  We demonstrate that when the frequency of the external field differs from the
lasing frequency of an autonomous spaser, the spaser exhibits stochastic
oscillations at low field intensity. The plasmon oscillations lock to the
frequency of the external field only when the field amplitude exceeds a
threshold value. We find a region of values of the external field amplitude and
the frequency detuning (the Arnold tongue) for which the spaser synchronizes
with the external wave.
