  Within the framework of the mean-field approximation, a coherently excited
two-dimensional system of weakly repulsive bosons is predicted to show a giant
loop scattering when the rotational symmetry is reduced. The considered process
combines (i) the parametric decay of the driven condensate into different
k-states and (ii) their massive back scattering owing to spontaneous
synchronization of several four-wave mixing channels. The hybridization of the
direct and inverse scattering processes, which are different and thus do not
balance each other, makes the condensate oscillate under constant one-mode
excitation. In particular, the amplitude of a polariton condensate excited by a
resonant electromagnetic wave in a uniform polygonal GaAs-based microcavity is
expected to oscillate in the sub-THz frequency domain.
