Robust <i>in situ</i> calibration of broadband terahertz time-domain polarimetry systems using a leaky wire grid polarizer

Polarization-sensitive measurements provide rich information about material properties and enable a wide range of applications. Accurate calibration of polarimetry and ellipsometry systems is especially important in broadband spectral imaging instruments, where the performance of polarizers varies with frequency and manufacturing tolerances can introduce systematic errors. Terahertz time-domain polarimetry (THz-TDP) offers amplitude and phase information across a broad spectral range that encompasses many low-energy resonances of chemicals, yet THz polarimetry remains less developed than its infrared and optical counterparts. In this work, we present a generalized in situ calibration technique for THz-TDP imaging systems that uses a rotating polarizer placed over a reference mirror. The method accounts for the leaky, frequency-dependent response of the wire grid polarizer and simultaneously extracts the polarizer response and system calibration parameters from the same measurement. We implement the approach on a handheld polarimetric THz scanner and demonstrate that including a leaky polarizer model yields more consistent and accurate calibration parameters across spectrum and the field of view, as compared to a simple model of an ideal polarizer. The method is validated using two wire grid polarizers with different extinction ratios, illustrating that an accurate in situ calibration can be achieved even when the polarizer response is imperfect and unknown a priori.