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

近几十年来，光学技术通过在通信、信息技术和医学方面提供巨大的改进，对我们的生活水平产生了显著的影响。通过先进材料的发展，许多这些应用已经成为可能，一个很好的例子是光纤的生产。这一建议是为了提高我们对在光学技术中起关键作用的材料物理学的认识。本提案将研究掺杂稀土元素的玻璃光学材料。玻璃在光学中的重要性是显而易见的，因为它们提供了一种光可以通过的透明介质。玻璃具有传导光（例如光纤）、对光进行整形（例如透镜）以及容纳与光相互作用的有源元件的能力。后者是一个重要的研究领域，因为它们对于提高光通信的效率和开发光学电子学的类似物至关重要。稀土元素是一种重要的活性元素。它们具有f电子的特性，f电子吸收和发射光，但也被高度屏蔽，因此（很大程度上）不受周围介质的干扰。这使得稀土元素对于光的受激发射非常有效，这种现象是激光器和放大器的基础。由稀土掺杂玻璃制成的激光器在上述应用中非常重要，并且在非常先进的科学实验中也是如此。例如，为核聚变创造条件的一种方法是使用非常强大的激光器，这种激光器包含几立方米的稀土掺杂玻璃。由稀土掺杂玻璃制成的放大器对于增强光纤中的信号至关重要，因此它们可以跨越海洋（例如）。使用量子力学理论描述稀土元素的光学特性。这是理解f电子改变轨道（或波函数）的方式所必需的，因此发射或吸收光，这个过程称为跃迁。也许令人惊讶的是，由于周围原子的小扰动通过它们对稀土f-电子波函数的影响在这些跃迁中起着关键作用。有一个完善的理论，说明f电子跃迁如何依赖于晶体中周围的原子，其中原子具有明确的位置。同样的理论也适用于眼镜。但玻璃本质上是非结晶的，因此原子位置的无序阻碍了充分解释稀土掺杂玻璃光学性质的尝试。这将为稀土掺杂玻璃的研究开辟新的途径。它将使用衍射实验技术，揭示玻璃结构，X射线吸收光谱，揭示稀土位点，结合分子动力学，创建原子位置模型。这些技术在以前的研究中是单独使用的，但本提案将结合使用它们来获得详细和准确的稀土掺杂玻璃模型。将分析稀土掺杂玻璃的模型，以了解原子位置的无序如何引起扰动。以前的研究集中在周围原子数量的变化上，但这并没有提供解释。这个提议将通过计算对称性的不同度量，集中研究周围原子对称性的变化。这一结果将填补玻璃中f电子跃迁物理描述的空白，从而提高对稀土掺杂玻璃光学性质的理解。这将间接有助于基于这些材料的光学技术的发展，从而促进它们的许多重要应用。

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.