Near-field imaging through plasmonic superlenses

Abstract

Near-field imaging through plasmonic 'superlensing' layers can offer advantages of improved working distance (i.e. introducing the equivalent of a focal length) and control over image intensity compared to simple near-field imaging. In a photolithographic environment at ultra-violet (UV) wavelengths the imaging performance of single- and multi-layer silver plasmonic superlenses has been studied both experimentally and via computer simulations. Super-resolution imaging has been demonstrated experimentally, with the sub-100 nm resolution currently being limited by issues of roughness in the silver layers and the ability to deposit high-quality silver-dielectric multilayers. The simulation studies have shown that super-resolved imaging should be possible using surprisingly thick silver layers (>100 nm), with the cost of much reduced image intensity, which is something that is yet to be shown experimentally. The use of multilayer plasmonic superlenses also introduces richness to the imaging behaviour, with very high transmission possible for certain spatial frequency components in the image. This has been widely touted as a means for improving image resolution, but the complexity of the spatial-frequency transfer functions for these systems does not make this a universal fact for all classes of objects. Examples of imaging situations are given where multi-layer superlenses are actually detrimental to the image quality, such as the case of closely-separated dark-line objects on an otherwise bright background.