  We study the use of exciton-polaritons in semiconductor microcavities to
generate radiation spanning the infrared to terahertz regions of the spectrum
by exploiting transitions between upper and lower polariton branches. The
process, which is analogous to difference-frequency generation (DFG), relies on
the use of semiconductors with a nonvanishing second-order susceptibility. For
an organic microcavity composed of a nonlinear optical polymer, we predict a
DFG irradiance enhancement of $2.8\cdot10^2$, as compared to a bare nonlinear
polymer film, when triple resonance with the fundamental cavity mode is
satisfied. In the case of an inorganic microcavity composed of (111) GaAs, an
enhancement of $8.8\cdot10^3$ is found, as compared to a bare GaAs slab. Both
structures show high wavelength tunability and relaxed design constraints due
to the high modal overlap of polariton modes.
