  We introduce a scheme of single photon emission based on four-wave mixing in
a three mode system with weak Kerr-type nonlinearity. A highly populated lower
energy mode results in strong stimulated scattering of particle pairs out of
the central mode, which consequently limits the central mode occupation. Thus,
the system can be reduced to a $\chi^{(2)}$ nonlinear medium with greatly
enhanced interaction constant. As a model setup we consider dipolaritons in
semiconductor microcavities. Using the master equation approach we show strong
antibunching under continuous wave pump, which largely exceeds the conventional
blockade mechanism. Finally, using a pulsed excitation we demonstrate
theoretically a triggered single photon emitter in a weakly nonlinear system
with 33% emission probability.
