Radiation from Coupled Plasma Oscillators*

The possibility of generating a plasma dipole oscillator (PDO) and using it as a narrowband radiation source has been studied for years [1]. One method to generate a PDO is by using two counterpropagating laser pulses with slightly different frequencies in underdense plasma. Electron bunches trapped in the beat ponderomotive potential of the pulses are displaced and released to initiate plasma oscillation. PDOs generated this way emit radiation, typically in the terahertz band, with a narrowband spectrum. Interestingly, the plasma has the potential to function as a more complex oscillator, where multiple PDOs are coupled or high harmonics are generated. Recently, we discovered in 2D PIC simulations that a single PDO could transversely split into two sub-PDOs when the driving laser spot size is large enough. These sub-PDOs couple with each other, splitting the oscillation frequency from the plasma frequency into two sidebands, a typical signature of coupled oscillators. Furthermore, second harmonic emission (relative to the plasma frequency) is observed, indicating the nonlinear properties of the large amplitude PDO. Potentially, the formation of double-PDOs can be related to Langmuir wave collapse, which occurs frequently in space plasmas and is pertinent to type III solar radio bursts. In this presentation, our new findings on double-PDO formation and their coupling are discussed, along with theoretical interpretations of the observed phenomena.

[1] K.B. Kwon, et al. Sci. Rep. 8, 145 (2018).