Developing terahertz sources for relativistic particle acceleration

We demonstrate the generation of terahertz radiation pulses with a sub-luminal phase velocity using ZnTe. The pulses are near-single cycle and have been measured over a distance of 4.8 mm. Tunable phase velocities as low as (0.77 ± 0.01) c have been measured and the method employed can readily be adapted to produce velocities over a wider range.