Optical absorption of composites with metallic nanosized spheroidal particles

The paper considers the problem of light absorption by a nanocomposite with randomly oriented metal spheroidal particles-inclusions, provided that the volume content of such inclusions is small. Expressions for the frequency dependences of the effective dielectric function and the absorption coefficient of the metal-dielectric nanocomposite are obtained within the effective medium model taking into account the axial symmetry of spheroidal inclusions. The effective relaxation rate of electrons is introduced using the kinetic approach. Numerical calculations are performed for the cases when inclusion particles have the form of elongated and flattened nanospheroids. The results of the calculations indicate the presence of two maxima of the absorption coefficient, which correspond to longitudinal and transverse surface plasmon resonance. The change in the position and magnitude of the maxima of the frequency dependences of the effective dielectric function and the absorption coefficient with varying the size and shape of the spheroidal particles-inclusions is analyzed. It is shown that the greater the difference in the lengths of the semi-axes of the spheroids, the greater the distance between the maxima of the effective dielectric function and the absorption coefficient, and the shape of the curves depends on the eccentricity of spheroidal inclusions. It has been found that the position of the maxima is significantly influenced by the choice of the material of the inclusion particles and the matrix medium, while the height of the maxima is largely influenced by the shape of the nanoparticles, as well as their volume content in the composite medium. It is proved that, dependent on the material of nanoparticles-inclusions, both maxima of the absorption coefficient can be found in the visible part of the spectrum (for Au inclusions) or in the ultraviolet (for Al inclusions). It is also possible that one maximum lies in the visible part of the spectrum, and the other in the ultraviolet, which is the case for inclusions of Pd, Pt, Cu, Ag.