The carrier dynamics and terahertz photoconductivity in the n-type silicon (n-Si) as well as in the p-type Silicon (p-
Si) have been investigated by using femtosecond pump-terahertz probe technique. The measurements show that the
relative change of terahertz transmission of p-Si at low pump power is slightly lower than that of n-Si, due to the lower
carrier density induced by the recombination of original holes in the p-type material and the photogenerated electrons. At
high pump power, the bigger change of terahertz transmission of p-Si originates from the greater mobility of the carriers
compared to n-Si. The transient photoconductivities are calculated and fit well with the Drude-Smith model, showing
that the mobility of the photogenerated carriers decreases with the increasing pump power. The obtained results indicate
that femtosecond pump-terahertz probe technique is a promising method to investigate the carrier dynamics of
semiconductors.
The ultrafast photoconductive characteristics of GaAs bulk were investigated by the optical-pump terahertz-probe
spectroscopy (OPTP) at room temperature. In our experiment, a significant decrease of the terahertz transmittance has
been observed when the time delay between the optical pump pulse and the terahertz probe pulse was adjusted. It can be
concluded that the electronic states of the GaAs became metallic, when the optical excitation occurred on the surface of
the GaAs material. Since we expect only the free carrier response in the terahertz range, the decrease of the transmittance
is obviously assigned to the appearance of the high electronic conductivity due to the increasing free carriers.
Furthermore, it was found that transmittance of the terahertz radiation decreased with the increase of the optical pump
power due to the more optical generated carriers under the higher power. We can also find that the carrier recombination
time of the GaAs became shorter while the power of the optical pump was lowered.
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