Polarization and amplitude tuning of terahertz radiation from laser-induced air filaments

Terahertz radiation with tunable polarization and amplitude can facilitate applications in many fields such as biological diagnosis and terahertz imaging. Using the photocurrent model and far-field coherent superposition method, we show that two-color laser-induced air filaments can generate terahertz radiation with independent control on both its polarization and amplitude when a circularly polarized fundamental wave and an elliptically polarized second harmonic are adopted. The polarization of the terahertz radiation, including polarization angle and polarization ellipticity, can be controlled independently by tuning the initial relative phase between two-color components and the filament length, respectively. Meanwhile, by increasing the polarization ellipticity of the second harmonic from $\ensuremath{-}1$ (with polarization opposite to the fundamental wave) to 1, the amplitude of the terahertz radiation can be tuned continuously from 0 to its maximum with its polarization fixed at a desirable state. Our studies provide an all-optical method for the manipulation of multiple parameters of terahertz radiation, which is useful for terahertz science.