  We report on the new optical gating technique used for the direct
photoconductive detection of short pulses of terahertz radiation with the
resolution up to 250 femtoseconds. The femtosecond optical laser pulse time
delayed with respect to the THz pulse generated a large concentration of the
electron hole pairs in the AlGaAs/InGaAs High Electron Mobility Transistor
(HEMT) drastically increasing the conductivity on the femtosecond scale and
effectively shorting the source and drain. This optical gating quenched the
response of the plasma waves launched by the THz pulse and allowed us to
reproduce the waveform of the THz pulse by varying the time delay between the
THz and quenching optical pulses. The results are in excellent agreement with
the electro-optic effect measurements and with our hydrodynamic model that
predicts the ultra-fast transistor plasmonic response at the time scale much
shorter than the electron transit time, in full agreement with the measured
data.
