This 2,000 years-old glass fragment reflects light across a range of different wavelengths, from red to blue and colours between, acting like a gold mirror. Credit: CCHT-IIT / National Archeological Museum of Aquileia / Italian Ministry of Culture.
A team of scientists from Italy and the United States has found that an ancient piece of glass unearthed near what was the Roman city of Aquileia in northern Italy is adorned with structures known as photonic crystals, which are usually found in cutting-edge optical technology1.
Photonic crystals consist of periodic nanoscale features that can filter and deflect different colours of light. Normally produced synthetically for applications such as optical communications, in this case they were laid down over centuries by the forces of nature, with hundreds of thin layers of silica that create a dazzling multi-coloured patina on the green glass.
Archaeologist, Arianna Traviglia, found the glass fragment in a field during an expedition in 2012 while a researcher for Macquarie University in Sydney. She has since analysed the object's physical and chemical properties with colleagues in her current lab at the Italian Institute of Technology in Venice, dating it to between the 1st century BC and 1st century AD.
Last year Fiorenzo Omenetto, an engineer at Tufts University in Massachusetts visited the Venice lab, and Traviglia recalls that when he saw the fragment on a desk, he got very excited, and then took a small portion of it back to America.
Using optical and electron microscopy, Omenetto and colleagues at Tufts discovered that the millimetre-thick iridescent patina consists of silica layers containing multiple stacks. Each stack, a one-dimensional photonic crystal, can reflect a specific wavelength of light thanks to its structure, consisting of dozens of alternating higher- and lower- density layers called nanolamellae. The thicker the higher density nanolamellae in each stack, the longer the wavelength of light that is reflected. The patina contains stacks reflecting light across a range of different wavelengths, from red to blue and colours between, causing it to act like a gold mirror.
Naturally-occurring photonic crystal have already been found, in butterfly wings for example. The novelty, says Traviglia, is to have established that the elements can form photonic crystals by working the surface of glass over long periods of time. As Omenetto puts it, these structures require no clean rooms but emerge after "spending a few thousand years in the dirt".
It's still not clear how that happens, but the researchers reckon that the alkaline soil slowly breaks up the amorphous molecular structure of glass to create silica nanoparticles that grow and coalesce to form nanolamellae – with variation in the soil's pH levels creating the alternating density.
Traviglia says that she and her team plan to artificially age ancient Roman-like glass more quickly than occurs in nature. Being able to create photonic crystals like this, she adds, might also provide a new way of fabricating materials for telecommunications and controlling light.
