Exploring third-order NLO response of Cu-doped Ag ₂ S/PMMA thin films under CW laser excitation

L-alanine modified Cu-doped Ag2S nanoparticles were synthesised via a simple co-precipitation method, and subsequently incorporated into PMMA thin films on optical glass substrates. Structural analysis by XRD confirmed the monoclinic acanthite phase (Ag2S, space group P121/n1(14) for all samples. EDS verified the elemental purity and successful incorporation of Cu dopant, while HR-TEM revealed uniform morphology with particle sizes in the 20–35 nm range, consistent with XRD data. UV-Vis-NIR spectroscopy demonstrated absorption features in the near-infrared region, with band gap values showing a tunable reduction upon Cu doping, indicative of strong quantum confinement and dopant-induced modifications. Third-order NLO properties of the thin films were evaluated using the Z-scan technique under continuous-wave He–Ne laser excitation. The Cu-doped samples exhibited pronounced nonlinear refraction (positive nonlinearity, self-focusing) and reverse saturable absorption, attributed to nonlocal thermal, surface state, and dielectric effects. These findings highlight the potential of Cu-doped Ag2S/PMMA nanocomposite thin films as promising candidates for photonic and optoelectronic applications, particularly in optical limiting and all-optical switching devices.