Optical properties and applications to production of plasmonic thin film nanostructures of self-ordered columnar alumina arrays on glass

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Conference Proceeding
Proceedings of SPIE - The International Society for Optical Engineering, 2006, 6038
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In this paper we report on new techniques for making self-ordered porous layers of alumina of varying aspect ratios on glass, without the use of lithographic or masking techniques. Use of RF etching in one of the hole forming steps and also when filling the holes with sputtered metal is shown to be advantageous over additional anodisation. These hole arrays have intrinsically interesting optical responses which will be reported, but their main use is for nano-patterning of subsequent deposited layers either as templates or as masks. High resolution images demonstrate the uniformity in nanohole diameter and in the spacing between holes, which can be achieved when care is used in production. While many nanostructured materials can be deposited using these Porous Anodic Alumina (PAA) templates we focus here on filling the vertical cylindrical holes with silver. Etching during hole filling leads to better-controlled structures and more efficient processes. Novel optical data on the resultant conducting columnar rings will be presented. Spectrally much sharper plasmon resonant features are found than those reported for classical and more random silver column and island arrays. The optical properties are analysed from an effective medium perspective using data from spectrophotometry and ellipsometry. Fitting this data gives modelled layer thickness and the vertical profile in close agreement with direct SEM imaging. The effective refractive indices of the silver columnar layer have interesting and potentially useful dispersion characteristics.
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