Lawrence Berkeley National Laboratory

The electric field profiles of broad-bandwidth coherent terahertz (THz) pulses, emitted by laserwakefield-accelerated electron bunches, are studied. The near-single-cycle THz pulses are measured with two single-shot techniques in the temporal and spatial domains. Spectra of 0 - 6 THz and peak fields up to ~; 0.4 MVcm-1 are observed. The measured field substructure demonstrates the manifestation of spatio-temporal coupling at focus, which affects the interpretation of THz radiation as a bunch diagnostic and in high-field pump-probe experiments. A ray-based model confirms the coupling.