Glasses under light: exploring the extremes
Glasses with identical composition can have very different properties depending on the preparation protocol followed to prepare them: they are, in fact, materials out-of-equilibrium. Within this project, we aim at using light to modify and control the properties of chalcogenide glasses, a family of glasses with many applications as, e.g. materials for detectors, optics and optical fibers in the infrared range. Chalcogenide glasses are indeed known to be photosensitive. We aim here at exploiting this characteristic to prepare glasses with extreme stability. The importance of these extreme glasses is not only fundamental: they are of interest for technological applications, as for high-performance phase-change materials for applications as memories.
A short introduction
In the everyday life we are in contact with several objects made out of glass, from the windows that keep out the cold air but let in the sunlight to bottles that contain water and whine, spectacles that facilitate our sight or even the screens from which we are reading these words. One of the reasons of this abundance of items made out of this class of materials is the fact that we are able to give it almost every shape while maintaining a solid and transparent object. This peculiar combination originates from the fact that at the atomic level glasses are a class of materials which falls in-between crystals and liquids. Like in crystals, the atoms are locked in place by the obstacles represented by their neighbors, while like in liquids there is no long-range repetitive unit with atoms and molecules joined together in an apparently random fashion. From a physicist perspective the existence of glasses is puzzling, in fact as everyone knows from textbooks and everyday experience, matter under atmospheric pressure typically present itself either as a gas, a solid or a liquid. Changing temperature or pressure it will eventually lead to a change of state which is an event that happens exactly at well defined temperature-pressure values, for example at ambient pressure pure water will always freeze/melt at exactly 0 °C. For glass-formers nothing of this happens. They never experience a phase transition like other materials but instead they retain the amorphous liquid structure even at temperatures well below the melting point. But there is a moment in which we can say that the material is changing from a liquid to a proper glass and this moment is called glass transition temperature (Tg). In contrast with the melting temperature, Tg is not always the same but depends on several parameters especially the cooling rate at which the glass is produced. The glass transition is then more of a range of temperatures in which the materials stops behaving more like a liquid and starts behaving more like a solid.
A powerful theoretical tool that can be used to describe the glassy state is the potential energy landscape, which is the energy associated to the atomic configuration of the system. As the name implies, this object can be seen as a landscape filled with valleys and hills in which the glass occupies only one point at a time. The altitude of this point is determined by the temperature of the system. If it is still a liquid, then the system occupies the points above the highest hills of the landscape, and can freely manifest every possible configuration without restrictions. Lowering the temperature, the system will start to feel the highest hills and will then avoid certain configurations. Further lowering the temperature, the system will find itself trapped in a valley and it will no longer capable to change its configuration, we have then created a glass. The way in which the temperature is lowered (or raised) determines the final properties of the glass. In this multidimensional space there are two notable configurations. One is the deepest minimum, which represent the most stable configuration and corresponds to the crystalline phase. The other is the second deepest minimum, which corresponds to a glass configuration which is almost as stable as the crystalline phase.
Representation of a potential energy landscape. The circle represents the configuration of the glass at a certain time. The temperature allows the glass to freely sample all the positions at equal height in the landscape.
The research team of GLASS@EXTREMES