Novel Highly Efficient Buried Gratings for Selective Coupling of SPP Waves onto Single Interfaces.

Diffraction gratings have always been used to effectively couple optical radiation within integrated waveguides. This is also valid for plasmonic structures that support Surface Plasmon Polariton (SPP) waves. Traditional gratings usually excite SPP waves at the interface where they are located or, for thin metal nanostrips, at both interfaces. But reducing the thickness of the metal layer in the presence of a grating has the handicap of increasing the tunnelling of light towards the substrate, which means higher losses and reduced coupling efficiency. In this paper, we design and optimize novel gratings buried within the metallic thin films for selective coupling of SPP waves onto individual interfaces. Compared with traditional superficial gratings, the novel buried ones demonstrate higher efficiency and much lower residual tunnelling of light through the coupling structures.