  The ability to control the asymmetric propagation of light in nanophotonic
waveguides is of fundamental importance for optical communications and on-chip
signal processing. However, in most studies so far, the design of such
structures has been based on asymmetric mode conversion where multi-mode
waveguides are involved. Here we propose a hybrid plasmonic structure that
performs optical diode behavior via breaking polarization symmetry in single
mode waveguides. The exploited physical mechanism is based on the combination
of polarization rotation and polarization selection. The whole device is
ultra-compact with a footprint of 2.95*14.18 {\mu}m2, whose dimension is much
smaller than the device previously proposed for the similar function. The
extinction ratio is greater than 11.8 dB for both forward and backward
propagation at {\lambda} = 1550 nm (19.43 dB for forward propagation and 11.8
dB for the backward one). The operation bandwidth of the device is as great as
70 nm (form 1510 to 1580 nm) for extinction > 10 dB. These results may find
important applications in the integrated devices where polarization handling or
unidirectional propagation is required.
