Manipulating four-photon absorption of ZnO via Ga doping

Multi-photon absorption in the second near-infrared (NIR-II) regime has attracted extensive attention due to biological imaging and frequency-upconverted lasing applications. We report the dispersion of four-photon absorption (4 PA) response in pristine and Ga-doped ZnO single crystals over the spectral range 1180-1350 nm. Femtosecond Z-scan results demonstrate that Ga doping can significantly enhance the 4 PA coefficient β₄ of ZnO. Interestingly, the wavelength dependency of β₄ in Ga-doped ZnO shows a strong resonance around 1215-1250 nm, which is correlated with the PL peak of Ga-doped ZnO at 405 nm. Femtosecond pump-probe measurements validate that Ga doping has no profound impact on the ultrafast carrier relaxation of ZnO, indicating Ga doping leads to a shallow state rather than a deep trap within the bandgap. The possible mechanism of 4 PA enhancement induced by degeneracy with multi-photon absorption resonance to the Ga-doped state is discussed. Our results verify the strong potential of Ga-doped ZnO with tunable nonlinear optical properties as a promising candidate for nonlinear optical and nanophotonic devices in the NIR-II region.