V. Martinez

Femtosecond laser induced density changes in GeO 2 and SiO 2 glasses: fictive temperature effect [Invited]

Density changes of GeO2 and SiO2 glasses subjected to irradiation by tightly focused femtosecond pulses are observed by Raman scattering. It is shown that densification caused by the void formation in GeO2 glass is very similar to the changes under hydrostatic pressure. In contrast, the experimental observations in SiO2 glass could be explained by pressure effect or by the fictive temperature anomaly, i. e., a resultant smaller specific volume of the glass (a denser phase) at a high thermal quenching rate. Density changes of GeO2 and SiO2 glasses are opposite upon close-to-equilibrium heating; this gives new insights into the mechanisms of densification under highly non-equilibrium conditions: fs-laser induced micro-explosions, heating and void formation. The pressure and temperature effects of glass modification by ultra-short laser pulses are discussed considering applications in optical memory, waveguiding, and formation of micro-optical elements.

Anorthite and CaAl2Si2O8 glass: low frequency Raman spectroscopy and neutron scattering

Abstract Raman spectra of anorthite crystal and glass have been obtained on a wide frequency range from 5 to 1200 cm −1 . Medium-range order has been pointed out in this study: numerous low frequency peaks in anorthite have been observed and a comparison with the well known band around 60 cm −1 in the Raman spectrum of anorthite glass is made. In order to probe the medium-range structure of CaAl 2 Si 2 O 8 glass, a comparison between neutron inelastic scattering, Raman low frequency vibrations and heat capacity data is done for both glass and crystal.

Structural studies of germanium doped silica glasses: the role of the fictive temperature

Abstract We studied the influence of germanium atom introduction into the silica network and the influence of heat treatements on the glass structure by doing Raman and infra-red spectra of vitreous germanium doped silica (5% wt GeO2, 95% wt SiO2). Germanium incorporates into the network by substituting Si atoms and consequently, we observed band shifts and broadening of defect peaks D1 and D2. The peak positions are directly correlated with the structural state characterized by the glass fictive temperature TF of the different samples.

Radial distribution of the fictive temperature in pure silica optical fibers by micro-Raman spectroscopy

Micro-Raman spectra are performed at different radial positions in order to determine the fictive temperature distribution in pure silica optical fibers. As-received and annealed optical fibers are compared and Raman results reveal a large decrease in the fictive temperature after annealing. The fictive temperature gradient between the core and the edge of the optical fiber can be eliminated after annealing and the spectroscopic decoupling between stress and the fictive temperature variation is discussed. The micro-Raman is a good probe in determining the spatial distribution of the fictive temperature in the micrometer length scale.

Density and concentration fluctuations in SiO2–GeO2 optical fiber glass investigated by small angle x-ray scattering

Density and concentration fluctuations have been investigated in a 3mol% GeO2 doped silica glass as a function of the fictive temperature (the temperature at which the structure of the supercooled liquid has been frozen-in to form the glass) by small angle x-ray scattering measurements. The fluctuations increase in a way is quite similar to that observed for pure silica glass as a result of density fluctuation fictive temperature dependence. Fluctuations have also been studied in glasses containing different amounts of GeO2 up to 21mol% GeO2. The fluctuations are shown to increase very strongly with germanium amount as a result of strong concentration fluctuation increase. This observation is in agreement with already observed excess losses in light scattering measurements.

Density fluctuations in oxide glasses investigated by small-angle X-ray scattering

The structure of glasses is characterized by the existence of density and composition fluctuations on the nanometre scale. We present three examples of the use of small-angle X-ray scattering to get information about these density fluctuations. The thermal history and OH content were observed to have a huge influence. The static compressibility decreases when the OH content or fictive temperature increase. We showed that temperature scanning small-angle X-ray scattering can provide an accurate description of the position, width and shape of the glass transition.

Relaxation processes of densified silica glass

Densified SiO2 glasses, obtained from different pressure and temperature routes, have been annealed over a wide range of temperatures far below the glass transition temperature (500 °C-900 °C). Hot and cold compressions were useful to separate the effects of pressure and the compression temperature. In situ micro-Raman spectroscopy was used to follow the structural evolution during the thermal relaxation. A similar glass structure between the non-densified silica and the recovered densified silica after the temperature annealing demonstrates a perfect recovery of the non-densified silica glass structure. While the density decreases monotonically, the structural relaxation takes place through a more complex mechanism, which shows that density is not a sufficient parameter to fully characterize the structure of densified silica glass. The relaxation takes place through a transitory state, consisting in an increase of the network inhomogeneity, shown by an increase in the intensity of the D2 band which is as...

Density and density fluctuations anomalies of SiO2 glass: comparison and light-scattering study

The objective of this work is to compare density and density fluctuations of silica as a function of temperature in the anomalous domain between 950 and 1480°C via Archimedes’ macroscopic measurements and light scattering. Parallel qualitative behaviour is also recorded between density fluctuations and macroscopic density. Density fluctuations of silica show a minimum as a function of temperature, as with macroscopic density. This correlation indicates a possible common origin of both observations.

High-Reflection Coatings for Gravitational-Wave Detectors: State of The Art and Future Developments

We report on the optical, mechanical and structural characterization of the sputtered coating materials of Advanced LIGO, Advanced Virgo and KAGRA gravitational- waves detectors. We present the latest results of our research program aiming at decreasing coating thermal noise through doping, optimization of deposition parameters and post- deposition annealing. Finally, we propose sputtered Si3N4 as a candidate material for the mirrors of future detectors.