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We have used electro-optic methods to investigate single-cycle THz pulses produced as coherent transition radiation from relativistic electron bunches. The pulses have energies up to 100 muJ such that electric fields approaching 100 MV/m should be attainable. We find that the combination of these large fields and their rapid variation (on a sub-ps time scale) results in significant time-dependent phase modulation of a laser pulse when co-propagated in a non-linear medium such as ZnTe.

We demonstrate that time-dependent electric field associated with intense single-cycle THz pulses can induce nonlinear phase modulation in electro-optical crystals, leading to spectral shift, broadening and modulation of co-propagating laser pulses.