Oxygen vacancy-driven bandgap tuning and ultrafast laser performance in Bi₂O₂Se

Bi₂O₂Se, with its excellent air stability, high mobility, and tunable bandgap, shows great potential for photonic modulators. This study proposes vacancy engineering as an effective method to refine its optical properties and enhance the nonlinear response. The optical properties of Bi₂O₂Se modified by oxygen vacancy defects were systematically investigated through theoretical simulations and experimental methods. Oxygen vacancies enhance photon-material interactions by introducing intermediate energy levels, altering the electronic structure, reducing the bandgap, inducing a redshift in the linear absorption spectrum, and forming a new absorption band near 1 μm. Argon annealing increased the concentration of oxygen vacancies, and the experimental absorption spectra showed excellent agreement with theoretical predictions. To evaluate the impact of oxygen vacancies on the nonlinear optical response, Bi₂O₂Se before and after annealing was employed as a saturable absorber in Q-switched and mode-locked lasers. The annealed Bi₂O₂Se exhibited a 4.42-fold increase in peak power, a 115.9 fs reduction in pulse width, and a 2.36 nm expansion in 3 dB spectral width. These findings indicate that vacancy engineering is a direct and effective strategy for optimizing the nonlinear optical properties of Bi₂O₂Se, which can contribute to advanced photon applications.