We consider interaction of a laser field with surface waves propagatin
g in a thin layer having a width much less than that of the laser fiel
d. It is assumed that the laser field interacts concurrently with a tw
o-level transition. We show that generation of the surface waves in th
is scheme may be described by the completely integrable Thirring model
. We use this model for study of the evolution of a steplike pulse. It
is found that a leading edge is described asymptotically by a sequenc
e of solitons. The modulation instability leading to the formation of
these solitons is treated in a hydrodynamic approximation. For this ai
m we find a solution to the Whitham equations associated with deformat
ion of the one-phase solution of the Thirring model. We show that the
results obtained here can be applied to other physical situations, for
instance, the study of the three-wave mixing in a bulk medium.