Terahertz time-domain spectroscopy and its application towards measurements in biology

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2016
Özer, Zeynep
Terahertz Time-Domain Spectroscopy (THz-TDS) has proved to be an exceptional tool in measurements of biological and non-biological materials. The main reason why this method is preferred is that terahertz waves are typically harmless on samples due to the low excitation energy. Furthermore, one is able to extract parameters such as the absorption and refractive index easily directly from the data without using modeling. The low energy of the terahertz wave also makes it a useful tool for characterizing dynamic behavior of a system such as done with pump/probe experiments. Also, outstanding properties of terahertz waves provide opportunity to investigate some important characteristics of biological samples like water content, vibrational spectra, and torsional modes. In this work, THz-TDS system was designed and tested using the facilities available at METU driven with an ultrafast sub 20fs Ti: sapphire mode-locked laser source. By performing experiments with the home-built terahertz time-domain spectrometer the behavior of a variety of biological materials were investigated. Speci cally the studies concentrated in developing an understanding the inter v action with terahertz radiation with hard tissues such as teeth and soft tissues such as skin. Furthermore, the interaction of the radiation with the speci c amino-acid L-trytophan was also investigated. Soft-tissue investigations focused on skin samples for development of techniques toward characterization of burn victims. Observation of changes in the burn injury's water content is one of the most important factors of the treatment. Because terahertz waves are highly absorbed by water and transparent through lots of visibly opaque materials, terahertz time-domain techniques are important to follow the healing process without removing the bandage. Burned rat-skin tissue on a glass plate was compared with glass plate with distilled drop of waterunderneathburnedrat-skinsampleandfromthemeasurementsabsorption coe cients of these samples showed a large di erence between 0.6THz and 1THz. Hard-tissue studies focused on dental applications. Tooth samples, which were prepared by thinly slicing primary and adult teeth, were measured in the wet and dry states. Wet samples have higher absorption and lower refractive index compared to dry samples. In addition to that, half of the tooth samples showed signs of decay, and the refractive index as well as absorptive properties of these samples showed clear di erences when compared to intact ones. Between 0.1 THz and 0.5 THz, the decayed tooth samples have lower refractive index and higher power absorption than the intact ones. Results show that there are clear di erences between dentine and enamel sections of each sliced tooth. In addition, decays tend to drastically change the refractive index and absorption pro le which suggests that terahertz techniques can be an invaluable tool in dentistry. Finally, sensitivity of our measurement techniques and its potential in identifying minute concentrations of biological materials was tested in studies focused on discerning the concentration of a known biological sample in a background environment. In the rst step of the concentration based L-tryptophan study, the characteristic tryptophan peaks in the frequencies of 1.42 THz and 1.82 THz in the 0.1 2 THz frequency region were observed for pure samples. The explanation of these peaks is given as the torsional motion of C11 and C12 and the ring vi torsions of C1 C9 in the tryptophan molecule, respectively. When mixed with varying concentrations of polyethylene in a pellet sample, clear changes were observed in both the absorption and refractive index pro les of samples with di erent L-tryp to polyethylene ratios. The results show that THz-time domain spectroscopy is a useful tool to monitor the change in concentration of certain amino acids like tryptophan in a background not only the frequency domain but also in the time-domain. The measurements as outlined throughout this thesis shed light on understanding the limits and detection capabilities of THz pulsed techniques when applied to biological materials.

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Citation Formats
Z. Özer, “Terahertz time-domain spectroscopy and its application towards measurements in biology,” M.S. - Master of Science, Middle East Technical University, 2016.