Sub-picosecond topological phase transition in nonlinear exciton–polariton superlattices

Ultrafast, on-demand manipulation of topological phases is crucial for advancing the study of complex topological dynamics and for developing high-speed, high-performance topological devices. However, simultaneously achieving both high versatility for constructing topological structures and high tunability for the precise modulation of system parameters is challenging. Here we present a dynamic exciton–polariton topological insulator based on a monolayer WS₂–photonic crystal superlattice and demonstrate sub-picosecond phase reconfiguration. Specifically, by precisely aligning excitonic and photonic lattice patterns, we introduce sublattice symmetry breaking in the excitonic layer with respect to the photonic lattice, resulting in distinct topological phases determined by exciton–photon coupling. Using an angle-resolved pump–probe technique, we observe an ultrafast topological phase transition exploiting the exciton-bleaching effect. Such a phase transition leads to a real-time reversal of the group velocity of the topological interface state. By interweaving the light and matter degrees of freedom, this study offers insights into tunable polaritonic devices integrated on chip.