Frozen Optical Fiber Makes Light and Sound Interact 1,000 Times More Strongly

Researchers from the Max Planck Institute of the Science of Light (MPL) in Erlangen, Germany, along with Leibniz University Hannover (LUH) and the Leibniz Institute for Photonic Technologies (IPHT) in Jena, have frozen the liquid core inside a specialized optical fiber, making light and sound interact more than 1,000 times more strongly than in standard optical fibers.

Optical fibers normally carry light through a solid glass core. The team instead used liquid core optical fibers (LiCOF), which have a liquid rather than solid core, and cooled the liquid inside with nitrogen to -196 degrees Celsius, turning it from a liquid into a solid. Surprisingly, freezing the core did not stop the fiber from guiding light; both the frozen and still-liquid sections also carry hypersonic sound waves.

The frozen core creates an exceptionally dense, tightly confined environment that dramatically strengthens the interaction between light and sound, a phenomenon called Brillouin-Mandelstam scattering. "The key point is that the frozen section of the LiCOF retains its ability to guide light," said Simon Seiderer, one of three lead authors of the study and a researcher in the Quantum Optoacoustics group led by Prof. Birgit Stiller, who heads the project.

The strong light-sound interaction allowed the team to demonstrate optoacoustic memory, in which information carried by a fast-moving light wave is transferred to much slower sound waves, temporarily stored, and then converted back into light. "By freezing the liquid core, we have created an entirely new physical platform that provides extreme nonlinearities while being easy to handle," said Stiller, who added that this level of light-sound coupling could open possibilities for neuromorphic computing, quantum information processing, microwave photonics and high-precision sensing.

The work builds on a long-running collaboration with Prof. Markus Schmidt of MPL and Prof. Mario Chemnitz of IPHT Jena, who pioneered research on liquid core optical fibers; the team's addition of the freezing step allowed them to achieve much greater nonlinear effects inside the fiber.