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25 Nov 2005
A material made from an array of gold nanopillars on glass exhibits unique optical properties.
Scientists in the UK and Russia have succeeded in fabricating a material that has a negative permeability at visible wavelengths (Nature 437 335). The development is important because it could lead to so-called “left-handed” materials which exhibit a negative refractive index and function as a perfect lens, focusing light to a smaller spot than is usually possible.
Such left-handed materials have previously been demonstrated at terahertz, microwave and infrared parts of the electromagnetic spectrum but never before in the visible.
“This discovery could be a milestone for optics and could help realise the visible left-handed materials which promise the perfect lens,” said Alexander Grigorenko from the University of Manchester, UK. “It also provides the possibility for making new optical devices such as spacers and nanolasers.”
The artificial material consists of an array of pairs of tiny gold pillars (about 100 nm high and wide) on a glass substrate. The structures occupy a total area of about 1mm2, which contains about 1 million pillars. The material is the result of a collaboration between Manchester scientists and the Chernogolovka Institute of Microelectronic Technologies in Russia and Aston University, UK.
Illuminating the pillars with green light (TM polarization) excites a so-called “antisymmetric plasmon resonance” between each pillar pair which induces a high frequency electric current. The magnetic moment associated with this current gives rise to negative permeability. Although the team has yet to observe negative refraction they have seen some other curious behaviour, such as optical impedence matching -- a cloaking effect where reflection from the sample is totally suppressed.
“In our case, this phenomenon resulted in a total invisibility of our structured films at green resonance frequencies for TM polarization of incident light, while the films could still be seen by using phase contrast imaging or TE polarization,” say the authors in their paper.
Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.
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