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Introduction
It is many years since Veselago gave the recipe for a material that refracts light in a negative direction. His recipe required a material in which both the electrical permittivity, ε, and magnetic permeability, µ, were both negative. At the time this seemed an impossible demand. Whereas in many metals ε < 0 in the visible region of the spectrum, few materials have µ < 0 and those that do have other disobliging properties which render them impractical. There matters rested for 30 years until in 1999 a group of scientists at the Marconi Company and myself suggested that µ < 0 could be realised by manufacturing a metamaterial. The so called ‘split ring’ structure proved that µ < 0 was possible at least in the GHz region. Shelby Smith and Schultz were the first to make a negatively refracting structure and their paper launched a torrent of activity in the new subject of negative refraction. Figure 1 shows the take-off in publications on the subject since 1999.
Why such enthusiasm? Figure 2 shows the four possible combinations of ±ε, ±µ. In each quadrant new phenomena are observed. For example when ε < 0 surface plasmons are observed. Therefore it is as if a new door has been opened into ‘third quadrant’ electromagnetism. One of the phenomena seen here is negative refraction, but there are other remarkable effects to be found such as subwavelength imaging, and I do not doubt that there will be more discovered in the coming months.
All of which is to say that this is a very good time to gather together a special issue before the subject is so big that it fragments into many parts. Even so not every aspect is covered, but we do have a good spread of articles. Computational techniques have been central to progress, Here P. Markoš and C. M. Soukoulis demonstrate the power of the transfer matrix, well adapted to these dispersive materials. P. Kolinko and D. R. Smith and R. W. Ziolkowski present simulations which confirm the concept of negative refraction, and J. B. Pendry shows how to design lenses using negative materials. Experimental evidence is vital to confirm all our theorising: microwave and optical experiments designed to test theoretical predictions are presented by Iyer et al. and by Greegor et al., whereas Fang et al. demonstrate that a silver film can act as an optical amplifier, a key prediction of theory. Meanwhile others are pressing on into new areas: Luo et al. show that in periodic media negative refraction is possible by means other than negative refractive index. Podolsky et al. present a scheme which would realise negative refraction at optical frequencies; Wiltshire et al. apply the concepts to magnetic resonance imaging at 21MHz; A. Lakhtakia shows the effects of chiral media and Lu et al. make a computational demonstration of the backward radiating Čerenkov effect predicted by Veselago.
