Laser-driven excitation regimes in microwave-engineered NiO nano-thin films for ultrafast nonlinear applications

The development of multifunctional nanomaterials is central to advancing light-based technologies through enhanced optical performance and versatility. In this study, nickel oxide (NiO) nano-thin films were synthesized via spray pyrolysis and subsequently modified by microwave (MW) irradiation to tailor their third-order nonlinear optical (NLO) properties. The role of MW-induced surface and structural modifications was systematically investigated, and the NLO modifications induced in the MW-irradiated NiO nano-thin films were analyzed under picosecond, nanosecond, and continuous-wave (CW) laser excitations. Z-scan analyses under CW excitation revealed pronounced reverse saturable absorption (RSA) with an enhanced nonlinear absorption coefficient of ~10⁻¹ m/W, attributed to sequential two-photon absorption, excited state absorption, and free-carrier absorption processes in the MW irradiated nano-thin films. Fluence-dependent third harmonic generation (THG) studies using nanosecond excitation demonstrated that 400 W-MW irradiated films achieved higher efficiency at elevated fluence, whereas films irradiated at 640 W and 800 W maintained strong responses even at lower fluence regimes. These trends highlight the influence of MW-induced defect states on carrier dynamics. Further investigations using the Maker fringe technique under picosecond excitation confirmed the angular dependence of THG intensity in MW-irradiated NiO nano-thin films and revealed enhanced excitation–relaxation dynamics within the lattice. The third-order nonlinear susceptibilities reached values of 8.46 ×10⁻²¹ m²/V² (480 W) and 8.06 ×10⁻²¹ m²/V² (800 W), surpassing those of pristine samples. Overall, the findings demonstrate that MW irradiation provides an effective strategy to engineer defect-mediated pathways and significantly enhance nonlinear optical responses in NiO nano-thin films. This approach deepens the understanding of MW-nanomaterial interactions and establishes a pathway toward the rational design of advanced NLO materials.