In nonlinear, multimode optical environments, light is typically too chaotic to be routed in a predictive way. Conventional optical routers require complex arrays of switches and electronic controls to toggle multiple pathways. Both of these approaches are technically challenging and provide limited speed and performance. To remove these constraints and smooth the path to universal optical routing, researchers at the University of Southern California (USC) developed a way for light to self-direct itself along designated paths by applying the principles of thermodynamics. Their approach could increase efficiency in computing and data processing and provide insight into the physics of light-matter interactions in nonlinear systems. Specifically, it could enable innovative functionalities including all-optical beam-steering, multiplexing, and nonlinear beam-shaping in high-power regimes. The researchers recognized that nonlinear, multimode optical systems undergo a process analogous to the process that systems use to reach a state of thermal equilibrium. Based on this knowledge, they developed a framework to capture the behavior of light in an array of nonlinear lattices, using the optical equivalent of thermodynamic processes like expansion, compression, and phase transitions. When light is launched into any input port of the nonlinear array, it universally channels itself into a tightly localized ground state. This phenomenon, arising from the interplay between the lattice structure and the way in which the kinetic and nonlinear components unfold, leads to two optical thermal processes. Light in the array undergoes the optical equivalent of a?Joule-Thomson thermodynamic expansion, followed by the optical equivalent of thermal equilibrium. This two-step process results in a self-organized flow of photons into the appropriate output channel, without the need for external switches. The team demonstrated this phenomenon in nonlinear, time-synthetic mesh lattices where the optical temperature was near zero, causing light to condense at a single spot, regardless of the initial excitation position. The intricate interplay of multiple modes in nonlinear, multimode optical systems makes these systems unpredictable. But at the same time, the chaotic nature of nonlinear systems makes it possible for them to achieve responses that cannot be attained with conventional linear systems. By mimicking Joule-Thomson expansion in synthetic photonic lattices, the team funneled light universally into a single output, regardless of the input. Then, using an optical thermodynamic approach, the team members exploited the complexity of nonlinear optical systems to make them predictable enough to use for optical routing — instead of too chaotic to direct light. An optical thermodynamic device emulates conventional thermodynamics to universally route light in nonlinear systems in a self-directed way. Light naturally finds its way through the device, guided by thermodynamic principles. Courtesy of the University of Southern California, Viterbi School of Engineering. Optical thermodynamics, broadly, could be used to build photonic devices that move information more efficiently than traditional electronics, for example, in semiconductor chips. Beyond chip-scale data routing, the researchers' optical thermodynamic framework could be used to direct and control light for telecommunications, high-performance computing, and secure information processing. “Beyond routing, this framework could also enable entirely new approaches to light management, with implications for information processing, communications, and the exploration of fundamental physics,” researcher?Hediyeh Dinani said. To the best of the team’s knowledge, the nonlinear device for optical thermodynamics is the first of its kind. “What was once viewed as an intractable challenge in optics has been reframed as a natural physical process — one that may redefine how engineers approach the control of light and other electromagnetic signals,” professor Demetrios Christodoulides said. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-025-01756-4).