British Scientists Working of Metamaterials for Invisibility Cloaks and Perfect Lens

Imperial College London researchers designing and building unique 'metamaterials'  received a $8.1 million (£4.9 M) funding boost from The Leverhulme Trust. Metamaterials can be used for invisibility 'cloaking' devices, sensitive security sensors that can detect tiny quantities of dangerous substances, and flat lenses that can be used to image tiny objects much smaller than the wavelength of light.

The new grant has been made to a team of Imperial College London scientists and engineers, who, in collaboration with scientists at the University of Southampton, will develop new applications for metamaterials that can bend, control and manipulate light and other kinds of electromagnetic waves. Metamaterials is a new, emerging field of science lying at the borders of physics and materials science. The concept relies not on clever chemistry, which is normally used to create new materials, but instead on creating clever patterns on the surface of existing materials, particularly metals.

The new grant is one of two The Leverhulme Trust is awarding for 'embedding emerging disciplines'. The project team is led by two of Imperial College London's Professors: Sir John Pendry, a world-leading physicist and pioneer in the field, who first proposed that metamaterials could be used to build an invisibility 'cloak' in 2006, and Professor Stefan Maier who is a leading experimentalist in the field of plasmonics. Also collaborating in the Project is Professor Nikolay Zheludev's team at the University of Southampton.

Sir John says that the new grant will help British universities to develop real-world metamaterial applications based on his theories, including the much-coveted optical invisibility cloak, which would render an object invisible to the human eye:

"We've shown that an optical invisibility cloak is theoretically possible: the big challenge now is to build it. This is just one of the many extremely exciting potential uses of metamaterials that we'll be exploring with our colleagues at Southampton, thanks to this new grant from The Leverhulme Trust," he explained.

The Leverhulme Trust's grant will provide funding for Imperial College London to recruit three top researchers, including a new Leverhulme Professor to join their world-class teams in physics, materials science and optoelectronics. The University of Southampton will be provided funding to recruit two Leverhulme Advanced Fellows. There will also be opportunities for 10 PhD research students and many postdoctoral researchers to join the group at Imperial College London.

Metamaterials have a carefully designed internal structure that interacts with light and other electromagnetic waves in unique ways, producing effects not seen in nature. For example, scientists can design them so that they control the movement and direction of all kinds of radiation - from visible light to microwaves and terahertz radiation. Being able to control radiation using materials in this way is a relatively new scientific development and opens up a realm of potential applications in diverse fields including medicine, security, imaging, telecommunications and data processing.

In the case of a true cloaking device, the aim is to design a metamaterial cloak that 'grabs' light as it approaches and forces it to flow smoothly around the cloak instead of striking it, in the same way that water in a river flows round a stick, rendering the object concealed beneath it invisible to the human eye.

In imaging, metamaterials could be used to build a 'perfect lens' microscope that would enable scientists to look at objects smaller than the wavelength of light being used - something that has never been achieved using an off-the-shelf optical microscope before.

One of the other areas in which metamaterials could have a big impact is in security technology. The airport scanners of the future could use 'T-ray' radiation to detect very small quantities of poisons and explosives, and advances in metamaterials could make them work.

Professor Stefan Maier from Imperial's Department of Physics, co-leader of the project, explains: "With metamaterials, we can devise completely new ways of controlling radiation, from visible light all the way down to terahertz radiation and beyond. What we are aiming at are structures that are easy to make, but that can give us a level of control over the flow of radiation thought impossible until now. For example, we can make surfaces that guide terahertz or even radiofrequency waves along them, with their energy highly concentrated right there at the surface, extending only a tiny fraction of the wavelength away from it. This might greatly improve the sensitivity of terahertz sensing devices and allow new ways to harness low-frequency radiation."

The Imperial College London and University of Southampton research teams will also be focusing on developing metamaterials for use in energy harvesting, optoelectronics, fibre optics, as well as bionanotechnology and imaging.

The images below were part of a demonstration of pudding camouflage technology developed at Tokyo University sponsored by JELL-O® in 2003 and are examples of metamaterials.  The technology is the invention of Professor Susumu Tachi. Military organizations have expressed interest in the technology.
Photo credits ©Tachi Laboratory, the University of Tokyo

Cloak of Invisibility Advancing Towards Reality

Purdue University (West Lafayette, IN) researchers are working to perfect a "cloak of invisibility" made from nano metamaterials. An apparatus and method of cloaking are described in U.S. Patent Application 20080165442. The application is based on research sponsored by the U.S. Army Research Office under 50342-PH-MUR. Photonic metamaterials are engineered nano-composite structures with optimized responses to electromagnetic fields at optical frequencies that lead to super- and hyper-lens and invisibility cloaks.

The cloak may be designed to decrease scattering (particularly "backscattering") from an object contained within, while at the same time reducing the shadow ("forward scattering"), so that the combination of the cloak and the object contained within it have a resemblance to free space, according to inventors Wenshan Cai, Vladimir M.; Shalaev, Uday K. Chettiar and Alexander V. Kildishev.

The materials used for the cloak may have properties where, generally the permeability and permittivity tensors are anisotropic and where the magnitudes of the permeability and permittivity are less than one, so that the phase velocity of the electromagnetic energy being bent around the cloaking region is greater than that of the group velocity.

Materials having such properties have not been discovered as natural substances, but may be produced as artificial, man-made materials, where the permittivity and permeability are less than unity, and may be negative. Metamaterials, an extension of the concept of artificial dielectrics, were first designed in the 1940s for microwave frequencies. They typically consist of periodic geometric structures of a guest material embedded in a host material. Analogous to the circumstance where homogeneous dielectrics owe their properties to the nanometer-scale structure of atoms, metamaterials derive their properties from the sub-wavelength structure of its component materials. At wavelengths much longer than the unit-cell size, the structure can be assigned parameters that may be used describe homogeneous dielectrics, such as electric permittivity and refractive index.

An object to be cloaked is disposed such that the cloaking apparatus is between the object and an observer. The appearance of the object is altered and, in the limit, the object cannot be observed, and the background appears unobstructed. The cloak is formed of a metamaterial where the properties of the metamaterial are varied as a function of distance from the cloak interfaces, and the permittivity is less than unity. The metamaterial may be fabricated as a composite material having a dielectric component and inclusions of particles of sub-wavelength size, so as to have a permeability substantially equal to unity.

A brief video of how the cloak interacts with light can be found at Purdue University as well as more details on the research.