Frozen fiber couples light and sound 1,000 times more strongly than standard glass fibers

Researchers have developed a new type of optical fiber by freezing a glass capillary filled with liquid. It guides light and sound waves simultaneously and enables highly efficient coupling between them. The high coupling strength lowers the energy consumption of photonic neuromorphic computing schemes and quantum signal processing applications by several orders of magnitude.

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When volcanoes erupt, one can observe streams of liquid lava cool and solidify into rock formations at the bottoms of volcanoes. The same physical process—a liquid changing into a solid phase upon cooling—can be observed when lakes start freezing during cold winters. These phase changes always come with changes in the physical properties of the material, such as the density or refractive index that govern how sound and light move through it.

This fundamental physical process is also used during the melting of glass preforms to loosen their structure while drawing optical fiber. These fibers then guide light through their cores, allowing the transmission of information via light very quickly over long distances, which is why they are widely used for telecommunications applications.

For more specialized applications, such as fiber lasers, fiber endoscopes or fiber sensors, other types of optical fibers have been developed. Hollow-core fibers, for instance, can be filled with different gases or liquids to measure temperature distributions or act as microscopic chemistry labs.

Freezing the fiber core

Researchers at the Max Planck Institute of the Science of Light (MPL) in Erlangen, Leibniz University Hannover (LUH) and the Leibniz Institute for Photonic Technologies (IPHT) in Jena have developed a new type of optical fiber by freezing such liquid-core optical fibers (LiCOF) in nitrogen at -196°C (-321°F), leading to a phase change in the fiber core from liquid to solid. The research is published in the journal Optica.

"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 sections 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 Dr. Birgit Stiller, who leads the project.

The researchers use the extremely efficient coupling between light and sound in their new fiber, an effect known as Brillouin-Mandelstam scattering. The effect is already well known in traditional optical fibers. However, with the phase transition to the frozen LiCOF, the researchers create an extreme, highly confined and dense environment.

Stronger light-sound interactions

Here, the optoacoustic coupling becomes more than 1,000 times stronger than in standard optical fibers. By harnessing this efficient coupling, the researchers demonstrated optoacoustic memory. This fundamental building block for photonic neuromorphic computing in fibers works by using the drastic differences in velocities between light and sound waves. Information is transferred from the fast light wave to the much slower sound waves and later converted back into light. The efficient optoacoustic coupling in the frozen LiCOF opens new avenues for drastically reducing the energy consumption of photonic computing architectures.

The project was possible thanks to established cooperation with Markus Schmidt and Mario Chemnitz from IPHT Jena, who pioneered research with liquid-core optical fibers. With the additional step of freezing the LiCOF's core, higher nonlinearities became possible.

"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."

More information: Simon Seiderer et al, Giant Brillouin gain in frozen CS ₂ capillaries, Optica (2026). DOI: 10.1364/optica.600056

Provided by Max Planck Institute for the Science of Light