Optical properties of metallic systems
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The continuous improvement of nanoscale fabrication techniques will ultimately result in a situation where the performance of plasmonic devices is not dependent on engineering defects, but rather on the fundamentally limiting behaviour of the underlying metals. This thesis addresses the following questions: Are silver and gold the best metals for plasmonics? What other materials are available? and finally; Can we design better plasmonic materials? To answer these questions, classical electrodynamics calculations are performed using tabulated dielectric functions from the literature. Starting from a comparison of nanoshells made of various free electron metals, it is shown that the low plasma frequency metals sodium and potassium perform well. However, these metals are not suitable for many common uses of nanoshells. As such, the material choice is extended to all non fgroup metals in the periodic table and a variety of additional geometries are studied, including nanorods, superlenses and a number of guiding structures. It is shown that gold, silver, the alkali metals and aluminium outperform all other metals, each over a range of frequencies and permittivities. None of the reviewed elements performs better than silver and gold. As none of the elements seem to offer any advantage over silver or gold, the search is extended to alternative materials with tabluated dielectric functions. A review of the plasmonic properties of these materials is presented, including alloys, intermetallic compounds , high pressure materials as well as silicides, metallic glasses, and liquid metals. It is discovered that liquid sodium outperforms its solid elemental counterpart. Additionally, several materials with simple crystal structures seem to perform well, but none to the extent of silver or gold. The number of compounds for which tabulated optical constants are available is severely limited. In order to evaluate the performance of a large number of materials, first principles quantum mechanical calculations must be performed. It is shown that the plasmonic performance can be approximately gleened from the relationship between the optical gap and the plasma frequency. However, in order to compare the calculated optical response of materials with experimental data for the elements, the Drude phenomenological scattering rate must be known. Here, for the first time, calculations of the real and imaginary components of the dielectric function including the electron-phonon scattering rate are performed in order to gauge the plasmonic performance of materials with no tabulated optical data. A list of publications associated with this work is presented on page iv.
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