Researchers have revealed how photonic crystals have been created through corrosion and crystallization over centuries. About 2,000 years ago, in ancient Rome, glass vessels containing wine, water or exotic perfumes rolled off market tables and shattered in the streets. Over the centuries, the fragments became covered in layers of dust and dirt, exposed to constant cycles of temperature, humidity and surrounding minerals.
Now, these tiny shards of glass have been uncovered from construction sites and archaeological digs and reveal something extraordinary. Their surfaces are studded with colorful colors such as blue, green and orange, and some display shimmering gold mirrors.
These beautiful pieces of glass are often set in jewelry as pendants or earrings, while larger, more complete pieces are displayed in museums.
For Omenetto and Guidetti, Silklab engineering professors at Tufts University and materials science experts, what's fascinating is how the molecules in the glass have rearranged and recombined with minerals over thousands of years to form what are called photonic crystals -- ordered arrangements of atoms that filter and reflect light in very specific ways.
Photonic crystals have many applications in modern technology. They can be used to make waveguides, optical switches and other devices that enable fast optical communications in computers and the Internet. Because photonic crystals can block certain wavelengths of light while allowing other wavelengths to pass through, they are used in filters, lasers, mirrors, and anti-reflective (cloaking) devices.
In a recent study published in the Proceedings of the National Academy of Sciences (PNAS), Omenetto, Guidetti and their collaborators report unique atomic and mineral structures formed from the original silicate and mineral components of the glass, which are affected by the pH of the surrounding environment and fluctuations in water levels in the soil.
The project began with a chance visit to the Cultural Heritage Technology Center of the Italian Institute of Technology (IIT). "This beautiful piece of shimmering glass on the shelf caught our attention," Omenetto said.
This is a shard of Roman glass found near the ancient city of Aquileia, Italy. Center Director Arianna Traviggia said her team affectionately calls it 'Wow Glass'. They decided to take a closer look.
The researchers soon realized that what they were seeing was nature's nanofabrication of photonic crystals. "It's really remarkable that glass that has been deposited in mud for two thousand years has ended up being a textbook example of a nanophotonic device," Omenetto said.
According to chemical analysis by the IIT team, the glass shard is dated to between the 1st century BC and the 1st century BC and was produced in the sands of Egypt - an indication of global trade at the time. The main body of this glass shard retains its original dark green color, but has a millimeter-thick patina on its surface, giving it an almost perfect mirror-like golden reflection. Omenetto and Guidetti used a new type of scanning electron microscope that not only shows the material's structure but also provides elemental analysis. This instrument can tell us at high resolution what a material is made of and how the elements fit together.
They could see that the patina had a layered structure made up of highly regular, micrometer-thick layers of silicon dioxide that alternated between high and low densities, similar to reflectors known as Bragg stacks. Each Bragg stack strongly reflects a different, relatively narrow wavelength of light. The vertical stacking of dozens of Bragg stacks creates a patina-gold mirrored appearance.
How did this structure gradually form? Researchers have proposed a possible mechanism that has been patiently studied for centuries. "This is most likely a process of corrosion and reconstruction," Guidetti said. "The surrounding clay and rainwater determine the diffusion of minerals and the periodic corrosion of the silica in the glass. At the same time, 100-nanometer-thick layers combining silica and minerals are also assembled cyclically. The result is an incredibly ordered arrangement of hundreds of layers of crystalline material."
"While the age of the glass may be part of its charm, in this case, if we could speed up the process significantly in the laboratory, we might find a way to grow optical materials instead of making them," Omenetto added.
The molecular processes of decay and reconstruction have some similarities to the city of Rome itself. The ancient Romans were keen on building long-lasting structures such as aqueducts, roads, amphitheaters and temples. Many of these buildings became the basis of the city's topography.
Over the centuries, the city has grown in layers, with buildings rising and falling in response to war, social unrest and the passage of time. In the Middle Ages, people used dilapidated and abandoned ancient building materials to build new buildings. In modern times, streets and buildings are often built directly on ancient foundations.
"The crystals growing on the glass surface also reflect changes in surface conditions during urban development and are a record of the history of the urban environment," Guidetti said.
Compiled source: ScitechDaily