  The electric field ionization of gallium acceptors in germanium was studied
by using terahertz time-domain spectroscopy after single-cycle terahertz pulse
excitation. As the peak electric field of the excitation pulse increases, the
distinct absorptions due to acceptor transitions centered at 2.0 and 2.2 THz
decrease, and simultaneously, absorption emerges in the lower frequency region.
These behaviors clearly show that the terahertz pulse ionizes neutral
acceptors. The electric field dependence of the released hole density is well
reproduced by a model assuming direct field-assisted tunneling of acceptors.
