Optical properties of nanostructured thin films

Because of their spontaneous nanostructure thin films behave differently from bulk materials of equivalent chemical composition. Depending both on material and deposition technique, optical thin films present structures, which when observed with an electronic microscope, may appear as columnar, polycrystalline, amorphous or lacunar. However, as these structures are in a nanometric scale they do not scatter light whereas they change the mean refractive index of the material. The lacunar structure also leads to water adsorption which induces shifts in the spectral properties of multilayer filters. A review of the work in this field is presented.Thanks to the progress in photolithography techniques, materials can now be artificially nano structured, and the mean refractive index can be controlled in this way. Thin films nano-structured in one dimension are anisotropic. A comparison between measured anisotropy and calculated anisotropy using homogenization models is given. Ion implantation is also shown to be a useful means of locally changing the refractive index and to control the mean refractive index. Calculation of polarizing multilayer filters made with such anisotropic layers is presented.