  We study theoretically the influence of the surface plasmon excitation on the
Goos-H\"{a}nchen lateral shift of a $p$-polarized Gaussian beam incident
obliquely on a dielectric-metal bilayer in the Otto configuration. We find that
the lateral shift depends sensitively on the thickness of the metal layer and
the width of the incident beam, as well as on the incident angle. Near the
incident angle at which surface plasmons are excited, the lateral shift changes
from large negative values to large positive values as the thickness of the
metal layer increases through a critical value. For wide incident beams, the
maximal forward and backward lateral shifts can be as large as several hundred
times of the wavelength. As the width of the incident Gaussian beam decreases,
the magnitude of the lateral shift decreases rapidly, but the ratio of the
width of the reflected beam to that of the incident beam, which measures the
degree of the deformation of the reflected beam profile, increases. In all
cases considered, we find that the reflected beam is split into two parts. We
also find that the lateral shift of the transmitted beam is always positive and
very weak.
