Infinitely fast light waves in a silver nano tube
Researchers at the FOM Institute AMOLF and the University of Pennsylvania in the US have developed a material in which light waves propagate almost infinitely fast. At the same time light itself is brought to a standstill. They publish their surprising discovery on December 17, 2012 in the prestigious journal Physical Review Letters.
Light waves in vacuum propagate with a speed of almost 300,000 km/s. In materials such as glass or plastic light always travels slower than in vacuum. Physicists use the 'refractive index' as a measure of that speed. For example, the refractive index of glass is 1.5, which means that light waves propagate 1.5 times slower than in vacuum. The refractive index determines, among others how light is focused in a lens or how it travels along a fiber optic cable in an internet connection. Almost all natural materials have a refractive index between 1 and 5. The researchers have now circumvented nature by designing a structure that has a refractive index close to zero. That means that the light waves propagate almost infinitely fast. In addition to the speed of the waves (the 'phase velocity') light also has a speed at which information travels, for example in a communications network, the so-called 'group velocity'. Surprisingly, in the new material the group velocity becomes almost zero.
Metamaterials
The Amsterdam researchers and their US colleagues investigate so-called metamaterials. These are materials with special properties that do not exist in nature, that are made by structuring them with units that are much smaller than the wavelength of light. The new metamaterial consists of a 2 micrometer long glass rod, in which light propagates, surrounded by a thin layer of silver. If the tube is made very narrow light gets a very strong interaction with the metal wall of the tube. For very narrow tubes, with a diameter of about 100-200 nanometers, that interaction is so strong that the refractive index of light becomes close to zero.
Imaging light waves
To study these unusual light waves the researchers used the 'cathodoluminescence spectroscopy' measuring technique they previously developed at AMOLF. In this technique, an electron beam in an electron microscope is used to excite the light waves in the silver tube. In this way, they could create images of the light waves inside the tube. They then discovered that in narrow tubes they could increase the wavelength of the light until the waves completely disappeared. While the tubes still support oscillations of light, their wavelength becomes nearly infinite, so the refractive index approaches zero. The new invention is not in conflict with Einstein's limit, because no information is transferred in the experiments with a speed greater than the speed of light. Indeed, the speed of information transfer in the material becomes almost zero.
The article was selected by Physical Review Letters as Editor's Suggestion. The American journal Science also devoted a news item to the article.
Reference
Experimental verification or n = 0 structures for visible light
E.J.R. Vesseur, T. Coenen, H. Caglayan, N. Engheta, and A. Polman
Physical Review Letters 109, in press (2012)
Science news article
Contact
Albert Polman, tel. + 31 (0)20 754 7402
Ernst Jan Vesseur, + 1 626 240 9794
Website: www.erbium.nl/arcis.html