  We recently formulated the canonical boundary-value problem of propagation of
surface plasmon-polariton (SPP) waves along the direction of periodicity of a
one-dimensional photonic crystal. Here we present the general formulation of
that canonical problem supporting the oblique propagation of SPP waves in the
interface plane. The general dispersion equation has been obtained using the
rigorous coupled-wave approach for the oblique propagation and numerically
solved using the Muller's method. A periodicity in the wavenumbers of the SPP
waves was observed. Furthermore, the regions of high losses for the SPP waves,
dubbed as plasmonic bandgaps, were observed in the photonic band diagram of the
SPP waves. These plasmonic bandgaps can be used to construct optical filters
for the SPP waves.
