Very high and very low Fisher information for small displacements of a plasmonic silver nanowire

In this paper, we study the effects of plasmonic resonances on the Fisher information in the far field, of the position of a silver nanowire with respect to the center of a focused spot. We study theoretically a nanowire embedded in a high refractive index substrate that is illuminated by a dark-field focused spot. The position of the nanowire is determined from the scattered far-field intensities. The Fisher information is computed for both a lateral and longitudinal displacement of the nanowire, and its dependence on the illumination frequency is analyzed. The resonance frequencies of the nanowires are determined. We find that frequencies near a plasmonic resonance can enhance the Fisher information. However, at the resonance frequency itself and very close to it, the scattered far field corresponds to an . The phenomenon is demonstrated for silver, and the underlying physical mechanism is explained with an analytical model. As is shown, the dark state can be converted into a state with very high Fisher information about the positions of the nanowire by modifying the focused spot.