A hierarchical supramolecular assembly strategy for remarkably amplifying third-order nonlinear optical responses of clusters

Developing practical optical materials with strong third-order nonlinear optical (NLO) responses attracts increasing attention. Efforts in this area extend to the activation of appropriate materials and the magnification of their low NLO responses. Herein, we propose a stepwise assembly-amplification strategy to enhance the NLO responses of low NLO-active metal sulfide clusters. Through a self-assembly process, W/Cu/S cluster-based tetrahedral and octahedral cages are successfully obtained, both exhibiting weak NLO responses in solution that are not apparent for the unassembled components. Their incorporation into polyvinyl alcohol (PVA), followed by spin-coating, results in the formation of cluster@PVA composite membranes with controlled thicknesses. Remarkably, the effective nonlinear absorption coefficients β for 12-layered membranes are 10⁴ times higher than those of the respective cage solutions. The octahedral cage delivers a stronger response than the tetrahedral cage, which likely reflects the higher degree of aggregation of the clusters. This work showcases a new approach for amplifying NLO responses, greatly facilitating the development of integrated photonic devices.[Figure not available: see fulltext.]