  We consider dynamically controllable periodic structures, called Raman
induced gratings, in three- and four-level atomic media, resulting from Raman
interaction in a standing-wave pump. These gratings are due to periodic spatial
modulation of the Raman nonlinearity and fundamentally differ from the ones
based on electromagnetically induced transparency. The transmission and
reflection spectra of such gratings can be simultaneously amplified and
controlled by varying the pump field intensity. It is shown that a transparent
medium with periodic spatial modulation of the Raman gain can be opaque near
the Raman resonance and yet at the same time it can be a non-linear amplifying
mirror. We also show that spectral properties of the Raman induced grating can
be controlled with the help of an additional weak control field.
