Thickness-dependent nonlinear absorption behaviors in polycrystalline ZnSe thin films

Polycrystalline ZnSe thin films have been deposited with different thicknesses by using sputtering evaporation method. Their nonlinear absorption behaviors were investigated by open aperture Z-scan and pump-probe techniques. Linear absorption measurements show redshift in energy as the film thickness increases. This can be attributed to Urbach tail effect. All films exhibit nonlinear absorption for 65 ps pulse duration at lower input intensities while they exhibit saturable absorption at higher input intensities. The life time of localized defect states on grain boundary was found to be ∼ 3 ns from ultrafast pump-probe spectroscopy. Nonlinear absorption coefficients and saturation intensity thresholds were extracted from the fitting of the experimental data for 65 ps pulse duration. They increase with increasing film thickness. This behavior can be attributed to the increasing localized defect states on grain boundaries as the film thickness increases.