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

Terahertz optics enables powerful spectroscopy and imaging thanks to its non-ionizing nature and sensitivity to molecular and material properties. It is also beneficial to next-generation communications, since it offers vast bandwidth for ultra-fast, low-latency data transfer. Polarization control of terahertz waves is vital for applications in chiral spectroscopy, spin dynamics, ultrafast material studies, and communication multiplexing. Despite progress, full polarization control—particularly for ultra-broadband terahertz pulses—remains a challenge. In this work, we introduce a simple method for generating circularly polarized terahertz radiation with an ellipticity of 0.99. By optimizing key parameters—BBO crystal tilt, rotation, position, and pump chirp—we achieve broadband circular polarization across 30 THz, with indications of further extent to 40 THz. This approach advances fundamental studies in terahertz optics and paves the way for adaptive sensing and next-generation terahertz communications.