Tunable spatial mode converters and optical diodes for graphene parallel plate waveguides

We introduce compact tunable spatial mode converters between the even and odd modes of graphene parallel plate (GPP) waveguides. The converters are reciprocal and are based on spatial modulation of graphene's conductivity. We show that the wavelength of operation of the mode converters can be tuned in the mid-infrared wavelength range by adjusting the chemical potential of a strip on one of the graphene layers of the GPP waveguides. We also introduce optical diodes for GPP waveguides based on a spatial mode converter and a coupler, which consists of a single layer of graphene placed in the middle between the two plates of two GPP waveguides. We find that for both the spatial mode converter and the optical diode the device functionality is preserved in the presence of loss.