  By monitoring changes in excitonic photoluminescence (PL) that are induced by
terahertz (THz) radiation, we observe resonant THz absorption by
magnetoexcitons in GaAs/AlGaAs quantum wells. Changes in the PL spectrum are
explored as a function of temperature and magnetic field, providing insight
into the mechanisms which allow THz absorption to modulate PL. The strongest
PL-quenching occurs at the heavy hole $1s\to 2p^+$ resonance where heavy hole
excitons are photo-thermally converted into light hole excitons.
