Optical pumping of excited exciton states in semiconductor quantum wells is a tool for the realization of ultracompact terahertz (THz) lasers based on stimulated optical transition between excited () and ground () exciton states. We show that the probability of two-photon absorption by a exciton is strongly dependent on the polarization 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 polarization dependence of the THz emission and identify photon polarization configurations for achieving maximum THz photon generation quantum efficiency.