The transformation of a liquid into a solid is familiar in nature. Lava flowing from a volcano eventually cools and hardens into rock, while water in a lake can freeze into ice during a cold winter. When materials undergo these phase changes, important physical properties can change as well, including density and refractive index, which affect how sound and light travel through them.
A related process plays an important role in manufacturing optical fibers. Glass preforms are heated until their structure becomes loose enough to be drawn into extremely thin fibers. Light travels through the fiber core, allowing information to move rapidly across long distances. This ability has made optical fibers essential to modern telecommunications.
Scientists have also developed specialized optical fibers for technologies including fiber lasers, fiber endoscopes, and fiber sensors. Some hollow core fibers can be filled with gases or liquids, allowing them to map temperature distributions or even function as tiny laboratories for chemical experiments.
Freezing a Liquid Core at Extreme Temperatures
Researchers from the Max Planck Institute of the Science of Light (MPL) in Erlangen, the Leibniz University Hannover (LUH) and the Leibniz Institute for Photonic Technologies (IPHT) in Jena have now taken liquid core optical fibers (LiCOF) in a new direction.
The team cooled the liquid inside the fibers with nitrogen at -196 °C, causing the material in the core to change from a liquid into a solid. Surprisingly, freezing the core did not prevent the fiber from carrying light.
“The key point is, that the frozen section of the LiCOF retains its ability to guide light. Not only that, but both the liquid and the frozen section of the fiber also guide hypersonic sound waves,” says Simon Seiderer, one of the three lead authors of the article and a researcher in the “Quantum Optoacoustics” research group of Prof. Dr. Birgit Stiller, who leads the project.
Light and Sound Interact More Than 1,000 Times More Strongly
The researchers took advantage of the unusually strong interaction between light and sound inside the frozen fiber. This phenomenon, called Brillouin-Mandelstam scattering, is already known from conventional optical fibers.
Freezing the liquid core, however, creates an exceptionally dense and tightly confined environment. Under these conditions, the interaction between light and sound becomes more than 1000 times stronger than it is in standard optical fibers.
That dramatic increase allowed the team to demonstrate optoacoustic memory, a key component that could be used in photonic neuromorphic computing inside fibers.
The approach relies on the enormous difference between the speeds of light and sound. Information carried by a rapidly moving light wave can be transferred to much slower sound waves, where it is temporarily retained, and then converted back into light.
Because the frozen LiCOF enables such efficient interaction between light and sound, the technology could provide new ways to sharply reduce the amount of energy required by future photonic computing systems.
A New Platform for Photonic and Quantum Technology
The work builds on a long-standing collaboration with Prof. Markus Schmidt and Prof. Mario Chemnitz from the IPHT Jena, who pioneered research involving liquid core optical fibers. Adding the freezing step allowed the researchers to achieve much greater nonlinear effects inside the fiber.
“By freezing the liquid core, we have created an entirely new physical platform that provides extreme nonlinearities while being easy to handle,” says Stiller. “While demonstrating a highly efficient optoacoustic memory is a fantastic first step, this level of light-sound coupling not only opens up exciting new possibilities for neuromorphic computing, but also for quantum information processing, microwave photonics and high-precision sensing.”