Terahertz electromagnetic pulses as probes for transient velocity overshoot in GaAs and Si

Ultrafast dynamics of the transient velocity overshoot in GaAs and Si have been studied through the use of measurements of radiated and guided terahertz pulses whose time-domain waveforms are proportional to the acceleration and the velocity of carriers, respectively. These measurements have been compared with each other and also with a Monte Carlo simulation. The field-dependent free-space and guided waveforms are qualitatively consistent with Monte Carlo calculations. By correlation of the calculated velocities with a sampling gate function the observed ratio of peak-to-steady-state velocities is found to be in reasonable quantitative agreement. Also, by detecting the free-space radiation from a Si p-i-n structure, we have made the first, to our knowledge, direct experimental observation of transient velocity overshoot in Si.