Ultrafast shift and rectification photocurrents in GaAs quantum wells: Excitation intensity dependence and the importance of band mixing

A microscopic approach that is based on the multisubband semiconductor Bloch equations formulated in the basis of a 14-band ${\mathbf k} \cdot {\mathbf p}$ model is employed to compute the temporal dynamics of photocurrents in GaAs quantum wells following the excitation with femtosecond laser pulses. This approach provides a transparent description of the interband, intersubband, and intraband excitations, fully includes all resonant as well as off-resonant excitations, and treats the light-matter interaction non-perturbatively. For linearly polarized excitations the photocurrents contain contributions from shift and rectification currents. We numerically compute and analyze these currents generated by the excitation with femtosecond laser pulses for [110]- and [111]-oriented GaAs quantum wells. It is shown that the often employed perturbative $\chi^{(2)}$-approach breaks down for peak fields larger than about 10~kV/cm and that non-perturbative effects lead to a reduction of the peak values of the shift and rectification currents and to temporal oscillations which originate from Rabi flopping. In particular, we find a complex oscillatory photon energy dependence of the magnitudes of the shift and rectification currents. Our simulations demonstrate that this dependence is the result of mixing between the heavy- and light-hole valence bands. This is a surprising finding since the bandmixing has an even larger influence on the strength of the photocurrents than the absorption coefficient. For [110]-oriented GaAs quantum wells the calculated photon energy dependence is compared to experimental results and a good agreement is obtained which validates our theoretical approach.