Microwave-engineered Mn–NiO nanostructures with tunable nonlinear responses for optoelectronics applications under multi-timescale laser probing

Nonlinear optical (NLO) processes such as second- and third-harmonic generation (SHG and THG) underpin modern photonics applications. This study demonstrates that microwave (MW) irradiation significantly influences the nonlinear optical (NLO) behavior of Mn-doped NiO thin films, evidenced by SHG and THG measurements and corresponding changes in nonlinear absorption and refraction obtained from Z-scan analysis. Microwave irradiation modifies the defect landscape, dipolar configurations, and charge-carrier dynamics of Mn-doped NiO thin films, leading to a pronounced enhancement in third-harmonic generation (THG) efficiency under different laser excitation regimes. Fluence-dependent THG analysis confirms that MW irradiation enhances photoexcitation and relaxation processes, thereby strengthening the third-order nonlinear optical response. Angle-dependent THG measurements reveal an increase in third-order nonlinear susceptibility from 11.81 × 10⁻²¹ m²/V² to 13.04 × 10⁻²¹ m²/V² after MW irradiation, with optimal enhancement observed at 2 min exposure due to improved charge transfer and defect concentration. Z-scan measurements under continuous-wave excitation indicate dominant thermally induced third-order nonlinearities, where all samples exhibit reverse saturable absorption in open-aperture configuration and self-defocusing behavior with a negative nonlinear refractive index in closed-aperture measurements. Notably, the thin film irradiated with MW for 10 min demonstrates the highest nonlinear refractive index and superior optical limiting performance. Overall, MW irradiation emerges as an effective route for tailoring and enhancing the third-order nonlinear susceptibility of Mn-doped NiO thin films. These enhancements, governed by MW-induced modifications in dipole moments and relaxation pathways, highlight the potential of MW-irradiated Mn-doped NiO thin films for next-generation nanophotonic and ultrafast laser applications.