Electrodynamic effects in plasmonic nanolenses

Full electrodynamic analysis of self-similar cascades of plasmonic nanospheres (nanolenses) that exhibit nanofocusing is performed. Electrodynamic resonances are identified and a significant local field enhancement (by a factor of hundreds) is found for different sizes and fractal ratios of the cascade system, and for different polarizations and directions of incidence of the excitation radiation. Supporting the earlier findings, the nanofocus ("hottest spot") is found to be located in the gap between the two smallest nanospheres of the cascade. Somewhat surprisingly, field enhancement is maximized when the incoming wave is polarized at an angle to the symmetry axis. The influence of the dielectric substrate on field enhancement is also evaluated. The results obtained show ways to optimize the nanolenses for their applications in spectroscopy and sensing.