MILESTONES Image courtesy of John fabricated by Richard Shelby, Sheldon Pendry and David Smith Schultz and David Smith. The capabilities of metamaterials are not limited to homogeneous structures. In 2006, Ulf Leonhardt, and independently Pendry, Schurig and Smith, realized that metamateri- als with spatially varying properties could be powerful tools for guiding light along almost arbitrary paths. Leonhardt used the ray approxima- tion to make his mapping. Pendry, Schurig and Smith developed a theoretical description that was based on a coordinate transformation of Maxwell’s equations, and was hence known as transformation optics. MILESTONE 21 Consequently, transformation optics has broken down many barriers in the field of optics. The masters of light In 2006, one of the most widely publicized metamaterial devices was revealed — the optical cloak, using The materials parameters of any All this changed in 1999, when which, for a specific set of parameters, optical apparatus, such as a lens John Pendry and colleagues demon- an object can be concealed from an or a prism, determine how the strated a ‘swiss roll’ structure that had observer. Leonhardt later extended properties of light can be controlled. negative permeability. The key was to the transformation-optics approach Unfortunately, the range of options tailor the structure of the material on to curved, non-Euclidean coordinate available in natural materials is a scale smaller than the wavelength transformations, which allow opera- surprisingly limited. Light–matter of light passing through it, so that tion across a broader range of wave- interactions are described by the optical waves do not resolve the lengths. Equipped with the possibility Maxwell’s equations (MILESTONE 2), underlying features. Such devices to manipulate the optical properties and the values of the parameters that are now known as metamaterials. of matter at will, further exciting enter those equations — dielectric Importantly, as negative permittiv- designs will undoubtedly emerge. permittivity and magnetic perme- ity occurs in metals close to their Joerg Heber, ability — mean that the refractive plasmon resonance, the realization Senior Editor, Nature Materials index of a natural material is always of a negative refractive index seemed positive, even though the equations within reach. ORIGINAL RESEARCH PAPERS Veselago, V. G. The electrodynamics of substances with allow the possibility of a negative Metamaterials then received simultaneously negative values of ε and µ. Sov. refractive index. widespread attention in 2000, when Phys. USPEKHI 10, 509–514 (1968) | Pendry, J. B., The investigation of negative Pendry published his landmark paper Holden, A. J., Robbins, D. J. & Stewart, W. J. Magnetism from conductors and enhanced refractive index, for which permit- on a perfect lens, which was made nonlinear phenomena. IEEE Trans. Microw. Theory tivity and permeability are both using a negative-refractive-index Tech. 47, 2075–2084 (1999) | Pendry, J. B. Negative refraction makes a perfect lens. Phys. Rev. Lett. 85, negative, began years ago. Notably, material. This work staggered many 3966–3969 (2000) | Shelby, R. A., Smith, D. R. & in 1968, Victor Veselago worked out in the field for its seemingly unbe- Schultz, S. Experimental verification of a that a planar slab of a material with lievable prediction of perfect imaging negative index of refraction. Science 292, 77–79 (2001) | Leonhardt, U. Optical conformal a negative refractive index would capability, and was the first concrete mapping. Science 312, 1777–1780 (2006) | focus light in the same way as would step towards metamaterials with Pendry, J. B., Schurig D. & Smith, D. R. Controlling curved lenses made from conven- functionality that is impossible for electromagnetic fields. Science 312, 1780–1782 (2006) | Schurig, D. et al. Metamaterial tional materials. In the absence of any natural materials. The following year, electromagnetic cloak at microwave frequencies. material that had such properties, the predictions made by Pendry were Science 314, 977–980 (2006) | Leonhardt, U. & Tyc, T. Broadband invisibility by non-Euclidean however, these early studies largely confirmed when the first negative- cloaking. Science 323, 110–112 (2009) fell into oblivion. refractive-index metamaterial was NATURE MILESTONES | PHOTONS MAY 2010 © 2010 Macmillan Publishers Limited. All rights reserved.