Universal routing of light via optical thermodynamics

Understanding and exploiting the dynamics of complex nonlinear systems is nowadays at the core of a broad range of scientific and technological endeavours. Within the optical domain, light evolution in a nonlinear multimode environment presents a formidable problem, as its chaotic evolution often hinders predictive insights. Recently, an optical thermodynamic framework has been put forward that, in a systematic manner, can not only predict but also harness the intricate behaviour of these systems. By deploying entropic principles, here we demonstrate a counter-intuitive optical process in which light, launched into any input port of a judiciously designed nonlinear array, universally channels into a tightly localized ground state, a response that is completely unattainable in linear conservative arrangements. This phenomenon arises from the interplay between lattice structure and the way the kinetic and nonlinear Hamiltonian components unfold, leading to two optical thermal processes: Joule–Thomson-like expansion followed by mode thermalization. Experimentally, this effect is demonstrated in properly configured nonlinear time-synthetic mesh lattices, where the optical temperature approaches near zero, causing light to condense at a single spot, regardless of the initial excitation position. The effect demonstrated here opens new avenues for applying the principles of optical thermodynamics in realizing new optical functionalities, such as all-optical beam-steering, multiplexing and nonlinear beam-shaping in high-power regimes, while also offering a greater understanding of the notable physics of light–matter interactions in multimode nonlinear systems. By exploiting an optical thermodynamic framework, researchers demonstrate universal routing of light. Specifically, light launched into any input port of a nonlinear array is universally channelled into a tightly localized ground state. The principles of optical thermodynamics demonstrated may enable new optical functionalities.