Terahertz emission in single quantum wells after coherent optical excitation of light hole and heavy hole excitons

We report on the first observation of coherent terahertz radiation, tunable from 1.4 to 2.6 THz, emerging from GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As single quantum wells after the coherent optical excitation of both light hole and heavy hole excitons. We attribute the radiation to charge oscillations following the coherent excitation of the excitons in an electric field. Terahertz radiation is also emitted when no electric field is present in the sample suggesting that valence band mixing leads to a significant far-infrared transition dipole moment between the light hole and heavy hole subbands.