Rectification properties of geometrically asymmetric metal-vacuum-metal junctions: a comparison of tungsten and silver tips to determine the effects of polarization resonances

We simulate with a transfer-matrix methodology the rectification properties of geometrically asymmetric metal-vacuum-metal junctions in which one of the metals is flat while the other is extended by a tip. We consider both tungsten and silver as the material for the tip and we study the influence of the dielectric function of these materials on the rectification properties of the junction. We determine in particular the power that these junctions could provide to an external load when subject to a bias whose typical frequency is in the infrared or optical domain. We also study the rectification ratio of these junctions, which characterizes their ability to rectify the external bias by providing currents with a strong dc component. The results show that these quantities exhibit a significant enhancement for frequencies Omega that correspond to a resonant polarization of the tip. With silver and the geometry considered in this paper, this arises for h Omega values of the order of 3.1 eV in the visible range. Our results hence indicate that the frequency at which the device is the most efficient for the rectification of external signals could be controlled by the geometry or the material used for the tip.