Effect of a uniaxial strain on optical orientation in cubic semiconductors

The effect of a uniaxial strain on the optical spin orientation of a cubic semiconductor is investigated by calculating the valence wavefunctions and the optical oscillator strengths for circularly polarized light excitation from heavy- and light-valence levels. Typical strain orientations along the [001], [111], or [1¯10] crystal direction and a light excitation parallel or perpendicular to the strain are considered. For all these cases, the oscillator strengths do not depend on the magnitude of the strain but only on its sign. Although these oscillator strengths strongly depend on the geometrical configuration, the conduction electron spin polarization generated by optical transitions from both the heavy- and light-valence levels induced by σ+ light excitation is in all cases equal to −0.5 for a vanishing strain and for resonant light excitation, as predicted by the simple atomic model. We have generalized these results by developing an analytical model allowing us to determine the initial spin field photogenerated by optical pumping selectively the heavy- or light-valence states with a circularly polarized light propagating in an arbitrary direction. This allows us to discuss under which conditions the commonly used spherical approximation fails.