The Role of Chromium Doping in Tuning the Bandgap, Surface Morphology, and Third-Order Optical Nonlinearities of Mn2O3 Thin Films for NLO Device Applications

The objective of this work was to investigate the optical and structural characteristics of Mn₂O₃ thin films produced by spray pyrolysis at chromium doping concentrations ranging from 1 wt.% to 15 wt.%. The results showed that chromium doping caused a peak shift and deformation in strain and crystallite size. Curiously, no appreciable changes in bandgap were detected after chromium doping was applied, indicating that chromium doping mainly modifies the electronic structure of Mn₂O₃ thin films without appreciably changing their bandgap energy. A significant difference in Urbach energy in the ultraviolet (UV) region was found between chromium-doped Mn₂O₃ thin films and undoped samples, even though the bandgap remained mostly unaltered. The findings from the atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM) investigations demonstrated how doping with Cr reduced surface roughness and increased consistent grain size. Both the Hall coefficient and the doping concentration caused variations in the electrical characteristics. The analysis of third-order nonlinear optical (TONLO) characteristics, including susceptibility χ⁽³⁾, nonlinear refractive index (NRI) (n₂), and nonlinear absorption coefficient (β), produced values ranging from 4.89   ×  10⁻⁷ to 7.63  ×  10⁻⁷ (esu), 0.86  ×  10⁻¹³ to 1.34  ×  10⁻¹³ (cm²W⁻¹), and 0.769  ×  10⁻⁵ to 1.39  ×  10⁻⁵ (cm W⁻¹) respectively. The optical limiting (OL) threshold values for pure Mn₂O₃ and 1 wt.%, 3 wt.%, 5 wt.%, 10 wt.%, and 15 wt.% Cr-doped Mn₂O₃ were found to be 3.5, 3.1, 2.9, 2.5, 2.1, and 1.5 KJ/cm², respectively. These outcomes suggest that the films include self-defocusing nonlinearity. In summary, the results highlight how Cr-doped Mn₂O₃ nanostructures can be used to change the surface shape and improve NLO properties.