  We use linear response theory in order to compute the light absorption
spectrum, in the terahertz band, of a polariton system composed by excitons in
a quantum dot very strongly coupled to the lowest photon mode of a thin
micropillar. In a thermalized (Bose condensed) system at low temperatures, the
spectral function shows a peak (GDR) associated to a 1s - 2p exciton
transition, enhanced by polariton effects. On the other hand, in a
non-equilibrium system absorption is peaked at low energies. Thus, a
measurement of terahertz absorption could give an indication of the degree of
thermalization in the polariton system.
