  The dynamics of propagating polariton condensates in one-dimensional
microcavities is investigated through time resolved experiments. We find a
strong increase in the condensate intensity when it travels through the
non-resonantly excited area. This amplification is shown to come from bosonic
stimulated relaxation of reservoir excitons into the polariton condensate,
allowing for the repopulation of the condensate through non-resonant pumping.
Thus, we experimentally demonstrate a polariton amplifier with a large band
width, opening the way towards the transport of polaritons with high densities
over macroscopic distances.
