Near-field directive beams from passive and active asymmetric optical nanoantennas

Core-shell, plasmonic-based nanoantennas have been shown to be resonant, resulting in extremely large enhancements of their scattered power. Nevertheless, they primarily radiate as electric dipoles due to their subwavelength sizes. We report how, through the introduction of asymmetry into these nanoantennas, it is possible to achieve highly directive optical beams in their near fields. In particular, we perforate the outer metallic coating of a core-shell nanoparticle and add gain material into its core region. It is demonstrated that this active holey-coated nanoparticle configuration effectively captures the incident field, amplifies it, and then channels this power into an intense near-field beam. The performance characteristics of these nanobeamers are compared to the nanojet behaviors experimentally realized with multimoded, electrically large dielectric particles. The intensities of the near-field power flow generated by these highly subwavelength nanobeamers are shown to be significantly larger than those produced by their multimode, micron-sized nanojet counterparts. Because of these enhanced field localization properties, a sensor array example is considered to demonstrate the efficacy of augmenting its detector elements with these nanobeamers.