'Any sufficiently advanced technology is indistinguishable from magic', as the late Arthur C Clarke wrote. So what does it take to do magic by technology? Transformation optics has developed some tantalizing ideas and the first practical demonstrations of 'pure and applied magic'.

Transformation optics gathers an unusual mix of scientists, ranging from practically-minded engineers to imaginative theoretical physicists and mathematicians or hybrids of all three. The engineers have been developing new materials with extraordinary electromagnetic properties, from materials for microwaves, to be used in radar or wireless technology, to materials for terahertz radiation and visible light. These materials typically are composites—they consist of artificial structures much smaller than the wavelength that act like man-made atoms, apart being much larger in size. The properties of these artificial atoms depend on their shapes and sizes and so they are tunable, in contrast to most real atoms or molecules. This degree of control is what makes these materials—called metamaterials—so interesting. Such new-won freedom invites the other side of the spectrum of scientists, the theorists, to dream. Just imagine there are no practical limits on electromagnetic materials—what could we do with them? One exciting application of metamaterials has been Veselago's idea of negative refraction, dating back to the 1960s. Metamaterials have breathed life into Veselago's idea, culminating in recent optical demonstrations (see for example [1,2]). Another application is cloaking, developing ideas and first experimental demonstrations for invisibility devices [3]. It turns out that both negative refraction and cloaking are examples where materials seem to transform the geometry of space. Any optical material appears to change light's perception of space, as countless optical illusions prove, but the materials of transformation optics act in more specific ways: they appear to perform coordinate transformations. If the coordinates they conjure up run backwards one gets negative refraction, if they exclude some region of space one makes anything inside invisible [4]. In physics, general relativity has honed the theoretical tools for understanding curved space and curved-coordinate transformations. In transformation optics, general relativity has become a theoretical tool for solving practical engineering problems [4]. What an unorthodox connection! This focus issue represents a snapshot of this rapidly developing research area. It is not restricted to optics or electromagnetism, though. Metamaterials for acoustics also exist and can be applied in ways similar to optical metamaterials. So transformation optics not only attracts an unusual mix of scientists, but also spans a range of applications in optics and beyond.

Transformation optics has the potential to transform optics, for example by visualizing invisibility and making materials beyond materials—metamaterials. But before we transgress the boundaries to the hermeneutics of transformation optics [5], let the papers speak for themselves.

References

[1]   Yao J, Liu Z, Liu Y, Wang Y, Sun C, Bartal G, Stacy A M and Zhang X 2008 Science 321 930

[2]   Valentine J, Zhang S, Zentgraf T, Ulin-Avila E, Genov D A, Bartal G and Zhang X 2008 Nature 455 376

[3]   Schurig D, Mock J J, Justice B J, Cummer S A, Pendry J B, Starr A F and Smith D R 2006 Science 314 977

[4]   Leonhardt U and Philbin T G 2006 New J. Phys. 8 247

[5]   Sokal A D 1996 Social Text 14(46/47) 217

Focus on Cloaking and Transformation Optics Contents

Transformation optics for the full dielectric electromagnetic cloak and metal–dielectric planar hyperlens
D P Gaillot, C Croënne, F Zhang and D Lippens

Transmutation of singularities in optical instruments
Tomáš Tyc and Ulf Leonhardt

Electromagnetic cloaking with canonical spiral inclusions
K Guven, E Saenz, R Gonzalo, E Ozbay and S Tretyakov

Theory and potentials of multi-layered plasmonic covers for multi-frequency cloaking
Andrea Alù and Nader Engheta

Electromagnetic cloaking devices for TE and TM polarizations
Filiberto Bilotti, Simone Tricarico and Lucio Vegni

An aberration-free lens with zero F-number
D Schurig

Transformational optics of plasmonic metamaterials
I I Smolyaninov

An acoustic metafluid: realizing a broadband acoustic cloak
J B Pendry and Jensen Li

On the possibility of metamaterial properties in spin plasmas
G Brodin and M Marklund

A homogenization route towards square cylindrical acoustic cloaks
Mohamed Farhat, Sébastien Guenneau, Stefan Enoch, Alexander Movchan, Frédéric Zolla and André Nicolet

Transformation optics: approaching broadband electromagnetic cloaking
A V Kildishev, W Cai, U K Chettiar and V M Shalaev

Generalized field-transforming metamaterials
Sergei A Tretyakov, Igor S Nefedov and Pekka Alitalo

Electromagnetic beam modulation through transformation optical structures
Xiaofei Xu, Yijun Feng and Tian Jiang

Superantenna made of transformation media
Ulf Leonhardt and Tomáš Tyc

Material parameters and vector scaling in transformation acoustics
Steven A Cummer, Marco Rahm and David Schurig

Isotropic transformation optics: approximate acoustic and quantum cloaking
Allan Greenleaf, Yaroslav Kurylev, Matti Lassas and Gunther Uhlmann

Transformation optical designs for wave collimators, flat lenses and right-angle bends
Do-Hoon Kwon and Douglas H Werner

Alternative derivation of electromagnetic cloaks and concentrators
A D Yaghjian and S Maci

Solutions in folded geometries, and associated cloaking due to anomalous resonance
Graeme W Milton, Nicolae-Alexandru P Nicorovici, Ross C McPhedran, Kirill Cherednichenko and Zubin Jacob

Finite wavelength cloaking by plasmonic resonance
N-A P Nicorovici, R C McPhedran, S Enoch and G Tayeb