When the probe polarization angle θ is rotated, the coherent amplitudes of LO and LOPC modes of GaAs shows an interference between isotropic (A) and anisotropic (B) polarization components: A+B cos2θ.  The isotropic and anisotropic amplitudes are attributed to the dipole-forbidden and dipole-allowed Raman scattering processes.  

The Raman processes depend on the mode (LO or LOPC) as well as on the probe wavelength.  When pumped and probed at the E0 gap with 1.55 eV light, LOPC mode (right panel) exhibits large isotropic amplitude arising from forbidden charge-density-fluctuation.  The detection of unscreened LO phonons (left panel), by contrast, is dominated by the allowed deformation-potential interaction.

Coherent response from GaAs consist of unscreened LO phonon and the upper (L+) and lower (L-) branches of LO phonon-plasmon coupled (LOPC) mode.

When probed at the E1 gap with 3.1 eV light, we observe LO phonon coupled with photoexcited and spatially separated hole and electron plasmas in the depletion layer of n-type GaAs.  The isotropic amplitude of the LOPC (LO-hole and LO-electron) modes become almost as small as the anisotropic amplitude, because of the absence of the charge-density-fluctuation due to small charge density at the E1 gap.

For more detail see http://link.aps.org/doi/10.1103/PhysRevB.84.235202.