Refined Terahertz Ellipsometry Calibration for Optical Property Determination on Sapphire substrate

Abstract

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.

Popular Abstract

So Long, and Thanks for All the Light

At the beginning of the Universe, there was light. More than 13 billion years later, it is still everywhere, bouncing off surfaces, passing through materials, and quietly carrying information. We see because light reaches our eyes. You are reading this sentence because light has travelled from this screen to your retina. Yet light does far more than let us see: it helps us understand the world around us. From exploring distant galaxies to developing new technologies, light is one of science’s most powerful tools.

To learn from light, however, we must measure it carefully. Light can change when it reflects from a surface or passes through a material, and those changes can reveal hidden properties of what it interacts with. My bachelor’s thesis focuses on how to make such measurements more reliable by improving how an instrument that studies light is prepared and tested before it is used.

The technique I work with is called ellipsometry. In simple terms, ellipsometry studies how the orientation of light changes after it reflects from a surface. Light does not just travel forward; it also oscillates, and the direction of that oscillation is known as polarization. When light reflects from a material, this polarization usually changes in a characteristic way. By carefully measuring that change, we can learn about the material without touching or damaging it.

In my project, this method is applied using terahertz radiation, a type of light that lies between microwaves and infrared light on the electromagnetic spectrum. Terahertz light is particularly gentle and can pass through many materials that visible light cannot. This makes it useful for studying delicate or complex materials, but it also places high demands on measurement accuracy.

Before any meaningful measurements can be made, the instrument itself must be calibrated. Calibration refers to the process of verifying that an instrument behaves precisely as expected when measuring well-known reference samples. If this step is done poorly, even perfect data can lead to incorrect conclusions. My thesis investigates how different choices of calibration samples affect the quality of the final measurements.

By comparing several calibration strategies, I demonstrate that utilizing more informative reference samples, particularly those composed of more complex materials, yields significantly better results. In practice, this means the instrument can distinguish subtle changes in light more clearly and produce measurements that agree more closely with theory. My work demonstrates that careful calibration is not just a technical detail, but a key factor in improving the reliability of terahertz light measurements.

Why does this matter? Because modern technology depends on understanding the materials we work with, and on creating new materials for new applications. Ellipsometry uses light as a gentle probe, allowing us to uncover a material’s properties without cutting, touching, or altering it. This makes it especially useful for studying entirely new materials and determining where they can be put to practical use.

This bachelor’s thesis contributes a practical improvement to how terahertz light experiments are carried out. By refining how we prepare and test the instruments themselves, we can make better use of the information that light provides, and take one small step toward understanding life, the Universe... and everything.

• author: Sokolovic, Igor
• supervisor: Vanya Darakchieva; Viktor Rindert
• organization: Department of Physics; Solid State Physics
• course: FYSK04 20252
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Ellipsometry, Mueller Matrix, MME, Mueller Matrix Ellipsometry, Terahertz, Sapphire, Anisotrpic, Callibration, Callibration Matrix, PSG, Polarization State Generator, Rotating Waveplates, 3D Printed Waveplates
• language: English
• id: 9223807
• date added to LUP: 2026-03-09 12:27:43
• date last changed: 2026-03-09 12:27:43