  A novel method is presented for generating radiation using the beat wave
associated with a bi-frequency laser pulse, to excite plasma oscillations in a
plasma slab with a density gradient. By resonantly exciting a plasma wave, it
can be localised and transformed into a plasma oscillator that produces a beam
of radially polarised terahertz radiation. Particle-in-cell simulations and
analytic theory are used to demonstrate its main characteristics, which
includes narrow bandwidth. The radiator should have useful applications such as
terahertz-band particle accelerators and pump-probe experiments.
