  Optical pumping of excited exciton states in semiconductor quantum wells is a
tool for realisation of ultra-compact terahertz (THz) lasers based on
stimulated optical transition between excited (2p) and ground (1s) exciton
state. We show that the probability of two-photon absorption by a 2p-exciton is
strongly dependent on the polarisation of both photons. Variation of the
threshold power for THz lasing by a factor of 5 is predicted by switching from
linear to circular pumping. We calculate the polarisation dependence of the THz
emission and identify photon polarisation configurations for achieving maximum
THz photon generation quantum efficiency.
