  A laser pulse composed of a fundamental and properly phased second harmonic
exhibits an asymmetric electric field that can drive a time-dependent current
of photoionized electrons. The current produces an ultrashort burst of
terahertz (THz) radiation. When driven by a conventional laser pulse, the THz
radiation is emitted into a cone with an angle determined by the dispersion of
the medium. Here we demonstrate that the programmable-velocity intensity peak
of a spatiotemporally structured, two-color laser pulse can be used to control
the emission angle, focal spot, and spectrum of the THz radiation. Of
particular interest for applications, a structured pulse with a subluminal
intensity peak can drive highly focusable, on-axis THz radiation.
