  We report observation of coherent oscillations in the relaxation dynamics of
an exciton-polariton condensate driven by parametric scattering processes. As a
result of the interbranch scattering scheme and the nonlinear
polariton-polariton interactions, such parametric scatterings exhibit high
scattering efficiency, which leads to fast depletion of the polariton
condensate and periodic shut-off of the bosonic stimulation processes,
eventually causing relaxation oscillations. Employing polariton-reservoir
interactions, the oscillation dynamics in the time domain can be projected onto
the energy space. In theory, our simulations using the open-dissipative
Gross-Pitaevskii equation are in excellent agreement with experimental
observations. Surprisingly, the oscillation patterns are clearly visible in our
time-integrated images including many excitation pulses, implying the high
stability of the relaxation oscillations driven by polariton parametric
scatterings.
