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

ABSTRACT

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 Co ₃ O ₄ 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‐Co ₃ O ₄ Schottky junction into mixed‐valence intermediate states. This distinguishes the system from conventional monometallic optical limiters.