Evolution and modulation of terahertz waveforms and spectra in two-color plasma filamentation investigated by a hybrid simulation approach
Phys. Rev. A 112, 053514 – Published 17 November, 2025
DOI: https://doi.org/10.1103/hg6h-r9g3
Abstract
Terahertz (THz) pulses from the two-color laser-field scheme in air have been widely investigated due to THz broadband and the simplicity of the generation technology. The traditional unidirectional pulse propagation equation has usually been adopted to investigate THz generation and laser propagation. However, this simulation method needs to be examined when THz radiation propagation is considered since the THz frequency is close to the characteristic frequency of the plasma ionized by the two-color laser field. Here, we propose using the time-domain nonlinear wave equation (NLWE-t) to calculate THz propagation accurately and develop a hybrid algorithm to improve the computation speed. With the developed hybrid simulation and the NLWE-t, we study the temporal evolution of THz waveforms and observe that a typical THz waveform can be considered a combination of a main body and an oscillatory tail. The main body contains mainly lower-frequency components, while the tail contains higher-frequency components. This tail originates from the interactions between the main body and the plasma density gradient, and new higher-frequency components can be excited during this process. This process is essentially the cross-phase modulation of the THz wave, which is different from the self-phase modulation responsible for blueshifting in ultrafast pulses. Finally, we propose a method to modulate THz waveforms by a preformed plasma, which is similar to the backward-propagating THz radiation emitted from two-color plasma filamentation. With the plasma frequency being higher than the THz frequency, the plasma can be used as a phase modulator to adjust the spatial position relationship of different THz frequency components. The lower-frequency THz wave and the higher-frequency THz wave can be separated through the reflection at the boundary. In this way, one could modulate THz waveforms and frequencies by adjusting the plasma density and length.
