Broadband nano-focusing of light using kissing nanowires

A strategy has been proposed recently to design plasmonic nanostructures capable of efficient harvesting of light over a broadband spectrum. Applying a singular conformal transformation to a metal-insulator-metal infinite structure, the optical response of two kissing nanowires can be deduced analytically. This nanostructure is shown to exhibit a large and continuous absorption cross-section relative to its physical size over the whole visible spectrum. Considerable field enhancement and confinement at the nano-scale are also expected at the touching point. Actually, instead of transporting the energy out to infinity, like in a metal slab geometry, the surface plasmon modes here propagate towards the singularity of the structure where their velocity vanishes and energy accumulates. The field enhancement is then a balance between this energy accumulation and dissipation losses. The asymptotic case of a nanowire placed on top of a metal plate is shown to be of great interest for nanofocusing. Finally, numerical simulations are performed to investigate the effect of radiative losses when the structure dimension becomes comparable to the wavelength.