Klaus Rademann

Maik Eichelbaum

People have been coloring glass for centuries. Even the ancient Romans knew that adding gold to glass would convert it into a ruby-red material when heated in a controlled fashion.

The coloration is due to gold clusters that form within the glass. Light absorption depends on the concerted oscillation of the conducting electrons in the gold clusters, plasmon oscillation. Klaus Rademann and Maik Eichelbaum of the Humboldt University of Berlin in collaboration with Ralf Mueller, Martin Radtke, Heinrich Riesemeier, and Wolf Goerner of the German Federal Institute for Materials Research and Testing in Berlin, suspected that varying the conditions for making this kind of glass would alter the electronic environment

Color changing in red glass

of the clusters within it and so could be used to influence the frequency of the oscillation and so the color of the absorbed light. They suggest such a technique could allow them to make glassy materials suitable for use in nanophotonic components, including tiny optoelectronic circuits or optical storage devices.

The researchers began with soda lime-silica glasses containing gold trichloride. They irradiated these glasses for five minutes with intense synchrotron radiation, which triggered the photochemical reduction of the gold ions to elemental gold. This led to an even, brown tone in the irradiated areas of the glasses. These were then heated to over 550 Celsius for 30-45 minutes, which led to the development of the red color characteristic of plasmon oscillation. The longer the heat treatment the more the color shifted to higher wavelength, shorter frequency and so the deeper red region of the spectrum. Rademman and colleagues explain this red shift as being due to the change in size and shape of the gold clusters within the glass.

Angew Chem Int Edn, 2005, in press; http://dx.doi.org/10.1002/anie.200502174