a Conceptual image of power modulation in the plasmonic nanostripes by applying a bias voltage, which induces a refractive index change due to the electro-optic effect in the substrate. The inset shows the cross-section of the two identical parallel waveguides of width w = 350 nm, separated by a gap of g = 350 nm and placed on z-cut lithium niobate (LiNbO3, LN). b False-colored scanning electron microscope (SEM) image of the investigated plasmonic directional coupler switch. The scale bar represents 5 µm. The optimal length L of interaction is chosen to be 15.5 µm.