Berreman effect in bimetallic nanolayered metamaterials

Abstract 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 ) .