  We show that planar semiconductor microcavities in the strong coupling regime
can be used as sources of stimulated terahertz (THz) radiation. Emitted THz
photons would have a frequency of the spliting of the cavity polariton modes.
The optical transition between upper and lower polariton branches is allowed
due to mixing of the upper polariton state with one of excited exciton states.
This transition is stimulated in the polariton laser regime.
