From unexceptional to doubly exceptional surface waves

An exceptional surface wave can propagate in an isolated direction, when guided by the planar interface of two homogeneous dielectric partnering mediums of which at least one is anisotropic, provided that the constitutive parameters of the partnering mediums satisfy certain constraints. Exceptional surface waves are distinguished from unexceptional surface waves by their localization characteristics: the fields of an exceptional surface wave in the anisotropic partnering medium decay as a combined linear-exponential function of distance from the interface, whereas the decay is purely exponential for an unexceptional surface wave. If both partnering mediums are anisotropic then a doubly exceptional surface wave can exist for an isolated propagation direction. The decay of this wave in both partnering mediums is governed by a combined linear-exponential function of distance from the interface.