Self-Similar Plasmonic Nanolenses: Mesoscopic Ensemble Averaging and Chiral Light–Matter Interactions

We investigate how the near-field enhancement of self-similar nanolenses, made of three plasmonic nanospheres with decreasing sizes and separations, is affected by quantum corrections in the optical response of the metal and by fabrication imperfections related to the positioning of the spheres in the nanolens. In particular, we integrate mesoscopic phenomena, such as electron spill-in and -out and surface-enabled Landau damping, via the surface-response formalism, focusing particularly on the role of spill-out in alkali metals. In addition, we take realistic imperfections in the nanofabrication process into account, through numerical averaging of both far- and near-field spectra for large collections of nanolenses. Statistical analysis of hundreds of trimers shows that inevitable deviations from the perfectly aligned chain only slightly, if at all, impair the field enhancement, as long as the average misplacement is kept within 1 nm from the ideal situation. Wishing to explore whether such imperfections can be harvested for practical applications, we probe the potential for triggering chiral response. Our results verify that imperfect nanolenses can display considerable light-induced optical activity and circular dichroism, while providing a means to manipulate the optical chirality density. This highlights how promising the nanolensing effect is for chiral light–matter interactions. Nonetheless, we emphasize that quantification of chiral light–matter interactions can be largely affected by mesoscopic phenomena, which cannot be ignored when near-field quantities like optical chirality density are investigated.