Transition-metal azo Schiff base complexes: nonlinear optics across solutions, thin films and nanocomposites

This paper investigates the nonlinear optical (NLO) properties of azo-based Schiff base ligand and its corresponding complexes incorporating Cu(II) and Zn(II) metal cations, designed upon the previous synthesis work. Both 2nd and 3rd order NLO properties are examined, with a particular focus on their potential for optoelectronic and photonic applications. The Z-scan technique is employed to analyze NLO refraction and NLO absorption in solution. All samples exhibit positive NLO phenomena, with Zn(L)2 showing the highest values (𝝌(3) = 27.95 × 10−22 m2 V−2, 𝜸 = 54.44 × 10−47 m5 V−2), attributed to enhanced ligand-to-metal charge transfer (LMCT). Additionally, thin films deposited via the spin coating method undergo 3rd order NLO analysis through the Maker fringe technique. THG analysis further confirms superior 3rd order NLO performance in Zn(L)2, exceeding several benchmark transition-metal complexes. 2nd order NLO properties are also explored in hybrid nanocomposites designed from Zn(L)2 embedded in nanoporous pSiO2 membrane. This structure exhibits anisotropic SHG behavior, with 𝝌 (2) = 0.11 pm V−1 under s-p polarization, suggesting polarization confinement within nanochannels. The results clearly demonstrate that transition-metal azo Schiff base complexes, particularly Zn(L)2, exhibit strong NLO responses, positioning them as potential candidates for applications in all-optical switching and frequency conversion.