Optical impedance of metallic nano-structures

Impedance matching refers to the suppression of reflected radiation from an interface and is a concept that applies right across the electromagnetic spectrum. In particular it has come to prominence in relation to the propagation of light in metallic structures and associated meta-materials. Whilst established for microwaves and electrical circuits, this concept has only very recently been observed in the optical domain, yet is not well defined or understood. We present a framework to elucidate the concept of optical impedance. We describe using a scattering matrix approach the characteristic, iterative, image and wave impedances of an optical system. With a numerical model, we explore each form of impedance matching in metal-dielectric structures. Thin gold layers may extend the concept of Brewster's angle to normal incidence and s polarization. Optical impedance for recently realized metallic gold nano-pillars which has shown negative permeability is also explored and we show that current measurements are inconclusive to robustly state its characteristic impedance is matched to the vacuum.