Aperture probes of scanning near-field optical microscopes (SNOM) offer resolution which is limited by a sum
of the aperture diameter at the tip of a tapered waveguide probe and twice the skin depth in metal used for
coating. An increase of resolution requires a decrease of the aperture diameter. However, due to low energy
throughput of such probes aperture diameters usually are larger than 50 nm. A groove structure at fiber
core-metal coating interface for photon-to-plasmon conversion enhances the energy throughput 5-fold for Al
coated probes and 30-fold for Au coated probes due to lower losses in the metal. However, gold coated probes
have lower resolution, first due to light coupling from the core to plasmons at the outside of the metal coating,
and second due to the skin depth being larger than for Al. Here we report on the impact of a metal bilayer
of constant thickness for coating aperture SNOM probes. The purpose of the bilayer of two metals of which
the outer one is aluminum and the inner is a noble metal is to assure low losses, hence larger transmission.
Using body-of-revolution finite-difference time-domain simulations we analyze properties of probes without
corrugations to measure the impact of using a metal bilayer and choose an optimum bi-metal configuration.
Additionally we investigate how this type of metalization works in the case of grooved probes.