Refined Terahertz Ellipsometry Calibration for Optical Property Determination on Sapphire substrate

Accurate calibration is essential for reliable polarimetric measurements in frequency-domain terahertz ellipsometry. In this thesis, we investigate how different sets of calibration samples influence the quality of the extracted 4×4 Mueller matrix in an in-house built terahertz Mueller Matrix Ellipsometer operating in the 170-250 GHz range. Three calibration sets composed of various combinations of polarizers, silicon wafers, air measurements, and an anisotropic sapphire are implemented and evaluated. By comparing experimental Mueller matrices with corresponding model-calculated data, we show that calibration sets containing anisotropic samples measured at multiple azimuths significantly improve the accuracy of the reconstructed Mueller matrix, reducing the mean-squared error by up to 37%. Furthermore, we identify a systematic error in the M33 element originating from insufficient modulation of the ±45◦ polarization states and verify this through targeted polarizer-rotation experiments. A practical solution involving a motorised PSG polarizer is proposed to achieve full modulation of all Stokes parameters. The results provide clear guidelines for improving calibration strategies in terahertz Mueller-matrix ellipsometry and for enabling more accurate optical-material characterisation.