Ordinary window glass may seem frozen forever, but inside, atoms never stay still. In amorphous germanium telluride (GeTe)—a material used in rewritable discs and non-volatile memory — physicists have for the first time directly tracked how atomic bonds stretch and bend in trillionths of a second.
Using ultrashort electron pulses (lasting femtoseconds, or 10⁻¹⁵ seconds), the scientists filmed a "movie" of atomic motion. They found that after a laser pulse, Ge-Te and Ge-Ge bonds stretch in just 0.2 picoseconds — a hundred times faster than the blink of an eye. Then, over 0.5–2 picoseconds, the angles between atoms shift, as if the molecule were taking a breath in and out. These movements are accompanied by local vibrations at a frequency of 3.10 terahertz — a trillion oscillations per second. That frequency is comparable to the vibration of a tiny spring if it were squeezed to the size of an atom.
These results not only settle a long-standing question in glass physics but also have practical implications. By understanding exactly how atoms shift in the amorphous state, we can more precisely control the rates of crystallization and relaxation in materials for next-generation memory. Perhaps soon we will learn to "tune" glass to respond as desired, much like tuning a musical instrument.
3.10 terahertz is roughly a hundred billion times higher than the frequency of sound audible to humans. If such a vibration could be heard, it would sound like an unimaginably high-pitched ultrasound.