The plasmonic folded directional coupler

We study the properties of a novel type of plasmonic coupler, the folded directional coupler, composed of two MIM waveguides in silver. We calculate the transmission properties of three designs, and show that coupling lengths as small as λ/5 can be achieved at 1.55 μm. We show that the theory of the photonic crystal folded directional coupler can straightforwardly be modified to give a simple and quite accurate model for the plasmonic folded directional coupler, and that metallic absorption reduces the contrast between maximum and minimum transmittance significantly below that in the lossless case.

► Folded Directional Couplers (FDCs) are compact two-port devices.
► They combine the properties of directional couplers and Fabry-Perot cavities.
► We describe the properties of the FDC in a plasmonic geometry.
► We develop a simple and quite accurate, semi-analytic model for the device.
► We show that such this design can lead to very compact couplers.