Ultrafast and broadband all-optical switching in ZnO via Ga-defect modulation

By leveraging the defect properties of Ga doping, the excellent nonlinear optical (NLO) properties of zinc oxide (ZnO) can be extended to a broadband spectral range. A significant increase in the non-degenerate two-photon absorption coefficient was achieved in the visible region by systematically tuning the pump and probe wavelengths in ultrafast transient absorption spectroscopy. Simultaneously, a high modulation depth was achieved in the Ga-doped ZnO, even under excitation by an extremely low pump fluence. The specific energy level positions of Ga defects with different charge states in ZnO were determined by combining the non-degenerate resonance enhancement effect with photoluminescence spectroscopy. The improved Kerr nonlinearity in the near-infrared region resulting from Ga-related defects was also confirmed by Z-scan measurements. The real part of the figure of merit, evaluated based on the Kerr effect, was significantly superior to that of other wide-bandgap semiconductor materials. This work provides an effective strategy for designing resonance-enhanced NLO responses in wide-bandgap semiconductors via doping, as well as an important reference for developing all-optical switching across a broadband range.