A new approach to guiding and controlling light, based on the process of diffusion, can enhance light transmission by orders of magnitude, even along curved trajectories. The diffusion waveguiding approach could be used in future medical imaging technologies. It could also be adapted to guide heat, instead of light, and to confine particles like neutrons, instead of light waves. Developed by researchers at the University of Glasgow, in collaboration with the University of Arizona, the new waveguiding mechanism shows that photon density can propagate as a guided mode along a core structure embedded in a scattering, opaque material. It guides the flow of energy by transporting light through a solid core of weakly scattering material that is encased within a uniform, strongly scattering medium. The contrast between the scattering properties of the two materials causes the light to remain confined to the core and enables the light waves to be guided with a high degree of precision. A method of waveguiding based on diffusion allows light waves to be guided around curved paths tunneled through opaque materials that would typically scatter the light. Courtesy of the University of Glasgow. Like fiber optic cable, the waveguiding mechanism transports light through a core. In fiber optics, the core is surrounded by a cladding material with a lower refractive index, which allows the light to travel long distances with minimal loss. In the new waveguiding mechanism, a thin cylindrical element inside strongly scattering material guides the energy flow. To demonstrate the new waveguiding approach, the researchers used a 3D printer to build highly scattering, opaque, white resin structures with a low-scattering core. When the researchers directed light through the structures, they found that 100 x more light was transmitted through structures with a low-scattering core than through structures without such a core. They demonstrated this effect in both straight and curved structures. The researchers developed a comprehensive mathematical model to describe the physical processes of diffusion that underpin the new approach to waveguiding. The equations they used supported the existence of guided modes. The presence of guided modes was also demonstrated in Monte Carlo numerical simulations. The team found that the equations that governed the propagation of the photon density also applied to the transport of other forms of energy, such as heat and neutrons. The new technique could, therefore, provide an effective means to guide not only electromagnetic waves, but also particles, giving it a wider use beyond moving light. “We’re still learning new tricks about light, in this case by a process that we were surprised to discover has more in common with our understanding of the way heat travels than light,” professor Daniele Faccio said. “That means that we can expect to use this technique to find new ways to see inside opaque biological tissue using light, but also that we can apply it to guide more than just photons.” The diffusion-based approach could be used to transport heat energy through systems that need to be cooled, like data center computer networks, or to transport particles like the neutrons used in nuclear power plants. Photon density modes in bent waveguides. Courtesy of Nature Physics (2024). DOI: 10.1038/s41567-024-02665-z. The researchers’ inspiration for the diffusion waveguiding method came from “having their heads in the clouds,” so to speak. The team knew that cumulus clouds are often bright white at their highest point and dark grey at their lowest, because sunlight is scattered through the water droplets contained in the cloud. The light in the cloud decays exponentially as it scatters through the cloud, making the lower part appear darker. Light is reflected at the top of the cloud, making the upper portion of the cloud appear white. “We started to wonder whether it might be possible to harness that scattering effect in a controlled way, and to use it to create a path to guide light through scattering materials,” Faccio said. “When we began thinking about clouds, we weren’t necessarily expecting to discover an entirely new method of waveguiding, but that’s where our experiments have led us.” The guiding and transport of electromagnetic waves underlies many technologies, from long-distance optical fiber communications to on-chip optical processors. The results of the research could offer insight into how to control and harness photon density modes to access locations deep within the body for deep tissue imaging. It is also possible to clear thin channels using spatially shaped beams in scattering fluids or in fog with light filaments from high-power lasers, with applications, for example, in free-space telecommunications. “We started with a key question, demonstrated it experimentally, then proved it mathematically with real rigor,” researcher Kevin Mitchell said. “Now that we’ve built a strong practical and theoretical foundation, we’ll continue to explore how we can find new ways to use this in the future.” The research was published in Nature Physics (www.doi.org/10.1038/s41567-024-02665-z).