Negative Refraction in Bimetallic Layered Nanostructures*

We theoretically predicted and experimentally confirmed the presence of a negative dispersion pass band in the ultraviolet (UV) optical spectra of a bi-metallic layered nanostructure for p-polarization electromagnetic waves. The bi-metallic nanostructure is in fact a one-dimensional photonic crystal having two metallic layers in its unit cell, namely, aluminum (Al) and silver (Ag). In the lab, the metallic layers were deposited on a quartz substrate by sputtering and the optical spectra were obtained by using a fluororeflectometer for measuring the reflection and transmission of UV and visible light. In regards to the theoretical development, a homogenization technique to calculate not only the components of the nonlocal effective permittivity tensor for the inherently anisotropic crystal, but also its optical spectra was proposed and applied. A negative-refraction pass band was theoretically found and experimentally observed just above the Ag plasma frequency where the parallel and perpendicular components of the permittivity tensor have opposite signs. In this pass band the dispersion relation of the material is a hyperbolic equation in the wave vector space k and, therefore, such a nanostructure behaves as hyperbolic metamaterial with negative refraction.