Glass - Back to the Future!

Presenting Author:
Martina Stoica

article posted 31 March 2016

Martina Stoica Field of studies: Chemistry, July 2011 Graduation with Diploma (equ. To MSc) Since 2012 University Jena, Otto-Schott-Institut, 07743 Jena (Germany) PhD student under the supervision of Prof. Dr. Christian Rüssel Thesis tiltle: "Photo-thermo-refractive glass from a Na2O/ K2O/ CaO/ CaF2/ Al2O3/ SiO2 glass system"

Effect of UV exposure on the crystallization and optical properties in a CaF2 photo thermal refractive glass

Martina Stoica* & Christian Rüssel
Otto- Schott- Institut für Materialforschung, Jena University, Fraunhoferstraße 6, 07743 Jena, Germany

A PTR (photo-thermal-refractive) glass was developed from a Na2O-SiO2-Al2O3-K2O-CaO-CaF2-ZnO oxyfluoride glass system by doping with CeO2, Ag2O, SnO2, Sb2O5 and KBr. A permanent refractive index change was induced by UV irradiation and subsequent two-step heat treatment. This allows the holographic structuring of Bragg gratings with periodically varying refractive indices which permit a more precise wavelength adjustment of laser sources. This results in an increased beam quality at high power, and an increase in energy efficiency. The new PTR glass has a potential of much larger differences in the refractive indices of radiated and non-radiated parts of the glass in comparison to the conventional PTR glass. A non-isochemical glass system was chosen due to the ability for nanoscale crystallization of CaF2. This CaF2 nanocrystals are formed in a phase-controlled crystallization mechanism.

When the nuclei has reached the critical radius, further crystal growth leads to the enrichment of SiO2 in a small layer around the crystal. Due to the increasing diffusion coefficient in this layer, further crystal growth as well as Ostwald ripening is hindered, resulting in small crystals with narrow crystallite size distribution. Therefore, these glass ceramics are colorless and transparent.
In combination with the photo-induced crystallization process, the precise nanoscale CaF2 crystallization was now possible. Irradiation led to a yellow coloration due to the silver cluster.

The effect of UV irradiation at 308 nm was examined spectroscopically, since a large number of polyvalent ions is involved in this process. Crystallization of CaF2 and the change in refractive index were highly dependent on the UV irradiation dose.