High-precision measurements of terahertz polarization states with a fiber coupled time-domain THz spectrometer

This paper presents a high-precision method for measuring terahertz polarization rotation over 0.2–2.2 THz using a fibre-coupled time-domain THz spectrometer. A free-standing wire-grid polarizer splits the transmitted THz light into orthogonal components that are measured simultaneously by two separate detectors, avoiding moving optical parts. The authors model the systematic errors introduced by non-ideal optics and alignment, and show that anti-symmetrizing the response with respect to applied magnetic field reduces them by more than four orders of magnitude. Measurements of Faraday rotation in an InGaAs 2D electron gas demonstrated 0.8% accuracy and a precision of 20 µrad (1.1 mdeg) for small rotation angles between 0.3 and 1 THz, remaining below 0.1 mrad across the full 0.2–2.2 THz range.