Enhanced nonlinear optical response of Zn–Ni alloy nanostructures on ITO for optical limiting applications

This study reports the fabrication of Zn–Ni alloy nanostructures (ZN) on indium tin oxide (ITO) substrates via electrodeposition and investigates their nonlinear optical (NLO) and optical limiting behavior. Despite the promise of Zn-based nanostructures in NLO devices, their performance remains limited by weak field localization and inadequate surface engineering. This work addresses the knowledge gap by investigating how Ni alloying and surface oxidation in Zn–Ni nanostructures enhance local field effects and defect-mediated transitions, leading to improved nonlinear optical and optical limiting responses. Structural and spectroscopic analyses confirmed the γ-Zn–Ni alloy phase, ZnO/NiO surface oxides, and nanowall network morphologies. The nanostructures exhibited tunable band gaps between 2.95 and 3.17 eV, and high visible photoluminescence from defect states. The presence of surface oxidation suggests that the observed increase in local field effects may result from the high surface oxide density, as confirmed by Raman and photoluminescence data, and defect-mediated transitions, contributing to the observed strong optical nonlinearities. Specific NLO results include a two-photon absorption coefficient of ∼10⁻³ cm/W under continuous-wave excitation and a three-photon absorption coefficient of ∼10⁻²² m³/W² under nanosecond pulsed excitation, which are larger than/comparable to typical ZnO-based systems reported in literature. Optical limiting thresholds were also determined and compared with reported oxide nanostructures, demonstrating competitive or improved performance. These findings highlight the novelty of correlating alloy composition, nanowall morphology, and surface oxide density with enhanced third-order nonlinearities and efficient optical limiting, establishing Zn–Ni nanostructures as promising candidates for photonic and laser-protection devices.