Anyone who has looked through a glass lens, or at the bottom of a garden pond through the water, has at the very least an intuitive grasp of light refraction. Transparent materials such as glass and water bend light rays entering them, so that the position of an object, when viewed from the other side of the medium, is shifted by an amount dependent on the angle the incident light rays make with the surface. This can be quantified using Snell’s Law, which relates the angles of light incidence and refraction to a material’s refractive index.
In natural materials, the refractive index is always a positive quantity, and the materials are referred to as ‘right-handed’, as they refract light to the right of the incident beam. Metamaterials, also known as ‘left-handed’ materials, are artificial substances that gain their properties from their structure rather than composition. Photonic metamaterials can be thought of as nanoscale electronic circuits in which the building blocks, or ‘photonic atoms’, have scale sizes smaller than the wavelength of light.
The key difference between normal materials and metamaterials is that the latter possess negative refractive index, and so refract light to the left of the incident beam, or at a negative angle. Another way of looking at it is that in metamaterials, electromagnetic waves carry energy in the opposite direction to their phase velocity.
Researchers led by Costas Soukoulis, at the US Department of Energy’s Ames Laboratory, have for the first time succeeded in developing a material with a negative refractive index for visible light. Their discovery is reported in a recent issue of the journal Science (subscription required).
Metamaterials were hypothesised by Russian physicist VG Veselago back in 1968, and it was John Pendry, a professor at Imperial College, London, who first came up with practical ways of making such materials. Until recently, metamaterials were restricted to the microwave and far-infrared regions of the electromagnetic spectrum, and the challenge has been to fabricate metamaterials that refract light at even smaller wavelengths.
The peculiar characteristics of metamaterials allow light to be manipulated in a way similar to that in which semiconductors control electricity, and the applications are many and diverse. For example, metamaterials may lead to the development of flat superlenses that can capture details smaller than the wavelength of light. They could also direct light so that it passes around solid objects rather than reflect off them, thus rendering them invisible.
Soukoulis, working with fellow physicists Stefan Linden and Martin Wegener from the University of Karlsruhe, created a mesh-like material by etching an array of 100-nanometre wide holes into layers of silver and magnesium fluoride on a glass substrate, leading to a metamaterial with a refractive index of –0.6 at the red end of the spectrum (wavelength 780 nm).
The use of silver, rather than the gold commonly used in earlier metamaterials, means there is less resistance to electromagnetic radiation, but energy losses remain a major limiting factor. Added to this is the difficulty in manufacturing materials at a scale that will work with visible light as opposed to radio frequency waves.
There is much to be done before practical applications of photonic metamaterials can be realised, but we have come a long way in the six years since the first metamaterials were fabricated. ‘Lots of scepticism has been expressed as to whether negative-index metamaterials would ever make it to visible frequencies,’ says Martin Wegener. ‘After some years of work, it was quite a pleasure to be in the laboratory and literally see them work.’
Figure: Left-handed refraction of visible light through a silver and magnesium-fluoride-based metamaterial. Photo of laboratory setup supplied by Martin Wegener.
Article first published in Nanomaterials News.