  Vertical cavity surface emitting terahertz lasers can be realized in
conventional semiconductor microcavities with embedded quantum wells in the
strong coupling regime. The cavity is to be pumped optically at half the
frequency of the 2p exciton state. Once a threshold population of 2p excitons
is achieved, a stimulated terahertz transition populates the lower
exciton-polariton branch, and the cavity starts emitting laser light both in
the optical and terahertz ranges. The lasing threshold is sensitive to the
statistics of photons of the pumping light.
