Femtosecond Charge Transport in Polar Semiconductors

The transient current response of bulk GaAs and InP is investigated at ultrahigh electric fields. On ultrashort time scales, the electronic system is far from equilibrium and overshoot velocities as high as $8\times {10}^7\mathrm{cm}/\mathrm{s}$ are observed. Our studies also lead to a detailed understanding of the ionic response of polar semiconductors. For the first time, carrier motion is determined with a resolution of 20 fs at fields up to $130\mathrm{kV}/\mathrm{cm}$. The dependence of the ultrafast dynamics on material and electric field provides new insights into the microscopic mechanisms governing nonequilibrium transport.