Terahertz-induced interband tunneling of electrons in GaAs

Ultrafast high-field transport is studied in n-type GaAs with ultrashort terahertz (THz) pulses of an electric field amplitude of up to 300 kV/cm. At lattice temperatures between $T=300$ and 80 K, we observe coherent ballistic transport of electrons over a major part of the first Brillouin zone. At T=300 K, ballistic transport occurs at a constant electron density whereas at lower temperatures, the THz pulses generate additional electron-hole pairs by field-induced tunneling between valence and conduction bands. We show that the ultrashort decoherence time of superpositions of valence- and conduction-band states plays a crucial role for the efficiency of the tunneling process. The extremely fast interband decoherence at room temperature results in a negligible tunneling rate.