Transition-Metal Incorporation Enables Reverse Saturable Absorption in Gold Nanoparticles

Gold nanoparticles are renowned for intense localized surface plasmon resonances, featuring intense linear light extinction and low threshold saturable absorption, yet the reverse saturable absorption (RSA) in gold nanoparticles is typically limited to small clusters. Here, we show that alloying gold with iron or cobalt enables RSA at sizes readily achieved by standard nanoparticle synthesis methods. The size-dependent (5-34 nm) nonlinear optical response of colloidal bimetallic Au-Fe and Au-Co nanoalloys synthesized by laser ablation in liquid are investigated with open-aperture z-scan measurements at 532 nm and compared with monometallic Au nanoparticles. While pure Au exhibits no RSA, Au-Fe and Au-Co nanoalloys display RSA at larger sizes with remarkable nonlinear absorption cross sections up to 1012 GM. This behavior is attributed to alloy-induced transitions involving virtually bound states near the Fermi energy, providing a practical route to engineer nonlinear optical responses.