TRANSIENT VELOCITY OVERSHOOT DYNAMICS IN GAAS FOR ELECTRIC-FIELDS LESS-THAN-OR-EQUAL-TO 200 KV CM/

We have experimentally studied the transient velocity overshoot dynami cs of photoexcited carriers in GaAs for electric fields as great as 20 0 kV/cm. Time domain waveforms proportional to the velocity and the ac celeration of carriers have been acquired, respectively, from guided a nd free-space radiating signals which contain terahertz frequency comp onents. The measurements demonstrated that the degree of overshoot was maximized for an electric field on the GaAs between 40 and 50 kV/cm w hen 1.44-eV photons in an 80-fs laser pulse excited the sample. For ca rriers excited with higher initial energy (1.55 eV), the degree of ove rshoot decreased and the maximum degree of overshoot occurred at a hig her electric field.