Battling Retardation And Nonlocality: The hunt For The Ultimate Plasmonic Cascade Nanolens

We perform a study of the achievable field enhancement of plasmonic cascade nanolenses in the quantum and classical regimes, and explore the limits enforced at larger sizes by retardation and at smaller sizes by nonlocality and other quantum effects. We compare the near field response of a sodium nanolens within a local and nonlocal formalism and find that the increased electron- surface scattering decreases the field enhancements by over an order of magnitude. The large parameter space is explored, within the local approximation, for a number of plasmonic metals as well as the polar dielectric SiC. Using localised surface phonon polaritons, which can be excited in polar dielectrics, is an effective strategy for overcoming retardation due to the lower energy phonon frequency. Finally, we compare the nanolens against the more usual dimer configuration, we find that the superior geometry depends crucially on the material used, with noble metal nanolenses unlikely to offer better performance to equivalent dimers. Interestingly SiC nanolenses can offer a larger maximum field enhancement compared to the corresponding dimer configuration, suggesting that future endeavours in constructing a nanolens should be based on polar dielectrics.