  We suggest an indirect method of detection of photons with zero projection of
spin mediated by emission of terahertz photons. This terahertz source is based
on a system of microcavity exciton polaritons in the regime of polariton BEC
formation when the cavity photons acquire an effective mass being localised in
the cavity and therefore receive the third spin degree of freedom
(corresponding to the longitudinal polarization with chirality l = 0). The
optical transitions can occur between two polariton ground states based on the
light-hole and heavy-hole excitons, respectively, accompanied by the emission
of terahertz radiation with controllable characteristics. We calculate the
dipole matrix element of such transitions and corresponding rate of spontaneous
emission for a realistic cavity based on InAlGaAs alloys, investigate its
dynamics and estimate quantum efficiency of the terahertz source.
