Developing novel structural modelling methods for optical glasses

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Organic chemical bonds, such as C-O, C-H and O-H, absorb light strongly at specific mid-infrared (mid-IR) frequencies. This gives every molecule a distinctive 'chemical fingerprint' that can be detected. The development of sensors that can measure these interactions would have applications in a diverse range of fields, from monitoring pollution in manufacturing environments and detecting drugs and explosives, to ensuring food and drink production lines are not contaminated and monitoring cancer margins during surgery. However, established silica-based fibre technologies only transmit light to the near-IR, and to exploit this "fingerprinting" method for chemical identification new mid-IR transmissive glasses are required. Research into mid-IR light technologies tends to focus on device development, utilising a small set of glass compositions that are known to exhibit adequate behaviour. However, non-optimal material properties can result in unnecessary problems, from loss of light intensity through a fibre to non-linear optical (NLO) effects that change the light characteristics. These need to be addressed when constructing a working device, but could be avoided if the starting material were specifically designed to exhibit the functional properties needed. The aim of this project will be to address the fundamental gap in knowledge that links glass composition to structure and functional properties. Current compositional development is, perforce, trial-and-error and requires a significant investment in time and money. A better understanding of composition-structure-property relationships in optical glasses will provide a road map to allow new glasses to be predicted, providing a short cut to determining optimised compositions for optical applications. Once established, the research protocols can be applied to improve glass performance for other applications such as energy, biomedical devices, architectural glasses and nuclear waste forms.The aim of this project will be achieved by studying glass compositions in the tellurite (TeO2) and chalcogenide (Sb2Se3) glass families. These glasses have been chosen because they transmit light into the mid-IR and exhibit strong NLO effects that can interact with light in a number of potentially useful ways. The research will be comprised of two stages. The first stage will be to measure the functional properties of the glasses. It is well-established that the functional properties of a glass, such as softening and melting temperatures, densities, refractive indices and light transmittance windows, depend upon atomic structure. The second stage of the study will be a quantitative analysis of glass structures through the direct and computational analysis of data obtained using a range of techniques. These will include Neutron and X-ray scattering, X-ray Absorption Spectroscopy, Raman Scattering and Nuclear Magnetic Resonance. Preliminary results show that small changes in the composition of tellurite glasses alter the local environment of tellurium, changing the number of nearest neighbours. In comparison, variations in the composition of chalcogenide glasses can lead to changes in the types of nearest neighbours around antimony. The number and type of nearest neighbours can have a large impact on the glass properties, affecting how we make, shape and use the glass. However, our current understanding of these changes is qualitative, rather than quantitative, particularly in the complex multicomponent glasses required for applications.A determination of structure and functional properties for carefully chosen compositional series will allow robust relationships to be developed. These will be used to predict new glass compositions that exhibit specific properties, allowing the precise functional property requirements of a specific application to be fulfilled. The application of these new materials will result in a step change in the development of new devices that operate in the mid-IR.