  The canonical boundary-value problem for surface-plasmon-polariton (SPP)
waves guided by the planar interface of a dielectric material and a plasmonic
material was solved for cases wherein either partnering material could be a
uniaxial material with optic axis lying in the interface plane.Numerical
studies revealed that two different SPP waves, with different phase speeds,
propagation lengths, and penetration depths, can propagate in a given direction
in the interface plane; in contrast, the planar interface of isotropic
partnering materials supports only one SPP wave for each propagation direction.
Also, for a unique propagation direction in each quadrant of the interface
plane, it was demonstrated that a new type of SPP wave--called a
surface-plasmon-polariton-Voigt (SPP-V) wave--can exist. The fields of these
SPP-V waves decay as the product of a linear and an exponential function of the
distance from the interface in the anisotropic partnering material; in
contrast, the fields of conventional SPP waves decay only exponentially with
distance from the interface. Explicit analytic solutions of the dispersion
relation for SPP-V waves exist and help establish constraints on the
constitutive-parameter regimes for the partnering materials that support
SPP-V-wave propagation.
