Ballistic photocurrents in semiconductor quantum wells caused by the excitation of asymmetric excitons

We theoretically investigate the generation and dynamics of photocurrents induced by linearly polarized single-frequency light pulses in GaAs quantum wells. Our approach is based on the multiband semiconductor Bloch equations (SBE) formulated in the basis of eigenfunctions on the 14-band k·p model and includes excitonic effects and carrier longitudinal-optical-phonon scattering processes. By solving the SBE, we obtain both shift and ballistic currents. For a particular excitation geometry, we obtain a ballistic current which is absent if the electron-hole attraction is neglected. Whereas in other cases excitonic effects quantitatively modify photocurrents that originate from single-particle properties, here we demonstrate the existence of a ballistic current which is absent in single-particle calculations. This photocurrent is of second order in the light-matter interaction and is purely caused by the asymmetric electron-hole Coulomb attraction that results from the inversion asymmetry of GaAs. Furthermore, we show that the coherent dynamics of excitonic wave packets gives rise to oscillations in the photocurrent transients.