  Polarization-dependent two-dimensional Fourier-transform spectroscopy (2DFTS)
is performed on excitons in strained bulk GaAs layers probing the coherent
response for differing amounts of strain. Biaxial tensile strain lifts the
degeneracy of heavy-hole (HH) and light-hole (LH) valence states, leading to an
observed splitting of the associated excitons at low temperature. Increasing
the strain increases the magnitude of the HH/LH exciton peak splitting, induces
an asymmetry in the off-diagonal coherences, increases the difference in the HH
and LH exciton homogenous linewidths, and increases the inhomogeneous
broadening of both exciton species. All results arise from strain-induced
variations in the local electronic environment, which is not uniform along the
growth direction of the thin layers. For cross-linear polarized excitation,
wherein excitonic signals give way to biexcitonic signals, the high-strain
sample shows evidence of bound LH, HH, and mixed biexcitons.
