Ultrafast THz Probe of Phonon-Assisted Processes in Photoexcited bulk GaAs

Hot carrier dynamics in semiconductors governs the performances of optoelectronic devices. The out-of-equilibrium distribution of photoinjected carriers relaxes through scattering with the lattice phonons. The main relaxation path can be depicted with an initial hot LO phonon distribution that relaxes through the emission of acoustic phonon. These hot phonon population may lead to the enhancement of two-phonon absorption processes involving optical-acoustic and acoustic-acoustic transitions detectable by time-resolved FIR spectroscopy. By means of Optical Pump - THz probe experiments, we investigated the out-of-equilibrium properties of intrinsic bulk GaAs. By tuning the pump-photon energy it is possible to excite the carriers directly in the Γ and in the higher energy L and X valleys. Independently from the excitation wavelength, we observed a rich transient THz response where a broad Drude-Lorentz complex conductivity is dressed with several sharp Lorentzian features in the 0.5-2.5 THz spectral range. This response is highly fluence-dependent, suggesting a carrier density dependence of the observed behavior. Our outcomes may be related to the enhancement of two-photon absorption processes involving the optical and acoustical out-of-equilibrium phonon distribution.