Terahertz time-domain spectroscopy (THz-TDS) is a reliable technique used for studying the complex optical properties of materials. Its frequency range makes it suitable for detecting low-energy collective excitations such as phonons, magnons, and plasmons. THz-TDS in transmission geometry has gained much attention over the years. However, despite the need for exploring reflective samples, the advancement of THz-TDS in reflection geometry has faced several obstacles, mainly due to its strict requirement for sub-micron precision in the placement of the sample and reference. Here we demonstrate a technique for measuring samples in reflection geometry using THz-TDS which involves systematically resolving the misplacement issue by first isolating and correcting sources of error in the experimental setup. We then use a novel and robust phase retrieval method to detect and rectify the remaining misplacement with nanometer precision. This provides us with precise values for the phase of the THz pulse, which in turn allows us to accurately measure the complex optical properties of different types of materials. We use this technique to study bulk semiconducting samples such as Si and InSb. The experimental results of incident angle and polarization-dependent measurements are shown along with the retrieved complex refractive index of these samples. Our method immensely simplifies the procedure for obtaining the optical properties of samples in the THz range. We anticipate that our technique will be applied to spectroscopic studies of many more quantum materials with collective excitations in the THz range.

We analyze the distribution of coherent terahertz radiation from a regular hexagonal microstrip antenna (MSA) made from the high-Tc superconductor Bi2Sr2CaCu2O8+x (BSCCO). We discuss the C6v symmetry of the solutions of the wave equation on a hexagonal domain and distinguish between the closed-form and non-closed-form solutions. The closed-form wavefunctions of the transverse magnetic (TM) electromagnetic cavity modes are presented and formulas for the radiated power arising from the uniform part of the AC Josephson current and from the resonant cavity modes are derived. The wavefunctions and angular distribution of radiation from both sources are plotted for sixteen of the lowest-energy modes. Finally, we comment on the relevance of these power distributions to hexagonal arrays of equilateral triangular MSAs and propose a strategy for studying the non-closed-form modes.

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Terahertz (THz) has become a strong area for scientific research and commercial application in recent years. This research group has redesigned and optimized a THz photoconductive antenna, which currently operates with approximately 10x the power of a commercial antenna. It has been determined by this research that the THz signal emitted from a photoconductive antenna consists of coherent and incoherent signals. In addition to the improvement of the THz photoconductive antenna, I have optimized an electro optic THz detection system by characterizing the field dependency of an electro optic crystal, which enabled me to estimate the THz electric field strength. The high power THz source and optimized detection system were combined into a high power, high resolution time domain THz spectrometer. This spectrometer was used to conduct original measurements of the THz spectrum of water vapor, ionized air, and various chemical vapor including explosives. Most of these measurements were only possible with our improved THz spectrometer. In order to understand ionized air, an additional study was carried out to explore the ionization of several gases (e.g. N2, O2, Ar, CO2, and water vapor) which were ionized by radioactive isotopes. This unique study found that in addition to dose rate, the gamma energy of the radioactive isotopes and the sequential ionization levels of gases affect the equilibrium ion densities of these gases. This effect was especially pronounced for argon gas. The study of ion dynamics in gases has lead to the development of a prototype for stand-off detection and identification of radioactive isotopes. This prototype, despite being simple in design, can detect isotopes faster and more cheaply than a conventional gamma ray spectrometer. Throughout this thesis research I have successfully developed a high power, high resolution terahertz spectrometer and demonstrated that with the spectrometer I could identify characteristic resonances of water vapor, some chemicals including explosives, and even ionized air produced by nuclear isotopes. From the characteristic resonance frequencies one can understand the underlying physics or chemistry of molecules or atoms. / Physics

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23451号 / 理博第4745号 / 新制||理||1680(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 田中 耕一郎, 教授 佐々 真一, 教授 金光 義彦 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM