Enhanced nonlinear optical responses of 2D MoO_3-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 (MoO_3-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, MoO_3-x exhibits excellent saturable absorption both in the visible and in the near IR regions: e.g., with a large nonlinear absorption coefficient β_eff and a low saturation intensity I_s 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 MoO_3-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 MoO_3-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.