Researchers discover how molecules in ancient glass rearrange and recombine with minerals over centuries to form a patina of photonic crystals -- ordered arrangements of atoms that filter and reflect light in very specific ways -- an analog of materials used in communications, lasers and solar cells.
Some 2,000 years ago in ancient Rome, glass vessels carrying wine or water, or perhaps an exotic perfumes, tumble from a table in a marketplace, and shatter to pieces on the street. As centuries passed, the fragments were covered by layers of dust and soil and exposed to a continuous cycle of changes in temperature, moisture, and surrounding minerals.
For Fiorenzo Omenetto and Giulia Guidetti, professors of engineering at the Tufts University Silklab and experts in materials science, what's fascinating is how the molecules in the glass 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.
They could see that the patina possessed a hierarchical structure made up of highly regular, micrometer-thick silica layers of alternating high and low density which resembled reflectors known as Bragg stacks. Each Bragg stack strongly reflected different, relatively narrow wavelengths of light. The vertical stacking of tens of Bragg stacks resulted in the golden mirror appearance of the patina.
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