Radiation from Coupled Plasma Oscillators

J. Lee, M. Kumar, D. Park, M. S. Hur

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

[1] K. B. Kwon, et al. "High-energy, short-duration bursts of coherent terahertz radiation from an embedded plasma dipole." Sci. Rep. 8, 145 (2018).

[2] J. Lee, et al. "Intense narrowband terahertz pulses produced by obliquely colliding laser pulses in helium gas." Phys. Plasmas 30, 4 (2023).