Veni, vidi, crevi crystallis photonicis

The Roman Republic is known for many things. Besides influencing most of the western world with their politics, philosophies, and language, Romans were also early adopters of innovations that are still used today, such as traffic signs, aqueducts, and plumbing. Yet, with all the innovations and technologies it implemented, ancient Rome was only recently credited for one of its more unique contributions to the field of photonics.

In an article published in the Proceedings of the National Academy of Sciences USA, researchers from Tufts University’s Silklab in collaboration with the Italian Institute of Technology’s (IIT’s) Centre for Cultural Heritage Technology describe the process and chemistry that enabled photonic crystals to grow naturally on ancient Roman glass shards.

Once crafted into vessels containing anything from wine, water, or perfume, the ancient glass has been excavated from sites all over the Italian Peninsula. It exhibits iridescent sheens of orange, blue, and green, earning the name “wow glass” from Arianna Traviglia, the director of the IIT’s Centre for Cultural Heritage Technology. The glass shards, however, were not always so colorful. Buried under successive layers of sediment and cycled through seasonal temperatures, the glass rearranged over centuries and recombined with native minerals to form photonic crystals — uniquely ordered arrangements of atoms that filter and reflect light in very specific ways.

Using a correlated optical-morphological analytical approach, Silklab researchers Fiorenzo Omenetto and Giulia Guidetti were able to find a highly reflective metallic patina composed of highly ordered nanostructured domains resembling Bragg stacks. Basically, the patina is composed of layers that act as high-quality light reflectors for different wavelengths of light. These layers, composed of micrometer-thick silica, are what gives the glass its “wow” factor, sometimes emitting a golden, reflective appearance.

A close-up image of photonic crystals on the Roman “wow glass.”. Courtesy of Giulia Guidetti, Silklab, Tufts University.

Oddly similar to the way ancient Rome’s history played out, the researchers believe that the formation of this patina occurred through a multi-century process of corrosion and reconstruction. External factors such as rain, climate, and the chemical composition of the surrounding mud not only broke down the glass particles, but also influenced which minerals would diffuse into the glass’s matrix. This process continued cyclically and progressively until the glass underwent the proverbial sea change into something rich and strange.

While they agree that the aging process is what makes the Roman glass so beautiful, both Omenetto and Guidetti foresee the possibility that the process could be accelerated in a lab setting, though we would still owe credit to the ancient Romans for such uniquely iridescent photonic crystals.

The research was published in the Proceedings of the National Academy of Sciences USA (www.doi.org/10.1073/pnas.2311583120).