Anisotropic excitons and their contributions to shift current transients in bulk GaAs

Shift-current transients are obtained for near-band-gap excitation of bulk GaAs by numerical solutions of the semiconductor Bloch equations in a basis obtained from a 14-band k$\ifmmode\cdot\else\textperiodcentered\fi{}$p model of the band structure. This approach provides a transparent description of the optically induced excitations in terms of interband, intersubband, and intraband excitations which enables a clear distinction between different contributions to the shift-current transients and fully includes resonant as well as off-resonant processes. Using a geodesic grid in reciprocal space in our numerical solutions, we are able to include the electron-hole Coulomb attraction in combination with our anisotropic three-dimensional band structure. We obtain an excitonic absorption peak and an enhancement of the continuum absorption and demonstrate that the excitonic wave function contains a significant amount of anisotropy. Optical excitation at the excitonic resonance generates shift-current transients of significant strength; however, due to the electron-hole attraction the shift distance is smaller than for above-band-gap excitation. We thus demonstrate that our approach is able to provide important information on the ultrafast electron dynamics on the atomic scale.