Abstract
A numerical simulation of terahertz (THz) radiation by coherent longitudinal optical (LO) phonons in GaAs p–i–n diodes under high electric fields is presented, with emphasis on the important role of a velocity overshoot of free carriers in coherent phonon generation. The dynamics of non-equilibrium photoexcited carriers is simulated by adopting a self-consistent ensemble Monte Carlo method. The explicit comparison of THz signals between calculations and experiments is carried out by taking into account the electronic and ionic accelerations. It is shown that the coherent phonon oscillations emitting the THz radiation are excited resonantly by synchronization between the proper lattice oscillation and ultrafast transients of electronic polarization within the initial period of lattice vibration. The appearance of saturation in THz amplitude due to the coherent phonon above 150 kV/cm as well as a significant phase shift of the oscillatory THz signal is predicted.
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