Broadband plasmonic device concentrating the energy at the nanoscale: The crescent-shaped cylinder

A new strategy has been proposed recently to design broadband plasmonic nanostructures capable of a significant nanofocusing of light. Applying a singular conformal transformation to a thin slab of metal, a cylinder with a crescent-shaped cross section is obtained. In this study, the corresponding theory is derived analytically and different physical insights are provided to analyze the broadband light harvesting and nanofocusing properties of this device. The optical response of the crescent is deduced by solving the metal-slab problem. The nanostructure is shown to exhibit a continuous absorption cross section which redshifts for thin crescents due to a decrease of the surface plasmon velocity. The field enhancement induced by the nanostructure is also derived analytically. The nanofocusing performance is shown to result from a balance between dissipation losses and surface-plasmons velocity. This implies a strong dependence of the field enhancement on the frequency and the crescent geometry. Numerical simulations have also been performed to investigate the effect of radiative losses when the structure dimension becomes comparable to the wavelength. Radiative damping makes the absorption cross section saturate at the level of the physical cross section. The field enhancement decreases with the size of the device. The crescent structure is shown to be quite robust to radiation losses, which opens perspectives for applications such as single-molecule detection.