Third-Order Nonlinear Optical Response and Optical Limiting in Silver-Cobalt Bimetallic Nanoparticles

Developing materials with high third-order nonlinear susceptibility and low optical limiting thresholds remains a challenge in photonic device engineering. Bimetallic nanoparticles offer a compelling route to enhanced nonlinear optical (NLO) performance by coupling plasmonic field amplification with interfacial electronic structure. Herein, we report the synthesis of AgCo bimetallic nanoparticles with a surface-confined Co₃O₄ layer and a systematic investigation of their third-order NLO response under both continuous-wave (CW) and nanosecond pulsed excitation. The system exhibits strong reverse saturable absorption, with β_eff reaching 7.5 × 10^-3 cm/W at 638 nm under CW excitation, β = 0.054 m/GW, and a third-order susceptibility χ⁽³⁾ = 4.12 × 10^-11 esu at 532 nm under nanosecond excitation, which is higher than that of many reported molecular and nanostructured systems. Crucially, the intensity-dependent nonlinear absorption coefficient confirms a sequential excited-state absorption mechanism rather than two-photon absorption, driven by hot-carrier injection across the Ag-Co₃O₄ Schottky junction into mixed-valence intermediate states. This distinguishes the system from conventional monometallic optical limiters.