The structural origin of crystal field parameters in rare-earth doped glasses

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During recent decades optical technology has had a pronounced impact on our standard of living, by providing great improvements in communications, information technology, and medicine. Many of these applications have been made possible through the development of advanced materials, a good example being the production of optical fibres. This proposal is to advance our knowledge of the physics of materials which play a key role in optical technology. This proposal will examine optical materials which are glasses doped with rare earth elements. The importance of glasses in optics is obvious as they provide a transparent medium through which light can pass. Glass has the ability to conduct light (e.g. optical fibres), to shape light (e.g. lenses), and to host active elements which interact with light. The latter are an important area of research as they are essential for improving the efficiency of optical communications and developing optical analogues of electronics.Rare earth elements are an important type of active element. They have the property of possessing f-electrons, which absorb and emit light but which are also highly shielded, and so (largely) unperturbed by the surrounding medium. This makes rare earth elements very effective for stimulated emission of light, the phenomenon which is the basis for lasers and amplifiers.Lasers made from rare earth doped glasses are important in the applications discussed above, and also in very advanced scientific experiments. For example, one way to the create conditions for nuclear fusion is by using enormously powerful lasers, and such lasers contain several cubic metres of rare earth doped glass. Amplifiers made from rare earth doped glasses are crucial for boosting the signal in optical fibres, so that they can cross oceans (for example).The optical properties of rare earth elements are described using the theory of Quantum Mechanics. This is needed to understand they way in which f-electrons change their orbits (or wavefunctions), hence emitting or absorbing light, a process called transition . Perhaps surprisingly, small perturbations due to the surrounding atoms play a critical role in these transitions, via their influence on rare earth f-electron wavefunctions. There is a well-developed theory of how f-electron transitions depend on surrounding atoms in crystals, where the atoms have well-defined positions. The same theory has been applied to glasses. But glasses by their nature are non-crystalline, and the consequent disorder in atom positions has hindered attempts to fully explain optical properties of rare earth doped glasses. This proposal will carrying out new studies of rare earth doped glasses. It will use the experimental techniques of diffraction, which reveals glass structure, and X-ray absorption spectroscopy, which reveals rare earth sites, in combination with molecular dynamics, which creates models of atom positions. These techniques were used separately in previous studies, but this proposal will use them in combination to obtain detailed and accurate models of rare earth doped glasses.The models of rare earth doped glasses will be analysed to understand how perturbations arise from disorder in atom positions. Previous studies focussed on variations in the number of surrounding atoms, but this did not provide an explanation. This proposal will focus on variations in the symmetry of surrounding atoms, by calculating different measures of symmetry. The results will fill a significant gap in the physics describing f-electron transitions in glasses, and hence improve the understanding of optical properties of rare earth doped glasses. This will indirectly assist in the development of optical technology based on these materials, and hence their many important applications.