Plasmonic Coupling-Engineered MXene/Au Nanocomposites for High-Performance Saturable Absorption in Ultrafast Fiber Lasers

The pursuit of high-performance saturable absorbers (SAs) demands synergistic optimization of modulation depth, saturation intensity, and response speed─a challenge persisting in ultrafast photonics. While two-dimensional (2D) MXenes exhibit great potential as SA candidates, their intrinsic limitations, including weak surface plasmon resonance (SPR) and insufficient near-infrared nonlinear optical responses, hinder further practical laser applications. Herein, guided by the plasmonic coupling theory, we proposed a Ti3C2Tx/Au nanoparticle (T/A) nanocomposite synthesized via a facile ultrasonic-assisted strategy. By engineering size-controlled Au nanoparticles onto Ti3C2Tx MXene surfaces, we constructed T/A-tapered fiber (T/A-TF) SA devices to synergistically amplify light-matter interactions. Nonlinear transmission measurements demonstrated that the T/A nanocomposites exhibit significantly enhanced modulation depth alongside markedly reduced saturation intensity at 1.03 and 1.55 μm wavelengths. The improved saturable absorption can be attributed to localized surface plasmon resonance (LSPR) coupling of randomly distributed Au NPs and strong plasmon coupling and electron transfer between Au NPs and Ti3C2Tx, as illustrated by finite element method (FEM) simulations. Based on the exceptional nonlinear saturable absorption properties, the T/A-TF was integrated into ytterbium-doped fiber lasers (YDFL) and erbium-doped fiber lasers (EDFL), generating ultrashort mode-locked pulses of 335 ps (1030.6 nm) and 510 fs (1561.4 nm), respectively. These results demonstrate the critical role of plasmonic coupling in enhancing the optoelectronic performance of MXene-based heterostructures, opening broader avenues for engineering high-efficiency MXene nonlinear photonic devices.