Self-Saturable Absorption and Reverse-Saturable Absorption Effects in Diamond-Like Carbon Films with Embedded Copper Nanoparticles

Nonlinear optical properties (self-saturable absorption and reverse-saturable absorption) of diamond-like carbon films with variable amounts of embedded copper nanoparticle (DLC:Cu) films deposited by high power pulse magnetron sputtering were studied in dependence on deposition conditions. X-ray photoelectron spectroscopy was used to study elemental composition and He ion microscopy to study changes of the size, shape, and concentration of the Cu nanoparticles deposited using different pulse currents as well as argon and acetylene gas ratios. Raman scattering spectra of DLC:Cu films studied were typical of diamond-like carbon (including the films with a high Cu atomic concentration where it was up to 60 atom % and carbon atomic concentration was less than 20 atom %). In all cases, in the absorption spectra of DLC:Cu films, the surface plasmon resonance-related peak was recorded. The position of the plasmonic peak for all the deposited nanocomposite films was in a 590-620 nm range. For the samples containing lower amounts of copper, reverse-saturable absorption was found. Transient optical absorption measurements have shown that the reverse-saturable absorption is typical for the samples with a lower relaxation time. The longer relaxation times recorded during transient optical absorption measurements correlated with the presence of the saturable absorption effect. For DLC:Cu samples containing 58-62 atom % Cu, some optical inhomogeneities were observed. The samples in different places demonstrated either saturable absorption or reverse-saturable absorption. The presence of both saturable and reverse-saturable absorption effects in the same sample is explained by the lateral inhomogeneities of the DLC:Cu films.