Topological photonics has emerged as a powerful paradigm for achieving robust light transport that is immune to imperfections, disorder, and structural defects. By harnessing principles from condensed matter physics, topological photonic systems support edge modes that guide light along boundaries without backscattering — a feature that has significant implications for resilient optical communication and information processing. However, most demonstrations of topological photonics have been confined to linear and static settings, where the transport pathways are fixed once the device is fabricated. This rigidity presents a major limitation for practical applications that demand adaptive, multitasking purposes. Enabling flexible control over topological light flow, particularly on ultrafast timescales, remains one of the central challenges in advancing this field toward real-world technologies.
In a new paper published in Light: Science & Applications, a team of scientists led by Alexander Cerjan with co-workers from the Center for Integrated Nanotechnologies, Sandia National Laboratories, USA and the University of Wurzburg, Germany, report a dynamically reconfigurable topological photonic platform enabled by nonlinearities. By leveraging driven-dissipative exciton–polariton lattices, they show that local topological properties can be controlled and reshaped in real time using spatially patterned optical pumping. Crucially, the reconfiguration of the system's topology works at the telecommunication regime and occurs on picosecond timescales, making it technological relevant and viable for next-generation optical devices.
Unlike static systems, where the edge mode follows a predetermined path, the team's approach enables the topological edge mode to be steered, redirected, or blocked entirely by simply adjusting an external pump profile — no physical reconstruction needed. This reconfigurability is driven by the nonlinear interactions between the exciton-polaritons and an exciton reservoir, which is created via the non-resonant optical pumping. The reservoir exerts a repulsive interaction on the exciton-polaritons, effectively blueshifting their band structure locally. By selectively illuminating parts of the lattice, researchers can blueshift regions into a topologically trivial phase, thereby reshaping the interface between trivial and non-trivial zones — and with it, the path of the topological edge mode. Moreover, the driven-dissipative nature of exciton–polariton lattices is essential here, allowing the system to respond rapidly to the external non-resonant pump and settle into new topological configurations within picosecond timescales.
To rigorously capture the reconfigurable local topology, the authors introduce a new analytical framework based on the spectral localizer — a tool that extends beyond available topological classification methods. Traditional band theory requires large, translationally invariant systems to define global topological invariants like the Chern number. However, in spatially inhomogeneous systems with localized pumping, disorder, or nonlinearity, such assumptions no longer hold. Similarly, while methods like the local Chern marker can provide spatially resolved indicators of topology in Hermitian systems, they are typically limited to linear regimes with no generalization in non-Hermitian or nonlinear settings. In contrast, the nonlinear, non-Hermitian spectral localizer used here provides a mathematically rigorous and spatially resolved classification of topology even in small, strongly perturbed regions where only a few unit cells are involved. This allows the researchers to track how local topological phases evolve in time and space — precisely the kind of analysis needed for a reconfigurable photonic system where topology is actively controlled via external inputs. Crucially, the localizer operates without requiring a global band gap or crystalline symmetry, making it ideal for capturing the local, pump-induced topological transitions observed in the exciton-polariton lattices. As such, the spectral localizer framework provided direct evidence that the reconfiguration preserves the topological nature of the edge modes.
Altogether, the fast dynamically reconfigurable routing predicted in this study could underpin next-generation optical communication networks, reprogrammable quantum photonic devices, offering the resilience of topology combined with the flexibility of nonlinear control.
