Stacked Gold Nanorectangles with Higher Order Plasmonic Modes and Top-Down Plasmonic Coupling

We present stacked hollow nanostructures created using electron beam lithography (EBL) that act as optical scattering sites with a complex combination of local surface plasmon resonances and top-down electromagnetic hotspots due to the incorporation of the third dimension into their construction. These hollow rectangular gold nanotructures with gold caps show a significant red-shift in their main scattering peak as compared to the solid structures. Finite-difference time-domain modeling shows that the plasmonic response of these structures is dominated by higher order plasmonic modes and that the strength of these modes is shown to vary according to whether a cap is present. The higher order dipolar mode caused by the capped nanostructure results in manifold increase in the intensity of the electric field compared to the quadrupolar mode from a solid rectangle. This analysis provides important information on how complex plasmonic resonances respond to structural changes which will be useful in future studies that utilize these coupled resonances for detection or light manipulation. In addition, the stacking scheme presents a new route for modifying the optical response of plasmonic nanostructures through top-down plasmonic coupling which may yield plasmon resonance modes not observed in common 2D nanostructures along with significant increases in the local electric fields of these open "hotspots".