Third-order optical properties of Ag nanocubes and Ag@Au hollow nanostructures investigated by femtosecond Z-Scan measurements

Abstract

We investigate the third-order nonlinear optical properties of silver nanocubes (Ag-NCs) and Ag@Au hollow nanostructures using femtosecond Z-scan measurements at 1040 nm. Both systems exhibit negative nonlinear refraction (n₂∼10⁻¹⁶ cm²/W) attributed to hot-electron generation under ultrafast excitation. In contrast, saturable absorption (SA) is uniquely observed in Ag@Au nanoboxes, with SA intensity tunable via Au content. Ag@Au hollow nanostructures exhibit an order-of-magnitude enhancement in nonlinear refractive index (n₂) and saturable absorption (SA) performance compared to monometallic Ag nanocubes, highlighting morphology and composition as key design parameters. These results demonstrate the promise of Ag@Au hollow nanostructures for applications in ultrafast all‑optical switching and nonlinear photonic devices.