Nanolithography using Surface Plasmons

Surface plasmons have attracted renewed interest recently particularly because they can give rise to some interesting electromagnetic phenomena (www.surfaceplasmonoptics.org). Ebbesen et al. demonstrated high transmission through an array of sub-wavelength apertures due to coupling between surface plasmons on either side of the metallic film. This transmission exceeded the Bethe limit for single apertures (Bethe 1944) by several orders of magnitude. A large amount of work has since been carried out, and sophisticated sub-wavelength structures have been devised to enhance the throughput from single sub-wavelength apertures and to provide a highly collimated far-field radiation pattern from such apertures.

In addition, surface plasmons have been identified as having a very important role to play in designing new materials that exhibit a negative index of refraction (Shelby 2001). It has been predicted that a lens made from such material will have resolution beyond the conventional diffraction limit - the so-called 'Perfect Lens' (Pendry 2000). Such a material requires both negative permeability and negative permittivity, however a near-field demonstration of perfect lensing should be possible in a material with only one of these negative, such as a metal illuminated close to its plasma frequency. Recent theoretical work also suggests that metal nanowire composites could be used to realise negative refraction at optical wavelengths.

Surface plasmons play an important role in ENFOL, and by manipulating their generation and propagation some interesting new near-field lithography modes can be developed and explored (Blaikie 2001; Luo 2004). Some of these are reviewed here, although it should be noted that this field is very much in its infancy at this point in time.

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