An efficient terahertz polarization converter with highly flexible tunability over an ultra-broad bandwidth

The polarization control and conversion of electromagnetic waves is of vital importance in many practical applications and experimental systems. For terahertz waves that cover a broad frequency range of 0.3–10 THz, an operation with high spectral tunability is essential, especially for spectroscopic applications. In this paper, we propose a very simple yet efficient experimental apparatus to convert a linearly polarized beam into a circularly polarized or even an elliptically polarized beam, with high efficiency (>90%) and ultrabroad bandwidth (0.3–10 THz) tunability in the terahertz region. The proposed polarization converter, consisting of only a wire-grid polarizer and a mirror, is convenient and flexible to use in the experimental setup in the laboratory. We expect that it can find broad applications in advanced spectroscopy systems like terahertz ellipsometry where an elliptically polarized beam is needed.