Mazaheri, Zahra (2023) Terahertz Time-Domain Spectroscopy Ellipsometry. [Tesi di dottorato]

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Abstract

Ellipsometry is extensively used in the optical regime to investigate the properties of many materials as well as to evaluate with high precision the surface roughness and thickness of thin films and multilayered systems. Due to the inherent non-coherent detection technique, data analyses in optical ellipsometry tend to be complicated and require the use of a predetermined model in the case of multilayered structures, therefore, indirectly linking the sample properties to the measured ellipsometric parameters. On the other hand, in the time domain the ellipsometric technique utilizes a coherent detection technique. Hence, we have knowledge of the amplitude and phase of the signal interacting with the material, which provides a direct method for measuring the optical properties of the sample under test. This work aims to report the procedures taken to develop and calibrate a customized Terahertz time-domain spectroscopic ellipsometer (THz-TDSE), which is based on a coherent detection approach and allows in a simple and direct way the measurement of the material response. This thesis consists of four chapters as follows: • Chapter 1: In this chapter, we give an introduction to Terahertz technology, as well as Terahertz spectroscopy in time and frequency domains. In particular, we explain in detail the THz time-domain spectroscopy technique. Additionally, we present the results of measurements done on some bulk material with finite thickness and on polar liquids in a standard transmission configuration, which are considered challenging samples in the THz region as they highly absorb THz radiation. Chapter 2: In this chapter, we discuss the ellipsometry technique for spectroscopic purposes. The main focus of this chapter is on giving an insight into how we achieve information about the sample’s intrinsic properties by studying the change in polarization states of light after interacting with the sample. Additionally, information on how to analyze the ellipsometric data is presented. • Chapter 3: In this chapter, we present the heart of our study on Terahertz time-domain spectroscopic ellipsometry (THz-TDSE). We introduce the process of designing and building a customized Terahertz spectroscopy ellipsometer operating in time-domain along with a standard THz timedomain spectrometer capable of measuring at low temperatures both using the same laser source in the ultrafast spectroscopy laboratory of ”Federico II” University of Naples, and the procedure we bear on to calibrate the customized THz-TDSE to enhance the accuracy of measurements. In addition, we present the results of measurements on a wide variety of samples from bulk semiconductors to liquids. The results indicate that THz-TDSE is more reliable in measuring the extinction coefficient than optical ellipsometry, which makes this technique a potential candidate to investigate the absorption properties of lossy materials. The presented setup shows potential in measuring the dielectric response of liquids. • Chapter 4: This chapter is dedicated to studying binary mixtures of liquids, aqueous isopropyl alcohol solutions, with different volume fractions. The presence of hydrogen bonding makes the dielectric response of these solutions complicated. Modeling the dielectric function of such samples cannot be done using conventional models such as Maxwell-Garnett. The effective Debye relaxation model is presented to model experimental results. The difference between directly fitting the experimental retrieved dielectric function of each solution is compared with the theoretical response based on the effective Debye model corresponding to pure water and isopropyl alcohol for a selected volume fraction. Finally, we conclude this work by presenting the possible prospects for future work and limitations that we need to overcome in order to make this setup operate in a very wide frequency bandwidth and with high resolution.

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