Gold is prized in chip manufacture for its excellent electrical conductivity, but it also has unusual properties that could give it a role in new optical chips, scientists have discovered.

In a typical electronic chip, tiny gold wires link microscopic connecting pads on the silicon wafer to the terminals of the chip packaging. The gold wires are about 50 micrometres in diameter (about half as thick as a human hair), and at that thickness the gold still behaves like the shiny metal we know. A micrometre is one-millionth of a metre.

If you divide it into rods 2,500 times thinner, though, just 20nm (nanometres) across, the gold glitters in an entirely different way, according to scientists at the US Department of Energy's laboratory in Illinois.

At the nm scale, where distances are measured in billionths of a metre, it's not just optical properties that change: many materials respond differently to variations in temperature, and to the effects of electric and magnetic fields, when divided into such nanoparticles.

The nanoscale gold rods studied at Argonne National Laboratory emit light when electrons in them are stimulated, and the wavelength of the light depends on the length of the rod used, the scientists found. They tested gold rods with lengths between 70nm and 300nm.

Being able to control the wavelength of light, and to build light sources of a specific wavelength, is very important in optical communications. The discovery at Argonne could one day allow the fabrication of tuned light sources inside chips, leading to the creation of chips that can switch or route optical signals in fibre networks without having to convert them back to an electronic form first, the scientists said. However, they emphasised that they are only involved in basic research, not the development of products.

Researchers at the Nanotechnology and Optical Instrumentation Laboratory in Troyes, France, also participated in the research, which was published on 23 December in a paper in Physical Review Letters entitled Surface Plasmon Characteristics of Tunable Photoluminescence in Single Gold Nanorods. It can be viewed here.