  We show theoretically that an intense femtosecond optical pulse incident
normally on a metal surface tends to decay into a pair of counter-propagating
surface plasmon-polaritons (SPPs). The interference field heats the medium
periodically, which causes a periodic permittivity perturbation and resonantly
amplifies the magnitudes of SPPs. The instability growth time is only 10-50 fs
for typical metals at damaging laser fluences. This mechanism is promising for
the interpretation of laser-induced periodic surface structures formation in a
single-pulse pumping regime.
