  The spin-splitting energy bands induced by the relativistic spin-orbit
interaction in solids provide a new opportunity to manipulate the
spin-polarized electrons on the sub-picosecond time scale. Here, we report one
such example in a bulk Rashba-type polar semiconductor BiTeBr. Strong terahertz
electromagnetic waves are emitted after the resonant excitation of the
interband transition between the Rashba-type spin-splitting energy bands with a
femtosecond laser pulse circularly polarized. The phase of the emitted
terahertz waves is reversed by switching the circular polarization. This
suggests that the observed terahertz radiation originates from the
subpicosecond spin-polarized photocurrents, which are generated by the
asymmetric depopulation of the Dirac state. Our result provides a new way for
the current-induced terahertz radiation and its phase control by the circular
polarization of incident light without external electric fields.
