  We demonstrate the role of LO-phonon assisted polariton relaxation in
reducing the stimulation threshold in strongly coupled microcavities. When the
energy of the relaxation bottleneck is one LO-phonon above the ground polariton
state, we observe a ten-fold improvement of the polariton relaxation rate in
the linear regime, and a two-fold reduction of the threshold to the non-linear
polariton lasing regime.
