Researchers succeeded in designing a material with an isotropic negative index of refraction for visible light
Researchers at the FOM Institute AMOLF and the California Institute of Technology have succeeded for the first time to design a material with an isotropic negative index of refraction for visible light.
The material consists of a nanoscale grid of coupled silver tubes that guide light. In this material light will bend the 'wrong' way and light waves will run backwards. The new material is also a step towards the fabrication of an 'invisibility cloak' that would make objects invisible to light. The researchers published their work on April 18 in the journal Nature Materials.
The propagation of light in a material is described by the refractive index, a number that usually lies between 1 (air) and 1.5 (glass). The refractive index determines the speed of light and also how light refracts at an interface between two materials: the higher the index, the stronger the refraction. In recent years, physicists have focused on whether the refractive index can be also negative. In that case, light would move ‘backwards’, and light would bend the 'wrong' way at an interface.
The new material for the first time shows exactly this for visible light. The researchers studied the propagation of light in a material that consists of a bundle of tiny tubes which each consist of a core of silver with a coating of semiconductor material which in turn is wrapped in silver, a kind of coaxial cable for light. The secret of the new material is that the size of the tubes is much smaller than the wavelength of light. Light thus 'sees' a bundle of tubes, that each guide a small part of the light. The net effect is that the refractive index is negative, even over a large angular range. By varying the structure of the tubes the refractive index can be varied between 10 and -10.
Worldwide, many researchers are looking for materials with a negative refractive index. Because these materials do not occur in nature, so-called "metamaterials" are investigated of which the optical properties are determined by the structure of the material, rather than by the atoms of which it is built. A few years ago the first metamaterial with a negative refractive index was demonstrated for microwaves; more recently also for infrared light. The challenge however, remained to achieve negative refraction for visible light, because the most important applications are foreseen for visible light. Albert Polman, leader of the Amsterdam team: "All the research so far focused on resonant structures: small rings of light that briefly confine the light and cause as a net negative refractive index. However, it was clear that this principle is not applicable to visible light. We have now developed a completely new design, in which we couple narrow metal waveguides for light. In these structures, the propagation of light is based on a totally different principle, and interestingly enough it works very well for visible light."
The new structure enables many special applications. For example, as predicted by the British physicist John Pendry in 2000, using a material with a refractive index of -1 a 'perfect lens' can be made, with which extremely sharp images can be made. Also, by spatially varying the refractive index an 'invisibility cloak' can be designed, in which light is folded around an object so that the object becomes invisible. The new structure makes this all possible for the first time with visible light.
The newly published study is based on calculations and computer simulations. The next challenge is to actually make these materials. René de Waele, a PhD student who developed the design of the new metamaterial: "My first experiments in the cleanroom showed that it is possible to make these extremely small structures; I have now made tubes of only 20 nanometers wide and that is exactly what we need."
The study is part of a research program on plasmonics, extremely small light waves that propagate in metals. Polman’s group published earlier this year articles on the integration of optics and electronics on a computer chip using plasmons (Nature Materials, January 2010) and improving solar cells using plasmons (Nature Materials, March 2010).
Reference
The article "A single-layer wide-angle negative index metamaterial at visible frequencies", S.P. Burgos, R. de Waele, A. Polman and H.A. Atwater, is published on April 18, 2010 in the online edition of Nature Materials.
Information
For more information please contact Albert Polman, (020) 754 71 00.