Terahertz absorption by excitonic polaritons

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.