Applications in broadband THz spectroscopy towards material studies

Türkşen, Zeynep
The purpose of this work was to construct and analyze a THz time domain spectroscopy (THz-TDS) system by using a nanojoule energy per pulse ultrafast laser (non-amplified ultrafast laser or oscillator) source and a non-linear optical generation method for THz generation. First a THz-TDS system, which uses photoconductive antenna (PCA) method for THz generation, was built to understand the working principles of these types of systems. This THz-TDS system which used PCA for generation and a 2mm thick <110> ZnTe crystal for detection had a bandwidth up to 1 THz with a 1000:1 signal to noise ratio (S/N). Using this system, various materials were investigated to study the usefulness of the obtained bandwidth. Absorption coefficient and refractive indices of the sample materials were calculated. Results showed that the bandwidth of the system was not sufficient to obtain fingerprint properties of these materials. In order to improve the system, optical rectification method was used for THz generation. A different THz-TDS system was built with a 1mm thick <110> ZnTe crystal used for the method of non-linear generation of THz radiation. Theoretical calculations of radiated intensity and electric field were done to analyze the expected bandwidth of the system. Results showed that the generation and the detection crystal thicknesses affect the obtained bandwidth of the system in that the bandwidth limiting factor is the crystal thickness and not the ultrafast laser pulse duration. Especially for detection, measurements obtained with both a 1mm thick and 2mm thick <110> ZnTe crystal showed that there was not much difference in bandwidth as was predicted by theory. Also in order to increase the signal to noise ratio, the optics used in the system were optimized. It was found that by using same focal lengths for focusing and collimating optics around the generation crystal and by using a short focal length parabolic mirror, S/N could be improved. After these improvements this THz-TDS system which uses optical rectification for THz generation and electro-optic method for THz detection had a larger bandwidth up to 3 THz but with a lower 100:1 signal to noise ratio.
Citation Formats
Z. Türkşen, “Applications in broadband THz spectroscopy towards material studies,” M.S. - Master of Science, Middle East Technical University, 2011.