Thickness-dependent polarization contrast in Carbon–nickel hybrid thin films: An open-aperture Z-scan study

Polarization-resolved open-aperture (OA) z-scan measurements at 458 nm were used to examine carbon–nickel (C–Ni) hybrid thin films prepared on nominal Ni layers of 10, 20, and 40 nm. Atomic force microscopy, Raman spectroscopy, and spectroscopic ellipsometry revealed thickness-associated changes in surface morphology, carbon structure, and effective dielectric response. The horizontal and vertical z-scan responses were similar for the 10 and 20 nm conditions, with weak H-favored contrast, whereas the 40 nm condition showed a substantially larger V-favored contrast and a reversal in the sign of trace-derived polarization-contrast metrics. Pure-Ni and pure-carbon reference films showed comparatively limited and nonuniform polarization dependence. Because the physical carbon-layer and total hybrid-film thicknesses were not independently measured, the nonlinear response is discussed using measured transmittance and polarization-contrast metrics rather than absolute nonlinear absorption coefficients. The observed reversal is therefore reported as a preliminary thickness-associated response under the present continuous-wave OA z-scan conditions.