Polarization dynamics and optical selection rules for excitonic transitions in strained quantum wells

Dual-beam spectral interferometric techniques are used to time resolve the polarization state of the coherent emission from both intentionally and unintentionally strained ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ multiple quantum wells following irradiation with very weak femtosecond pulses. Any anisotropy in the in-plane strain is shown to result in dramatic oscillations in the intensity, the orientation, and the ellipticity of the coherent emission at the heavy-hole, light-hole beat frequency. This behavior is shown to be the result of replacing the circularly polarized optical selection rules with elliptically polarized selection rules in the uniaxially strained sample. When only the heavy-hole transition is excited with linearly polarized light, these elliptical selection rules lead to a linearly polarized coherent emission that is rotated with respect to the incident orientation. The degree of rotation of the coherent emission as a function of sample orientation is used to quantify the anisotropy in the strain. When the in-plane strain is uniform, no such rotation and no light-hole, heavy-hole beats in the polarization state of the coherent emission are observed.