Metamaterials are materials made up of tiny, repeating structures, which are capable of interacting with electromagnetic waves in unusual ways. For instance, the possibility of a real "invisibility cloak" is often discussed in relation to metamaterials, although their applications are far broader, spanning defence, communications and research.
A team of researchers based at Sandia National Laboratories have now created a metamaterial made up of a square-patterned array of nanocylinders of a semiconductor (gallium arsenide) which bends light strongly.
The researchers then selected two near-infrared lasers with wavelengths matching the resonant frequency of their metamaterial. The two frequencies of light from these lasers could be combined in different ways to produce 11 different colours spanning the rainbow in combination with the metamaterial.
This is an entirely different approach to that conventionally used to generate a coloured laser beam – such as for commercial laser pointers – which relies on the use of specially grown crystals.
"With this tiny device and two laser pulses, we were able to generate 11 new colours at the same time, which is so cool," said Dr Polina Vabishchevich, first author of the Nature Communications paper describing the technique. "We don't need to change angles or match phases."
Although at present the conversion efficiency for this colour-mixing technique is low – meaning that the resulting beam of light is far less bright than the lasers used to create it – the researchers believe that this could be improved with further experimentation, such as by stacking many layers of their metamaterial.
The material could be a first step towards a laser pointer equivalent of a multi-coloured ballpoint pen, where the colour of the light can be changed with the click of a button.
However, the material could have other, more practical applications, such as in archaeology, astronomy and telecommunications; tuning wavelength of light could allow for archaeological sites to be identified amid dense vegetation, to search the skies for signs of extra-terrestrial life, and to increase the capability of long-distance fibre-optic communication. Meanwhile, this technique could also prove valuable to researchers in an enormous range of fields, such as through the development of specialised microscopes.
