  We show that the coupled two-fluid gravitating system (e.g. stiff matter and
'vacuum energy') could trap nonlinear gravitational waves (e.g. Einstein-Rosen
waves). The gravitational wave amplitude varies harmonically in time
transferring the energy coherently to the stiff matter wave, and then the
process goes to the backward direction. This process mimics the behaviour of
trapped electromagnetic waves in two-level media. We have defined the limits
for the frequency of this energy transfer oscillations.
