Cu-engineered TiO₂ thin films: defect-driven nonlinear optical responses for ultrafast frequency conversion and...

Cu-doped TiO₂ thin films with varying Cu concentrations were fabricated by spray pyrolysis to investigate the influence of Cu incorporation on their nonlinear optical (NLO) properties. XRD analysis confirms the formation of anatase TiO₂ with the emergence of Cu-related peaks at higher doping levels. XPS analysis verified the successful incorporation of Cu into the TiO₂ matrix. Z-scan measurements under continuous-wave laser revealed an intensity-dependent transition from saturable absorption to reverse saturable absorption in the 1 and 2 wt% Cu-TiO₂ films, with the 2 wt% film exhibiting the highest nonlinear absorption. Under nanosecond excitation, the 1 wt% Cu-TiO₂ film displayed the strongest second-harmonic generation response, while the pristine TiO₂ film showed the highest third-harmonic generation (THG) intensity. Maker fringe technique analysis under picosecond excitation demonstrated that the 2 wt% Cu-TiO₂ film possesses the highest third-order nonlinear susceptibility (χ ⁽³⁾ = 3.85 × 10^-21 m² V^-2). Furthermore, the 1 and 2 wt% films exhibited superior THG performance under picosecond excitation, highlighting the role of ultrafast electronic nonlinearities. The enhanced NLO response is attributed to the synergistic effects of Cu-induced defect states, lattice distortion, and excitation-dependent nonlinear mechanisms. These findings establish Cu-TiO₂ thin films as promising candidates for applications in optical limiting and ultrafast frequency conversion devices.