Enhanced nonlinear optical responses of 2D MoO3-x nanosheets through defect engineering.

The rapid deployment of ultrashort pulsed lasers in biomedicine, spectroscopy, and micro/nanoprocessing has driven increasing demand for high-performance saturable absorbers (SAs). Despite rapid advancements, SAs remain limited by poor stability and low modulation depth. Wide-bandgap transition metal oxides with engineered oxygen vacancies have emerged as promising SA candidates, offering robust stability and tunable defect-mediated nonlinear absorption. In this work, we synthesized 2D MoO3 (MoO3-x) nanosheets with tunable oxygen vacancy concentrations through a controlled defect engineering strategy, which enables precise modulation of their electronic band structure and thus optical properties. Through Z-scan measurements, MoO3-x exhibits excellent saturable absorption both in the visible and in the near IR regions: e.g., with a large nonlinear absorption coefficient βeff and a low saturation intensity Is of -6.45 × 10-9 m W-1 and 8.89 × 1012 W m-2 at 532 nm, respectively. Femtosecond transient absorption (TA) spectroscopy reveals that the defect-state trapping process in MoO3-x (682 ps) is significantly shortened compared to those in pristine MoO3 (877 ps) and nearly defect-free MoO3-air (950 ps). Such rapid carrier trapping behavior effectively enhances the SA performance of MoO3-x. This work provides insights into the interplay among defect engineering, bandgap modulation, and nonlinear optical performance, establishing a fundamental understanding of how oxygen vacancies govern the saturable absorption behavior in 2D transition metal oxides.