Characterizing and mitigating scattering effects in terahertz time domain spectroscopy measurements.

Abstract

Terahertz research has came a long way since its inception in the mid 1980s when the first pulsed THz emission was reported using electro-optic sampling. With rapid advent in THz generation and detection techniques, research in terahertz time-domain spectroscopy (THz-TDS) has progressed to such a great extent that terahertz is finding potential use in real world applications such as biomedical sensing, security screening and defence related applications. While many researchers and commercial organizations have successfully demonstrated efficacy of terahertz, various challenges still exist before THz technology transitions from the realm of research into everyday life. This thesis focuses on the topical area of characterization and mitigation of scattering in terahertz time-domain spectroscopy measurements. Motivated by the lack of theoretical models and signal processing techniques, this thesis, presents several pieces of novel work that include theoretical models, numerical methods, signal processing techniques and experimental procedures to estimate and mitigate the scattering contribution in THz-TDS measurements of dielectric materials. The thesis is divided in to three main sections: Section I describes the various theoretical models developed for estimating and approximating the scattering cross-section, when an electromagnetic wave interacts with a random medium with characteristic particle dimensions comparable to the wavelength of the incident radiation. The section is divided in two main sub-sections, (i) scattering through a sparse distribution of particles, and (ii) scattering from dense media. Section II presents several signal processing based approaches for estimating and mitigating scattering effects in THz-TDS measurements for samples that exhibit sharp and sparse absorption features, without requiring a priori information such as its granularity, refractive index, and density. Section III discusses some common experimental techniques such as milling the material of interest into fine powder and time domain averaging spatially disjoint or multiple sample measurements, in order to reduce the presence of scattering features and effects in the THz-TDSmeasurements. Recognizing the invasive access and/or specializedmeasurement apparatus requirement for these techniques, we present our preliminary investigation in analysing multiple Fresnel echoes for estimating and mitigating scattering contribution in THz-TDS measurements. In addition to this, the thesis offers an introductory background to THz-TDS, in areas of hardware, applications, signal processing, and terahertz interaction with matter.