Cloak of Invisibility?
Exploring the properties of materials with negative refractive index
Have you ever wondered why completely transparent objects like wine glasses are so easy to see when windows made of the exact same material are almost invisible? The answer lies in a property of materials known as refractive index – their ability to bend light. Materials like glass have a high refractive index and therefore bend light as it enters or leaves them. Light hitting a flat window is all deviated in the same way, so the image remains undisturbed and consequently, the window is almost invisible. However the curves of a wine glass mean that light strikes the glass at different angles in some places than others and is therefore bent differently. In this case what we see is a warping of the surrounding image, which our brains conceptualize as a physical object – the glass.
But why do materials refract light in the first place? The refractive index of a material is created by a combination of the electric permittivity and magnetic permeability, which are themselves, dictated by the atomic structure and composition. In plain English, this means that a photon of light, which is made up of oscillating electric and magnetic fields, is perturbed by the rows of tiny electric and magnetic dipoles formed by the individual atoms making up the glass.
The mathematical analysis of refraction at the atomic scale is highly complex but the important end result is that light changes direction when entering and leaving refractive media. All materials found in nature have positive permittivity and positive permeability leading to positive refractive indices. There are however a class of artificially created substances called left handed metamaterials, that can be engineered to exhibit negative refraction.
Dr Ilya Shadrivov has spent many years with the Nonlinear Physics Centre investigating these exotic negatively refracting metamaterials, initially developing sophisticated theoretical models then later testing their predictions in the laboratory using real materials. The process of producing crystals with such negative refracting properties in visible light would be a highly complex and expensive exercise in nanotechnology and would not offer the control and flexibility required to work on developmental systems. So instead, the researchers use large-scale arrays of dielectrics such as fibreglass containing lattices of electronic components. Instead of visible light, the scientists irradiate these test arrays with microwaves that have far longer wavelength - in keeping with the scale of the lattice. “In optically transparent materials such as glass, the individual atoms are in effect tiny row of electric dipoles interacting with passing waves. In the large-scale lattices used in our research, the diodes serve the same purpose. What’s more, if we use variable capacitance diodes on wire loops whose electrical properties change with field strength, we can induce non-linear behaviour.” Dr Shadrivov explains.
”The use of these large-scale diode arrays gives us the flexibility we need to test and further develop our models, yet is on a scale easily constructed by humans. The electromagnetic microwaves are longer and the lattice is bigger, but the physics and mathematics is exactly the same as for visible light.”
The ANU group has for years been a world leader in the theoretical treatment of nonlinear matamaterials and has recently become one of the first groups in the world to practically demonstrate such behaviour in the laboratory. At the moment these exotic nonlinear metamaterials represent the forefront of materials physics. However it’s quite possible that within a few years they will lead to the realization of technology straight from the pages of science fiction. For example, an appropriately used shell of the right negatively refractive metamaterial may be able to bend light or radar around an object and redirect it to its original path on the other side. Such a device would render the object inside, completely invisible.