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Ultrafast carrier dynamics in semiconductors has attracted much attention due to the application in high speed
devices. Compared to the conventional experimental techniques, such as the time-resolved optical transmission
technique and the all-optical pump-probe spectroscopy, the optical pump-terahertz probe spectroscopy has a plethora of
advantages to provide the ability to temporally resolve phenomena at the fundamental timescales of carrier motion. The
distinct advantage of OPTP is being able to directly measure the photo-induced changes in the photoconductivity, which
contains the information of carrier density and mobility, with a temporal resolution of sub-picosecond. The ultrafast
carrier dynamics and transient terahertz photoconductivity in semi-insulating GaAs have been investigated under electric
field by using optical pump-terahertz probe technique with an unchanged pump power irradiating on the GaAs surface.
One-dimensional pump scan at the maximum value of the THz pulse under electric fields of 0 kV/cm, 6 kV/cm, and 15
kV/cm, respectively. The measurements indicate that the terahertz transmission change induced by the pump pulses at
high electric field is smaller than that without electric field. It is obvious that the threshold value of E, which begins to
enhance the transmission, is about 3 - 4 kV/cm. We attribute this phenomenon to carrier scattering into the L valley or
even X valley, which leads to a drop in carrier mobilities due to the large effective masses in those satellite valleys. The
calculated transient photoconductivities fit well with the Drude-Smith model, which attributes the negative imaginary
conductivity to the backward scattering of electrons. The negative value of c1 in our fitting implies that a fraction, but
not all, of the backward scattering is a result of the electron reflecting from surfaces. It could also result from a
Coulombic scattering between carriers. Due to the low mobilities of electrons in the L valley, the average mobility of all
electrons will decrease under high E. These fitting results are consistent with our intervalley scattering model. Our
investigation suggests that the OPTP technique is a very promising method for detecting the ultrafast dynamics in those
materials.
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