Metamaterials Generate Novel Electromagnetic Properties
The Blackett Laboratory,
Imperial College London, UK
Nature supplies a limited palette of electromagnetic properties and for this reason metamaterials have attracted a great deal of attention. The concept is a simple one: we recognise that a materialís response is an average over the individual responses of many sub-wavelength units. Conventionally these units would be the constituent atoms and molecules, but we could easily introduce larger structures and still stay within the sub-wavelength limit. At optical frequencies where the wavelengths are a few hundred nanometres, structures of dimensions of a few tens of nanometres could be introduced. Individually they would be invisible to light and only their average response would be evident. The extra freedom given by adding physical structure to chemical composition as a variable enables us to create materials with previously unheard of properties. Metamaterials like photonic crystals achieve their effects through structure however photonic crystals function by diffraction of light and hence have a complex response. In contrast metamaterials must eliminate diffraction through their sub-wavelength structure so that their response can be defined by a permittivity and permeability.
The most startling application of metamaterials has been to create negative refraction. Previously speculated on but practically unrealised, this novel property is now created routinely. Negative refraction has led to many remarkable possibilities including the ability to focus light unlimited by constraints of wavelength. Negative refraction follows when both the permittivity and permeability are negative and this inversion is achieved by introducing two classes of resonator: one electric the other magnetic in character. Tuning the frequency above the resonances, which must be close to one another, results in an inverted response and hence the negative values sought.
More recently metamaterials have been used to implement the concept of transformation optics: light can be fooled into thinking that space is distorted, as if by a giant gravitational field, by appropriate scaling of the permittivity and permeability. Hence the trajectory of a ray, or the location of magnetic lines of force can be bent almost at will. Implementation demands some unusual material properties: anisotropy for one, and is only feasible with access to metamaterials. The application that has attracted most attention is the cloaking of arbitrary objects by constructing a layer of metamaterials that guides light around the hidden object.