Defects-induced nonlinear saturable absorption mechanism in europium-doped ZnO nanoparticles synthesized by facile hydrothermal method

In the present work, the effect of europium (Eu) dopant on the third-order nonlinear optical (NLO) responses of ZnO nanoparticles (NPs) is investigated. The NPs are synthesized by facile hydrothermal technique using saponin as a capping agent. The samples are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Energy-dispersive spectroscopy (EDS), UV–visible spectroscopy and Room-temperature photoluminescence spectroscopy (RTPL). The synthesized NPs exhibit wurtzite crystal structure with good crystallinity. Williamson–Hall analysis of XRD pattern revealed the presence of dopant induced compressive strain in the crystal lattice of Eu doped ZnO NPs. The capping of saponin on the surface of NPs is confirmed by FTIR study. A significant decrease in the optical band gap of Eu-doped ZnO NPs attributed to the presence of extended defect states below the conduction band. An enhancement in the blue luminescence is observed for the highest doping level (5 at. wt% of Eu), and this is attributed to defects-related emission in the NPs. The third-order NLO properties of the NPs are explored by a single beam Z-scan technique by employing continuous-wave laser of wavelength 532 nm. The open aperture Z-scan curves showed a remarkable switching from reverse saturable absorption to saturable absorption in 5 at. wt% Eu-doped ZnO NPs is caused due to the ground-state bleaching effect. Further, the closed aperture Z-scan curves exhibit the positive nonlinear refractive index in the 5 at. wt% Eu-doped ZnO NPs attributed to the thermo-optical effects.