Effect of polarization and morphology on the optical properties of absorbing nanoporous thin films

This paper investigates the possibility of tuning the optical properties of thin films by introducing nanopores with different shape, size, and spatial distribution. The complex index of refraction of nanoporous thin films with various morphologies is determined for normally incident transverse magnetic (TM) and transverse electric (TE) absorbing electromagnetic waves by numerically solving the two-dimensional Maxwell's equations. The numerical results are compared with predictions from widely used effective medium approximations. For thin films with isotropic morphology exposed to TM waves, good agreement is found with the parallel model. For thin films with anisotropic morphology, the numerical results for TE waves are independent of the morphology and agree well with the Volume Averaging Theory model. By contrast, for incident TM waves, the retrieved effective optical properties depend on both porosity and film morphology. These results can be used to design nanocomposite materials with tunable optical properties and to determine their porosity and pore's spatial arrangement.