Allowed and forbidden Raman scattering mechanisms for detection of coherent LO phonon and plasmon-coupled modes in GaAs

Detection mechanisms of coherent phonons in variously doped GaAs are investigated by transient reflectivity method with photoexcitation near the E-0 gap and probe near either the E-0 or E-1 gaps. By varying the probe light polarization angle, the coherent amplitudes of both the LO phonon and the LO phonon-plasmon coupled (LOPC) modes show evidence for an interference between their anisotropic and isotropic dielectric response components. We attribute the anisotropic and isotropic components to the reflectivity modulation by lattice and electronic polarizations via dipole-allowed and dipole-forbidden Raman scattering processes. The forbidden processes are resonantly enhanced at the E-0 and E-1 critical points due to the relaxation of momentum conservation and the strong built-in electric field near the surface. The relative contribution of the anisotropic and isotropic components depends on the modes (LO or LOPC) as well as the probe wavelength (E-0 or E-1), because of the different Raman scattering mechanisms involved. Reflectivity measurements with the near-ultraviolet light, which is used to probe the E-1 gap, also enable highly surface sensitive detection of ultrafast LO phonon-plasmon dynamics, which is strongly depth dependent in the depletion layer of n-doped GaAs.