Manipulation of polarizations for broadband terahertz waves emitted from laser plasma filaments

Polarization control of broadband terahertz waves is essential for applications in many areas, such as materials science, medical and biological diagnostics, near-field communications and public securities. Conventional methods for polarization control are limited to narrow bandwidth and often with low efficiency. Here, based on theoretical and experimental studies, we demonstrate that the two-colour laser scheme in gas plasma can provide effective control of elliptically polarized terahertz waves, including their ellipticity, azimuthal angle and chirality. This is achieved with a circularly polarized laser at the fundamental frequency and its linearly polarized second harmonic, a controlled phase difference between these two laser components, as well as a suitable length of the laser plasma filament. Flexible control of ellipticity and azimuthal angle is demonstrated with our theoretical model and systematic experiments. This offers a unique and flexible technique on the polarization control of broadband terahertz radiation suitable for a wide range of applications.