A transparent crystal can create, all by itself, a surprising microscopic network inside its own material. It resembles a fabric whose threads cross by passing over one another. Even better, some parts can be rearranged with a laser. This behavior could one day be used to manipulate or store information with light.
The studied material is called KTN:Li. It notably contains potassium, tantalum, niobium and lithium. Its peculiarity appears when it cools sufficiently. Each of the crystal's small zones then adopts a particular electric orientation. Physicists call these zones "domains".
Laser-induced unraveling of the "domain" structure.
To make the network appear, the team cooled the crystal slowly. The change occurs around 292 K, i.e. about 19 °C. About 2 °C lower, around 17 °C, the domains begin to take an unusual shape. Instead of simply remaining side by side, they form filaments that interlace in the crystal's three dimensions.
The result truly evokes weaving at a minuscule scale.
At each crossing, a filament can pass above or below another. The organization is not identical everywhere and does not follow a perfectly repeated pattern. If the crystal is warmed and then cooled again, another arrangement appears. However, once the network had formed and the temperature was maintained, its structure remained stable during the experiments, for up to six hours.
To verify that the filaments were not just creating an illusion of crossing on a flat image, the scientists observed different depths of the crystal. They thus reconstructed the network in three dimensions. Other measurements made it possible to determine how the different zones were electrically oriented and how they joined together.
This way of intertwining helps the network stay in place. To modify a part, it is not enough for it to change slightly in orientation: some crossings must also be undone. The very shape of the network therefore gives it a certain stability. In physics, this type of resistance linked to geometric organization is called "topological protection".
This stability does not, however, prevent acting on the crystal. The researchers used a green laser with a wavelength of 514 nm. By targeting a small area, they managed to locally undo certain interlacing and reorganize the domains. An infrared laser at 1040 nm did not produce the same effect. No irreversible damage to the crystal was detected during these observations.
Being able to maintain a stable organization, then modify it precisely with light, is of particular interest to research on photonic memories. In these devices, light would take part in storing or processing information. The authors also consider research on systems inspired by neural networks. For now, this is a phenomenon observed in the laboratory: the next step is to learn to create and control these networks with sufficient precision to turn them into a real device.