Onion peel dye as a maintainable approach to modify the liner and non-linear optical characteristics of PVA polymer

The search for eco-friendly materials with tunable optical properties has encouraged the use of natural dyes in polymer composites. In this study, onion peel dye (OPD) extracted from peel wastes was incorporated into the polyvinyl alcohol (PVA) to improve its optical performance. Flexible PVA:OPD films were prepared using the simple casting technique, and their structural and optical properties were systematically investigated. FTIR analysis confirmed the presence of characteristic functional groups in OPDs and revealed strong interactions with the PVA matrix. UV–Vis studies showed that dye doping significantly altered the optical response. The optical bandgap decreased from 5.38 to 2.83 eV for direct transitions and from 4.55 to 1.95 eV for indirect transitions, while the Urbach energy increased from 0.33 to 0.66 eV, indicating enhanced localized states and structural disorder. The dispersion parameters derived from the Wemple–DiDomenico model also changed notably: Ed and Eo varied between 1.05–1.18 eV and 2.64–4.22 eV, respectively; the oscillator strength (N/m∗) increased from 0.457×1053 to 8.294×1053; and the plasma frequency (ωp) rose from 1.149×1013 to 4.898×1013 Hz. In addition, the resonance wavelength (λo) shifted from 305.9 nm to 560.6 nm at low energy and from 134.1 nm to 353.9 nm at high energy, confirming strong modification of dielectric behavior. These results demonstrate that natural OPDs effectively tailor both the linear and nonlinear optical properties of PVA. The developed films are eco-friendly, low-cost, and exhibit promising potential for use in optoelectronic devices, UV-shielding coatings, and nonlinear optical applications.