Non-absorbing metamaterial with dispersion of effective refractive index

Background. For many applications it is important to define the so-called effective material parameters that allow describing them in familiar terms of the refractive index or the dielectric permittivity and magnetic permeability. The article investigates the applicability of the method for determining the effective refractive index of a composite film consisting of ordered nano-objects. The authors have shown previously that the nanocomposite layer with spherical or cylindrical pores of the reflected wave can impart additional phase shift dependent on the frequency of the incident field, thus providing broadband illumination, which formally corresponds to the frequency dispersion of the effective refractive index nanostructure that is of interest, because the material components are non-dispersive. Materials and methods. In this paper, the researchers use the NRW-method (Nicholsson-Ross-Weir) to determine the effective refractive index of the nanocomposite films from reflection and transmission coefficients. Results. It is shown that homogenization procedure is not allways appropriate for describing the optical properties of composite films, having substantial inhomogenity even in condition when its optical thickness is smaller compared with the wavelength. However, the authors discovered a configuration for which this method works. For the found configuration the researchers showed the presence of the frequency dispersion of the refractive index in the absence of absorption in the film. It is also shown that the frequency dependence of the effective refractive index does not contradict to the Kramers-Kronig relation. Conclusions. It is shown that the previously proposed nanostructured antireflective coating, consisting of nano-pores in the surface of the medium, disposed in a lattice arrangement, has a dispersion of the effective refractive index that can be used to “tune” the optical properties and to provide broadband antireflection, than is not possible for the homogeneous films. The present effect can be used to construct the composite thin-film optical coatings for various purposes.