Tuning the excitation-regime-dependent nonlinear optical responses of Cu-doped NiO thin films for harmonic generation and ultrafast photonic applications

Abstract

Advanced nonlinear optical (NLO) materials with engineered structural and optical characteristics are increasingly vital for meeting modern technological demands due to their multifunctional capabilities. Cu doped NiO thin films were synthesized using spray pyrolysis and investigated for their structural, linear, and nonlinear optical properties. Cu incorporation significantly modified crystallinity, surface morphology, and the formation of defect states, influencing the multifunctional behavior of the films. The third-harmonic generation (THG) efficiency of the films displayed distinct excitation-dependent behavior. The 1 wt% Cu-doped film showed the highest THG efficiency under nanosecond excitation, attributed to enhanced linear and nonlinear interactions facilitated by longer pulse durations, while the 3 wt% Cu-doped film exhibited maximum THG response under picosecond excitation, reflecting the role of electronic structure and ultrafast carrier dynamics in short-pulse regimes. Z-scan analysis revealed that the films exhibit reverse saturable absorption from sequential two-photon processes and a negative nonlinear refractive index arising from thermally induced self-defocusing. The observed enhancement in both THG and Z-scan responses is attributed to the interplay of Cu-induced defect states, modified electronic transitions, and dynamic nonlinear interactions across different temporal excitation regimes. These findings establish Cu-doped NiO thin films as efficient and versatile NLO materials, suitable for applications in optical limiting, laser safety, and integrated photonic devices.