  A THz light amplifier and the corresponding methods are proposed, based on
the Raman interaction between a THz light and visible light lasers. Two lasers,
with their frequencies differing by that of a Langmuir wave, counter-propagate
inside a background plasma, exciting a rather strong Langmuir wave. The THz
light amplification occurs through the non-resonant Raman scattering between
the THz light and one of the lasers in the presence of the mentioned Langmuir
wave. It is normally the case that the non-resonant scattering does not
exchange the energy between E \&M fields but the author demonstrates that it is
possible under the pre-exciting Langmuir wave. In the presence of a
pre-existing Langmuir wave, the ponderomotive density perturbation between the
THz light and the visible-light laser is induced with the phase suitable for
transferring the energy from the visible light laser to the THz light. The
condition for achieving the favorable phase and the amplification strength,
when the condition is met, is estimated as an mathematical formula. The gain is
as high as or even higher than 100 per centimeter.
