Terahertz pulses are applied as an all-optical bias to ferroelectric thin-film $BiFeO_{3}$ while monitoring the time-dependent ferroelectric polarization through its nonlinear optical response. Modulations in the intensity of the second harmonic light generated by the film correspond to on–off ratios of 220× gateable on femtosecond timescales. Polarization modulations comparable to the built-in static polarization are observed.