Strong Radiation Emission from Plasma Dipole Oscillation

The plasma oscillation and Langmuir wave are usually regarded as non-radiating, because their electrostatic, curl-free nature of the electric field prohibits its coupling with the electromagnetic wave in plasma. However, in a non-uniform plasma, the plasma oscillation can take on a non-zero component of the curl of the electric field, leading to the emission of electromagnetic radiation by mechanisms of mode conversion or travelling wave antennae. These mechanisms, in non-uniform plasmas, inevitably yield broadband emission spectra. In contrast, we conceived a novel method of obtaining quasi-narrowband radiation from a localized plasma dipole oscillation, which is generated by colliding, detuned laser pulses in plasma. The radiation exhibits a quasi-narrowband peak around the plasma frequency, typically a few THz, which should yield diverse novel applications where strong terahertz fields are required for pump-probe studies of materials. Furthermore, we show that the field emitted from a plasma oscillation immersed in uniform plasma can transport its energy through the uniform plasma over a large distance without significant decay, in contradiction with the common wisdom of radiation cut-off in ambient plasma. The new discovery may be pertinent to solar radio bursts.