Wavelength-dependent tunable optical nonlinearities and beam shaping in GO-ZnFe₂O₄ nanocomposite

In this work, we aim to investigate wavelength-dependent tunable optical nonlinearities and concomitant beam shaping of graphene oxide-ZnFe₂O₄ (GO-ZFO) composite following the femtosecond pulse lasers with the wavelength of 532 nm and 800 nm. To begin with, narrow bandgap semiconductor ZnFe₂O₄ (ZFO) nanoparticles are decorated on the GO nanosheet with the hydrothermal method and then characterized by a variety of material characterization methods. Subsequently, the femtosecond Z-scan results reveal that, compared to the pristine GO, the GO-ZFO composite features nonlinear optical absorption switching from saturated absorption to reverse saturated absorption with 532 nm laser excitation, whereas its nonlinear refraction converts from self-focusing effect to self-defocusing with 800 nm wavelength illumination, thus leading to tunable nonlinear optical responses. More specifically, the strongest nonlinear absorption (refraction) coefficients are given to be 1.1 × 10^-9m/W (2.9 × 10^-16 m²/W) and 7.1 × 10^-10m/W (-1.6 × 10^-16 m²/W), respectively, with 532 nm and 800 nm laser excitations, which correspond to approximately several times of the pure GO. In principle, these exotic optical nonlinearities of the GO-ZFO composite may be due to the broadband absorption and narrow bandgap of doped ZFO and photoinduced electron and energy transfer between the GO and ZFO. Femtosecond pump-probe measurements reveal that the formation of the GO-ZFO heterojunction effectively induces a prolonged (39.7 ps) carrier decay lifetime following photoexcitation. Such dynamics indicate that the interfacial interaction hinders immediate recombination, thus leading to the enhanced optical nonlinearities. Furthermore, we find that beam shaping can be achieved by leveraging the optical nonlinearities of the GO-ZFO composite, in which the input Gaussian beams are reshaped into doughnut ones within the micrometer scale. The findings presented here demonstrate that the GO-ZFO composite can be regarded as an excellent nonlinear optical competitor, highlighting the versatility and potential in broadband nonlinear photonics devices and super-resolved imaging.