Light in reverse: Three-dimensional metamaterials with a negative index of refraction

AMOLF-researchers designed a new material with a negative index of refraction, in which light waves run backwards. The researchers unraveled the behavior of light in materials that look like 'sandwiches' of very thin stacked silver and dielectric films. The material 'puts visible light in reverse', irrespective of the direction under which light enters the material. With this breakthrough, the realization of a perfect lens for visible light comes within view. The researchers publish their results in Physical Review Letters on November 23.

The unusual properties of this so-called 'metamaterial' are caused by the fact that it is structured on a scale smaller than the wavelength of light. In naturally occurring materials, the refractive index, which is related to the velocity of light waves in the material, is always positive. But specially designed metamaterials can in principle exhibit a negative index. This means that the velocity of light waves is negative, so they flow towards a light source instead of away from it. This does not mean, however, that no light is transmitted: the light energy is still moving forwards, opposite to the wave. This remarkable feature causes a ray impinging on the metamaterial to refract at an angle opposite to that in a normal material.

Such materials could have spectacular applications: The English theoretical physicist John Pendry recognized that a negative-index material can be used to make a 'perfect lens', which, unlike regular lenses, could image even the smallest details of an object - smaller than the wavelength. In the past decade researchers have been searching for possible metamaterials. This has been successful for electromagnetic waves with long wavelengths, like microwaves. But it has proven to be very difficult for visible light: It is hard to properly structure materials on a subwavelength scale and the proposed designs all suffer from severe absorption. Moreover, they only show the desired behavior for one particular direction of light propagation.

The AMOLF-team consisting of Ewold Verhagen, René de Waele, Kobus Kuipers and Albert Polman describe a new class of metamaterials, operating at optical frequencies. The metamaterials are based on stacked layers of metallic and dielectric materials, and are in that respect surprisingly simple. Light waves can travel along the layers, but can also couple from layer to layer. In that way, light can propagate through the material in all three dimensions. The scientists show that by using layers of proper thickness (of several tens of nanometers) the index of refraction of the metamaterial is negative, and can be equal in all directions. Moreover, absorption in this metamaterial is a much smaller problem than in other approaches. These new metamaterials bring the application of negative-index materials, for example in microscopy and lithography, an important step closer to reality.

Reference
'Three-dimensional negative index of refraction at optical frequencies by coupling plasmonic waveguides', Ewold Verhagen, René de Waele, Kobus Kuipers and Albert Polman, Physical Review Letters 105, 223901 (2010).

Information
Dr. Ewold Verhagen, École Polytechnique Fédérale de Lausanne, Zwitserland,  telephone +41 21 693 05 59/+41 76 225 73 25.
Prof.dr. Albert Polman, FOM-instituut AMOLF, Amsterdam,  telephone +31 (0)20 754 71 00.