Narrowband Terahertz Emission from a Plasma Oscillator Imbedded in a Plasma Density Gradient

A novel method is presented for generating radiation using the beat wave of a bifrequency laser pulse to excite plasma oscillations in a plasma slab that has a density gradient. The plasma wave is localized where it is excited resonantly and becomes a plasma oscillator that produces a beam of radially polarized, terahertz radiation. Particle-in-cell simulations and a theoretical analysis are used to demonstrate its main characteristics, such as its narrow bandwidth. The radiator should have useful applications including driving terahertz-band particle accelerators and for pump-probe experiments.