measurements corroborate the conclusions from the Brillouin scattering results, namely that the SiO2 and GeO2 bond angles are changing with pressure or the oxygen angle distribution is changed without bond breaking.

Nitrogen doping of fused silica and silicate glasses: a study of transport and optical properties☆

Abstract Our work has demonstrated for the first time that relatively large amounts of atomic nitrogen can be incorporated in fused silica, rather than Vycor, by a high temperature NH3 cracking process. The nitrogen analysis process shows that the nitrogen is chemically dissolved in the molecular structure of the fused silica glass and we obtain a product which is a stable and transparent glass. A kinetic study of this nitriding process has shown that nitrogen incorporation in a glass will occur even at temperatures as low as 500°C, although the best efficiencies in the reaction are attained at temperatures near or slightly below the glass transition temperature of the glass. The kinetic study also shows that optimum nitriding conditions for fused silica are a NH3/N2 ratio of 3:1 at a nitriding temperature of ∼1000°C. We discuss the possible structural scheme that allows increased lattice stiffness of a nitrided glass relative to a non-treated glass. This in turn has important consequences for understanding the problem of glass stability (i.e. devitrification) in silicate based glasses.

Anomalous light scattering in the glass transition region

Abstract At the 6th Halide Glass Symposium, apparently anomalous results were reported on light scattering of a ZBLAN glass. The scattering intensity passed through a maximum as the sample was heated through the glass transition region. An explanation has been developed for these results and ascribes the anomalous scattering to relaxational effects on density fluctuations. The theory has implications for our understanding of the structural relaxation process in glass-forming liquids.

Intrinsic Brillouin linewidths and stimulated Brillouin gain coefficients in glasses studied by inelastic light scattering

Rayleigh–Brillouin scattering measurements on various multicomponent halide glasses and some selected oxide glasses were performed. The Brillouin linewidth measurements, the Brillouin intensities, and Brillouin frequency shifts allowed us to calculate the phonon attenuation, Pockels elasto-optic coefficients, and the stimulated Brillouin scattering gain coefficients. Brillouin linewidths obtained experimentally range between the limit of 21 MHz for an 88BeF2–12ThF4 glass and 213 MHz for a zirconium barium lanthanum fluoride glass. The parameter obtained in this study shows that the threshold power for the onset of stimulated Brillouin scattering in some halide glasses is greater than that for silicate glasses, which has important ramifications as a selection criterion for halide-based glasses as single-mode optical waveguide materials.

Non-exponential structural relaxation, anomalous light scattering and nanoscale inhomogeneities in glasses

Light scattering from glasses in the glass transition region exhibits an anomalous Rayleigh scattering. We observe a maximum in the scattering intensity vs. temperature during heating. It is shown that this behavior is consistent with the presence of nanometer scale inhomogeneities (density fluctuation) which relax at differing rates. Recent observations of anomalous light scattering were carried out on strong glass formers and fragile glass formers. In all cases a hysteresis effect is seen in the light scattering between heating and cooling each sample from room temperature through the glass transition region. From these measurements we suggest that this could be the source of non-exponential structural relaxation kinetics. A model based upon the modified Tool-Narayanaswamy model is found to be in agreement with the calculations based on the experimental results.

Inelastic light scattering in halide glasses at high temperatures: boson peaks and the degree of disorder☆

Abstract Inelastic light scattering measurements as a function of temperature were carried out for three ZrF 4 - or HfF 4 -based fluoride glass samples. The frequency regime probed is from close to the exciting line to 300 cm −1 , with a temperature range extended from ambient to above the glass transition region. The low frequency Raman scattering containing a dominant spectral line (the boson peak) is interpreted in terms of the amplitude and magnitude of the density fluctuations in the glass. The degree and range of disorder in a glass is obtained in a quantitative sense from the behaviour of the spectral form of the boson peak with temperature.

Strained Quantum Dots in Porous Silicon

On the basis of Raman, photoluminescence, and absorption studies of porous and nanoparticle silicon we propose that the strong luminescence in porous silicon results from strained silicon quantum dots. A silicon nanoparticle is a special Jahn-Teller system induced by extended electron states rather than localized state. Thus Raman scattering and photoluminescence in porous silicon are multi-phonon assisted free electronic transition processes, all observed anomalous properties of porous silicon can be clearly explained by using this strained quantum dot model.