Generation and modulation of fully circularly polarized ultrabroadband THz radiation using two-color gas plasma techniques

We present a novel, easy-to-implement, and cost-effective method for generating circularly polarized THz radiation using a single-arm setup, which achieves a record-breaking polarization ellipticity of 0.99. By systematically optimizing critical parameters, such as the tilting and rotation angles of the BBO crystal, its position, and the chirp applied to the fundamental harmonic wave, we demonstrate the generation of broadband circularly polarized THz radiation. Our method extends the spectral range of circular polarization up to 40 THz, with robust performance up to 8 THz. However, the lack of sufficient spectral components at the lowest frequencies causes a loss of ellipticity, consistent with theoretical predictions recently reported in the literature. Building on our findings, we also propose two types of high-speed THz modulators by acting on the fundamental beam polarization or its angle of incidence on the BBO crystal by means of a Pockels cell and an acousto-optic beam deflector, respectively. These modulators offer the advantage of simultaneous control over the polarization axis, ellipticity, and energy, enabling more responsive and precise modulation compared to conventional devices. Our work bridges theoretical predictions with experimental results, offering a deeper understanding of THz polarization mechanisms in two color plasma generation. The proposed techniques not only advance fundamental research but also open new avenues for adaptive sensing technologies and advanced communication systems.