Berreman effect in bimetallic nanolayered metamaterials

The visible and ultraviolet responses of a binary regular stack, composed of alternating Al and Ag nano-thin layers, are both experimentally and theoretically studied. It is found that the s- and p-polarization reflectivity spectra (R_s and R_p, respectively) of the inherently anisotropic bimetallic system are noticeably different due to the appearance of a prominent dip near the zero of the Ag permittivity. Such an effect is similar to the Berreman one, which is observed in thin polar-crystals films near the longitudinal-optical phonon frequency, as well as in thin metal films near plasma frequency. Using both the effective medium approach and a nonlocal homogenization formalism, being valid even beyond the long wavelength limit, it is demonstrated that the bimetallic multi-layer stack behaves as a uniaxial anisotropic metamaterial. As it is shown, the appearance of the dip in the p-polarization spectrum is associated to the zero of the longitudinal component of the effective permittivity (ϵ_z). The calculation of 45-degree reflectometry spectrum, namely the difference R_p−R_s ², from experimental and theoretical reflectivity data confirmed that the observed dip in R_p is connected to a plasma resonance of the energy-loss function ℑ(−1∕ϵ_z).