Defect-driven sequential two-photon absorption for enhanced optical limiting in Co:ZnO

The optical limiting study of intrinsic and cobalt doped ZnO has been carried out in the ns regime with varying dopant concentrations. The flower-like structured, polycrystalline wurtzite ZnO has been revealed by the structural and morphological data. Presence of cobalt ions was confirmed by the Raman peaks corresponding to the Co-O-Zn vibrations and absorption peaks indicating d-d transitions in cobalt (II) ions. The single beam Z- scan technique with nano pulsed Nd-YAG laser is employed to study the nonlinear optical responses and optical limiting studies. While 5 % doped samples showed the lowest optical limiting threshold value of 18.8 GW/cm², 1 % and 3 % doped samples exhibited higher values than intrinsic ZnO. Intensity-varying nonlinear optical studies have also been performed to confirm the defects-assisted sequential two-photon absorption (2PA), which promoted the reverse saturable absorption (RSA) characteristics of the materials.